ASTM C1553-21

Standard Guide for Drying of Spent Nuclear Fuel

SIGNIFICANCE AND USE
4.1 Drying of the SNF and fuel cavity of the SNF container and its internals is needed to prepare for sealed dry storage, transportation, or permanent disposal at a repository. This guide provides technical information for use in determining the forms of water that need to be considered when choosing a drying process. This guide provides information to aid in (a) selecting a drying system, (b) selecting a drying method, and (c) demonstrating that adequate dryness was achieved (see Annex A2).  
4.2 The considerations affecting drying processes include:  
4.2.1 Water remaining on and in commercial, research, and production reactor spent nuclear fuels after removal from wet storage may become an issue when the fuel is sealed in a dry storage system or transport cask. The movement to a dry storage environment typically results in an increase in fuel temperature, which may be sufficient to cause the release of water from the fuel. The water release coupled with the temperature increase in a sealed container may result in container pressurization, corrosion of fuel or assembly structures, or both, that could affect retrieval of the fuel, and container corrosion.  
4.2.2 Removal of the water associated with the SNF may be accomplished by a variety of technologies including heating, imposing a vacuum over the system, flushing the system with dry gases, and combinations of these and other similar processes.  
4.2.3 Water removal processes are time, temperature, and pressure-dependent. Residual water in some form(s) should be anticipated.  
4.2.4 Drying processes may not readily remove the water that was retained in porous materials, capillaries, sludge, CRUD, physical features that retain water and as thin wetted surface films. Water trapped within breached SNF may be especially difficult to remove.  
4.2.5 Drying processes may be even less successful in removing bound water from the SNF and associated materials because removal of bound water will only occur when the t...
SCOPE
1.1 This guide discusses three steps in preparing spent nuclear fuel (SNF) for placement in a sealed dry storage system: (1) evaluating the needs for drying the SNF after removal from a water storage pool and prior to placement in dry storage, (2) drying the SNF, and (3) demonstrating that adequate dryness has been achieved.  
1.1.1 The scope of SNF includes nuclear fuel of any design (fuel core, clad materials, and geometric configuration) discharged from power reactors and research reactors and its condition as impacted by reactor operation, handling, and water storage.  
1.1.2 The guide addresses drying methods and their limitations when applied to the drying of SNF that has been stored in water pools. The guide discusses sources and forms of water that may remain in the SNF, the container, or both after the drying process has been completed. It also discusses the important and potential effects of the drying process and any residual water on fuel integrity and container materials during the dry storage period. The effects of residual water are discussed mechanistically as a function of the container thermal and radiological environment to provide guidance on situations that may require extraordinary drying methods, specialized handling, or other treatments.  
1.1.3 The basic issues in drying are: (1) to determine how dry the SNF must be in order to prevent problems with fuel retrievability, container pressurization, or container corrosion during storage, handling, and transfer, and (2) to demonstrate that adequate dryness has been achieved. Achieving adequate dryness may be straightforward for intact commercial fuel but complex for any SNF where the cladding is breached prior to or during placement and storage at the spent fuel pools. Challenges in achieving adequate dryness may also result from the presence of sludge, CRUD, and any other hydrated compounds. These may be transferred with the SNF to the storage con...

General Information

• Status: Published
• Publication Date: 30-Sep-2021
• ICS: 27.120.30 - Fissile materials and nuclear fuel technology
• Technical Committee: C26 - Nuclear Fuel Cycle
• Drafting Committee: C26.13 - Spent Fuel and High Level Waste

Relations

• Refers: ASTM C859-24 - Standard Terminology Relating to Nuclear Materials
• Effective Date: 01-Jan-2024
• Refers: ASTM C1174-20 - Standard Guide for Evaluation of Long-Term Behavior of Materials Used in Engineered Barrier Systems (EBS) for Geological Disposal of High-Level Radioactive Waste
• Effective Date: 15-Feb-2020
• Refers: ASTM C1562-10(2018) - Standard Guide for Evaluation of Materials Used in Extended Service of Interim Spent Nuclear Fuel Dry Storage Systems
• Effective Date: 01-Sep-2018
• Refers: ASTM C1174-17 - Standard Practice for Evaluation of the Long-Term Behavior of Materials Used in Engineered Barrier Systems (EBS) for Geological Disposal of High-Level Radioactive Waste
• Effective Date: 01-Jul-2017
• Refers: ASTM C859-14a - Standard Terminology Relating to Nuclear Materials
• Effective Date: 15-Jun-2014
• Refers: ASTM C859-14 - Standard Terminology Relating to Nuclear Materials
• Effective Date: 15-Jan-2014
• Refers: ASTM C859-13a - Standard Terminology Relating to Nuclear Materials
• Effective Date: 01-Jun-2013
• Refers: ASTM C859-13 - Standard Terminology Relating to Nuclear Materials
• Effective Date: 01-May-2013
• Refers: ASTM C1174-07(2013) - Standard Practice for Prediction of the Long-Term Behavior of Materials, Including Waste Forms, Used in Engineered Barrier Systems (EBS) for Geological Disposal of High-Level Radioactive Waste
• Effective Date: 01-Apr-2013
• Refers: ASTM C859-10b - Standard Terminology Relating to Nuclear Materials
• Effective Date: 01-Nov-2010
• Refers: ASTM C859-10a - Standard Terminology Relating to Nuclear Materials
• Effective Date: 01-Aug-2010
• Refers: ASTM C859-10 - Standard Terminology Relating to Nuclear Materials
• Effective Date: 01-Feb-2010
• Refers: ASTM C1562-10 - Standard Guide for Evaluation of Materials Used in Extended Service of Interim Spent Nuclear Fuel Dry Storage Systems
• Effective Date: 01-Jan-2010
• Refers: ASTM C859-09 - Standard Terminology Relating to Nuclear Materials
• Effective Date: 15-Feb-2009
• Refers: ASTM C859-08 - Standard Terminology Relating to Nuclear Materials
• Effective Date: 15-Sep-2008

Overview

ASTM C1553-21: Standard Guide for Drying of Spent Nuclear Fuel provides critical guidance for the safe and effective drying of spent nuclear fuel (SNF) prior to sealed dry storage, transportation, or permanent disposal. Issued by ASTM International, this guide supports stakeholders involved in the nuclear fuel cycle by offering technical information to select appropriate drying systems and methods while ensuring adequate dryness is achieved. The standard applies to SNF of any design, including fuel from commercial power, research, or production reactors, and considers a range of fuel conditions and storage histories.

Drying SNF is a vital step to prevent issues such as container pressurization, corrosion of the fuel or container, and retrievability challenges. This guide details both the technical and practical considerations for removing various forms of water associated with SNF, aiding facilities in meeting safety, regulatory, and performance requirements.

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Key Topics

• Types and Sources of Water: The standard differentiates between free/unbound water, bound water (physisorbed and chemisorbed), ice, and water trapped in sludge or CRUD deposits. Identifying these forms is essential for assessing residual moisture and choosing the correct drying process.

• Drying Methods and Technologies:

  • Heating
  • Vacuum application
  • Flushing with dry gases
  • Combinations of these processes

• Fuel and Defect Categorization: SNF is classified by group (e.g., cladding type, fuel composition) and by conditions such as intact, breached, or containing sludge. Different categories require different drying protocols.

• Process Considerations:

  • Dependencies on time, temperature, and pressure
  • Effectiveness of water removal from porous materials, capillaries, and damaged fuel
  • Evaluation of dryness (e.g., vacuum pressure rebound testing)

• Safety and Compliance:

  • Emphasis on health, safety, and environmental precautions
  • Need to meet regulatory requirements (e.g., U.S. Code of Federal Regulations for packaging and disposal)

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Applications

ASTM C1553-21 serves as a foundational guide for:

• Nuclear Power Plants: Ensuring safe transition of spent fuel from wet pools to dry storage systems by minimizing corrosion risks and ensuring retrievability.

• Research and Production Reactors: Assisting facilities with specialized SNF in selecting drying methods appropriate for unique fuel designs and histories.

• Nuclear Fuel Storage and Transport Facilities: Providing procedures to demonstrate compliance with regulatory requirements for dryness before transportation or long-term storage.

• Repository Preparation: Informing steps needed prior to permanent disposal of spent nuclear fuel in geological repositories, minimizing risks of container degradation or gas generation from residual moisture.

• Regulatory Compliance: Offering a benchmark for licensing and inspection, validating that appropriate drying procedures have been implemented and documented.

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Related Standards

Facilities and professionals implementing ASTM C1553-21 should also be familiar with the following related standards and references:

• ASTM C859: Terminology Relating to Nuclear Materials
• ASTM C1174: Guide for Evaluation of Long-Term Behavior of Materials Used in Engineered Barrier Systems (EBS) for Geological Disposal of High-Level Radioactive Waste
• ASTM C1562: Guide for Evaluation of Materials Used in Extended Service of Interim Spent Nuclear Fuel Dry Storage Systems
• ANSI/ANS 8.1 & 8.7: Nuclear Criticality Safety in Operations and Storage
• ANSI/ANS-57.9: Design Criteria for Independent Spent Fuel Storage Installations
• Title 10 of the U.S. Code of Federal Regulations (10 CFR 60, 63, 71, 72, 961): Rules governing disposal, storage, and transport of spent nuclear fuel and high-level radioactive waste

For comprehensive implementation, users should consult these documents to ensure alignment with terminology, safety protocols, material behavior, and regulatory expectations.

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By adhering to ASTM C1553-21, nuclear industry stakeholders can optimize SNF drying processes, address fuel integrity and container longevity, and satisfy stringent safety and regulatory requirements throughout the storage and disposal lifecycle.

Standards Content (Sample)

This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the
Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
Designation: C1553 − 21
Standard Guide for
Drying of Spent Nuclear Fuel
This standard is issued under the fixed designation C1553; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision.Anumber in parentheses indicates the year of last reapproval.A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope compounds. These may be transferred with the SNF to the
storage container and may hold water and resist drying.
1.1 This guide discusses three steps in preparing spent
1.1.4 Units are given in both SI and non-SI units as is
nuclear fuel (SNF) for placement in a sealed dry storage
industrystandard.Insomecases,mathematicalequivalentsare
system: (1) evaluating the needs for drying the SNF after
given in parentheses.
removal from a water storage pool and prior to placement in
1.2 This standard does not purport to address all of the
dry storage, (2) drying the SNF, and (3) demonstrating that
safety concerns, if any, associated with its use. It is the
adequate dryness has been achieved.
responsibility of the user of this standard to establish appro-
1.1.1 The scope of SNF includes nuclear fuel of any design
priate safety, health, and environmental practices and deter-
(fuel core, clad materials, and geometric configuration) dis-
mine the applicability of regulatory limitations prior to use.
charged from power reactors and research reactors and its
1.3 This international standard was developed in accor-
condition as impacted by reactor operation, handling, and
dance with internationally recognized principles on standard-
water storage.
ization established in the Decision on Principles for the
1.1.2 The guide addresses drying methods and their limita-
Development of International Standards, Guides and Recom-
tionswhenappliedtothedryingofSNFthathasbeenstoredin
mendations issued by the World Trade Organization Technical
water pools. The guide discusses sources and forms of water
Barriers to Trade (TBT) Committee.
that may remain in the SNF, the container, or both after the
drying process has been completed. It also discusses the
2. Referenced Documents
important and potential effects of the drying process and any
2.1 ASTM Standards:
residual water on fuel integrity and container materials during
the dry storage period. The effects of residual water are C859Terminology Relating to Nuclear Materials
discussed mechanistically as a function of the container ther- C1174Guide for Evaluation of Long-Term Behavior of
mal and radiological environment to provide guidance on Materials Used in Engineered Barrier Systems (EBS) for
situations that may require extraordinary drying methods, Geological Disposal of High-Level Radioactive Waste
specialized handling, or other treatments. C1562Guide for Evaluation of Materials Used in Extended
1.1.3 The basic issues in drying are: (1) to determine how Service of Interim Spent Nuclear Fuel Dry Storage Sys-
tems
dry the SNF must be in order to prevent problems with fuel
retrievability, container pressurization, or container corrosion 2.2 ANSI/ANS Standards:
during storage, handling, and transfer, and (2) to demonstrate ANSI/ANS 8.1-1998Nuclear Criticality Safety in Opera-
that adequate dryness has been achieved. Achieving adequate tions with Fissionable Materials Outside Reactors
ANSI/ANS-8.7-1998Nuclear Criticality Safety in the Stor-
dryness may be straightforward for intact commercial fuel but
complex for any SNF where the cladding is breached prior to age of Fissile Materials
ANSI/ANS-57.9American National Standard Design Crite-
or during placement and storage at the spent fuel pools.
Challengesinachievingadequatedrynessmayalsoresultfrom ria for Independent Spent Fuel Storage Installation (Dry
Type)
the presence of sludge, CRUD, and any other hydrated
1 2
This guide is under the jurisdiction ofASTM Committee C26 on Nuclear Fuel For referenced ASTM standards, visit the ASTM website, www.astm.org, or
Cycle and is the direct responsibility of Subcommittee C26.13 on Spent Fuel and contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM
High Level Waste. Standards volume information, refer to the standard’s Document Summary page on
Current edition approved Oct. 1, 2021. Published November 2021. Originally the ASTM website.
approved in 2008. Last previous edition approved in 2016 as C1553–16. DOI: Available fromAmerican National Standards Institute (ANSI), 25 W. 43rd St.,
10.1520/C1553-21. 4th Floor, New York, NY 10036, http://www.ansi.org.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
C1553 − 21
2.3 Government Documents: The U.S. government docu- 3.2.3 disposal, n—in nuclear waste management, the em-
ments listed in 2.3 or referenced in this standard guide are placement of radioactive materials and wastes in a geologic
included as examples of local regulations and regulatory repository with the intent of leaving them there permanently.
guidancethat,dependingonthelocationofthedrystoragesite,
3.2.4 getter, n—in nuclear waste management, a material
may be applicable. Users of this standard should adhere to the
(typically a solid) used to chemically react with certain gases
applicable regulatory documents and regulations and should
(forexample,H,O,H Ovapor)toformasolidcompoundof
2 2 2
consider applicable regulatory guidance.
low vapor pressure.
Title10onEnergy,CodeofFederalRegulations,Part60,10
3.2.4.1 Discussion—Somefuelroddesignsincludeaninter-
CFR 60,U.S. Code of Federal Regulations, Disposal of
nal getter to remove residual hydrogen/moisture from the
High Level radioactive Wastes in Geologic Repositories
internal rod atmosphere.
Title10onEnergy,CodeofFederalRegulations,Part63,10
3.2.5 independent spent fuel storage installation (ISFSI),
CFR 63,U.S. Code of Federal Regulations, Disposal of
n—asystemdesignedandconstructedfortheinterimstorageof
High-Level RadioactiveWastes in Geologic Repository at
spent nuclear fuel and other radioactive materials associated
Yucca Mountain, Nevada
with spent fuel storage.
Title10onEnergy,CodeofFederalRegulations,Part71,10
CFR71,U.S.CodeofFederalRegulations,Packagingand
3.2.6 intact SNF, n—any fuel that can fulfill all fuel-specific
Transport of Radioactive Materials
and system-related functions, and that is not breached. Note
Title10onEnergy,CodeofFederalRegulations,Part72,10
that all intact SNF is undamaged, but not all undamaged SNF
CFR 72,U.S. Code of Federal Regulations, Licensing
is intact, since in most situations, breached spent fuel rods that
Requirements for the Independent Storage of Spent
are not grossly breached will be considered undamaged.
Nuclear Fuel and High-Level Radioactive Waste
3.2.7 packaging, or SNF storage container, n—in nuclear
Title 10 on Energy, Code of Federal Regulations, Part 961,
waste management,anassemblyofcomponentsusedtoensure
10 CFR 961U.S. Code of Federal Regulations, Standard
compliance with the applicable requirements for independent
Contract for Disposal of Spent Nuclear Fuel and/or
storage of spent nuclear fuel and high-level radioactive waste
High-Level Radioactive Waste SFST-IST-1, Damaged
or for transportation of radioactive materials.
Fuel
3.2.8 repository, geologic repository, n—in nuclear waste
management, a disposal site, a permanent location for radio-
3. Terminology
active wastes.
3.1 Definitions—For definitions of terms used in this guide
3.2.9 spentnuclearfuel(SNF),n—nuclearfuelthathasbeen
but not defined herein, refer to Terminology C859 or Practice
irradiated in a nuclear reactor and contains fission products,
C1174.
activation products, actinides, and unreacted fissionable fuel.
3.2 Definitions of Terms Specific to This Standard—Various
3.2.10 sludge, n—in nuclear waste management, a slurry or
terms are used internationally for the broad set of definitions
sediment containing nuclear waste materials; a residue, gener-
related to failed fuel (ref. IAEA Nuclear Energy Series, No
ally radioactive, that has usually been formed from processing
NF-T-3.6). In this drying guide, only two fuel conditions that
operations, corrosion, or other similar reactions.
can impact drying behavior are considered (1) intact or
non-breached fuel; and (2) breached or failed fuel.
3.2.11 waste container, n—in nuclear waste management,
3.2.1 breached spent fuel rod, or failed fuel, n—spent fuel
the waste form and any containers, shielding, packing, and
rod with cladding defects that permit the release of gas from
other materials immediately surrounding an individual waste
the interior of the fuel rod; a breached spent fuel rod may be
container.
such that the cladding defects are sufficient to permit the
3.2.12 water, n—in drying of spent nuclear fuel,referstothe
release of fuel particulate; water could enter a breached spent
variousformsofH Opresentinthefuelstoragecontainer.Itis
fuel rod of any severity, and thus may adversely impact the
the total amount of moisture (specified by weight, volume, or
ability to dry the fuel to remove this water.
number of moles) present in a container as a combination of
3.2.2 CRUD, n—in nuclear waste management, deposits on
vapor, free or unbound liquid H O, physisorbed H O,
2 2
fuel surfaces from corrosion products that circulate in the
chemisorbed H O, and ice. The following specific terms for
reactor coolant.
water are used in this guide:
3.2.2.1 Discussion—Compositions of the deposits reflect
3.2.12.1 chemisorbed water, n—water that is bound to other
materials exposed to coolant and activation products formed
species by forces whose energy levels approximate those of a
during irradiation.
chemical bond.
3.2.2.2 Discussion—The term CRUD was originally an
acronym for “Chalk River Unidentified Deposits.” 3.2.12.2 physisorbed water (adsorbed water), n—water that
is physically bound (as an adsorbate, typically by weak forces)
to internal or external surfaces of solid material; the binding
energy of the first monolayer of water on oxides (for example,
The Code of Federal Regulations is available from U.S. Government Printing
ZrO ) is strong with reduced binding energy of successive
Office, Superintendent of Documents, 732 N. Capitol St., NW, Washington, DC 2
20401-0001, http://www.access.gpo.gov. monolayers.
C1553 − 21
3.2.12.3 trapped water, n—unbound water that is physically response could then be associated with the residual water,
trapped or contained by surrounding matrix, blocked vent especially unbound water, in the system.
pores, cavities, or by the nearby formations of solids that 4.2.7 Residual water associated with the SNF, CRUD, and
prevent or slow the escape of water from the waste package. sludge inside a sealed package may become available to react
with the internal environment, the fuel, and the package
3.2.12.4 unbound/free water, n—water, in the solid, liquid,
materials under dry storage conditions.
or vapor state, that is not physically or chemically bound to
4.2.8 Thermal gradients within the container evolve with
another species.
time, and as a result water vapor will tend to migrate to the
cooler portions of the package. Water may condense in these
4. Significance and Use
areas. Condensed water will tend to migrate to the physically
4.1 Drying of the SNF and fuel cavity of the SNF container
lower positions under gravity such as the container bottom.
and its internals is needed to prepare for sealed dry storage,
4.2.9 Radiolyticdecompositionofhydratedandotherwater-
transportation, or permanent disposal at a repository. This
containing compounds may release moisture, oxygen and
guideprovidestechnicalinformationforuseindeterminingthe
hydrogen to the container.
forms of water that need to be considered when choosing a
4.2.10 Extended time at temperature, coupled with the
drying process. This guide provides information to aid in (a)
presence of ionizing radiation, may provide the energy neces-
selecting a drying system, (b) selecting a drying method, and
sary to release bound or trapped water to the container.
(c) demonstrating that adequate dryness was achieved (see
5. Considerations in Drying
Annex A2).
5.1 An effective approach to drying SNF will depend on
4.2 The considerations affecting drying processes include:
fuel type, fuel condition, fuel basket design, and associated
4.2.1 Water remaining on and in commercial, research, and
materials (such as the neutron absorber in the basket).There is
production reactor spent nuclear fuels after removal from wet
no single correct or even preferred approach. Intact commer-
storage may become an issue when the fuel is sealed in a dry
cial fuel may be dried by one approach, SNF with breached
storage system or transport cask. The movement to a dry
fuel rods by another approach, and research and production
storage environment typically results in an increase in fuel
reactor fuels by yet another approach. Furthermore, the vari-
temperature, which may be sufficient to cause the release of
ablesthatmustbeconsideredinselectingadryingapproachfor
water from the fuel. The water release coupled with the
one fuel type may differ significantly from those that are
temperature increase in a sealed container may result in
importantforanotherfueltype.Forexample,hydridebehavior
container pressurization, corrosion of fuel or assembly
should be considered in fuel systems clad with zirconium-
structures, or both, that could affect retrieval of the fuel, and
basedalloysbutisnotimportanttoaluminumorstainlesssteel
container corrosion.
clad SNF. An effective drying approach will minimize the
4.2.2 RemovalofthewaterassociatedwiththeSNFmaybe
potential for damage of the fuel during the drying operation
accomplished by a variety of technologies including heating,
and subsequent dry storage. Ref. (1) provides additional
imposing a vacuum over the system, flushing the system with
information regarding vacuum drying.
dry gases, and combinations of these and other similar pro-
5.1.1 Some forms of fuel degradation, such as cladding
cesses.
pinholes or cracks, may form before or during the dry storage
4.2.3 Water removal processes are time, temperature, and
period without violating design or licensing requirements.
pressure-dependent. Residual water in some form(s) should be
However, damage such as small cladding cracks or pinholes
anticipated.
formedduringthedrystorageperiodcouldcausethefueltobe
4.2.4 Drying processes may not readily remove the water
reclassified as failed fuel for disposal. Fuel is classified at the
that was retained in porous materials, capillaries, sludge,
time of loading, so the drying process should be chosen to
CRUD, physical features that retain water and as thin wetted
balance the risks caused by the presence of water in the
surface films. Water trapped within breached SNF may be
container and the risks incurred by removing the water.
especially difficult to remove.
5.2 Thermal cycling during drying of commercial light
4.2.5 Drying processes may be even less successful in
water reactor SNF may affect the hydride morphology in the
removing bound water from the SNF and associated materials
cladding (2). Heating the SNF during a drying operation may
because removal of bound water will only occur when the
dissolve precipitated hydrides, and subsequent cooling may
threshold energy required to break the specific water-material
result in hydride reprecipitation. The hydride orientation and
bonds is applied to the system. For spent nuclear fuel this
thereforethepropertiesofthefuelcladdingmaybeaffectedby
threshold energy may come from the combination of thermal
the dissolution-reprecipitation process.
input from decay heat, externally applied heat, or from the
5.3 Research reactor and other non-commercial SNF that is
ionizing radiation itself.
not treated or reprocessed may be stored in sealed canisters
4.2.6 Theadequacyofadryingproceduremaybeevaluated
within regulated dry storage systems. Such dry storage canis-
by measuring the response of the system after the drying
ters may be expected to contain the SNF through interim
operation is completed. For example, if a vacuum drying
storage, transport, and repository packaging.
technology is used for water removal, a specific vacuum could
beappliedtothesystem,thevacuumpumpsturnedoff,andthe 5.4 The following objectives of drying processes are com-
time dependence of pressure rebound measured. The rebound mon to most fuels and containers:
C1553 − 21
5.4.1 Preclude geometric reconfiguration of the packaged consequently, liquid water may undergo a considerable tem-
fuel, perature drop during drying. Since the heat of fusion of water
(79.7 cal/g) is relatively small, the energy removed from the
5.4.2 Prevent damage to the container from over-
pressurization, liquid by evaporation can cause the remaining water to freeze.
Additionally,icecanformduringwaterexpansionfromcracks.
5.4.3 Minimize damage to the internal components of the
Measures may be necessary to prevent the water from freezing
canister from material corrosion,
inthecontainerorinthevacuumlines.Dryingprocedureswith
5.4.4 Minimize hydrogen generation that presents problems
thermal homogenization steps such as a helium backfill or use
during storage, transport, or repository handling operations,
of other hot inert gases usually prevent ice formation. It is also
and
important to route vacuum lines to avoid low spots. Throttling
5.4.5 Minimize the risk of the formation of significant
of vacuum pumps to slow the rate of vacuum drying may also
quantities of potentially pyrophoric hydrides of metallic fuels
prevent ice formation (see Annex A2).
(particularly uranium metal).
5.7.3 Physisorbed Water—Physisorbedwaterisfoundonall
5.5 The selection of the drying methodology for treating
external surfaces of the SNF (for example, cladding and
fuelfordrystorage,transportation,ordispositioninageologic
assembly hardware) and the container internals (for example
repository will involve many factors including the following:
container walls, baskets, etc.). The mass of a single-molecule
5.5.1 Irradiationandstoragehistory(forexample,thedecay
thick (monolayer) physisorbed water layer is reported to be
heat output and the burnup),
between 0.187 and 0.3 mg/m (3). The layers of physisorbed
5.5.2 Nature and degree of fuel damage (for example,
water may be partial, or multiple layers, depending on the
quantity of breached rods or rods containing water),
relative humidity. Additionally, the binding force holding the
5.5.3 Forms and quantify of water in the container (for
water to the surface varies depending on the number of
example, absorbed water),
monolayerswiththeouterlayersbeinglessboundthanthefirst
5.5.4 Degree to which self-heating may contribute to the
monolayer (4). Cracks, open pores, and corrosion products
drying process,
may hinder evaporation and also increases the amount of
5.5.5 Impactofresidualwateroncorrosionanddegradation
physisorbed water by virtue of additional surfaces (4).
of the fuel and container material during storage,
5.7.4 Chemisorbed Water—Chemisorbedwatermayexistin
transportation, and disposal,
a hydroxide or hydrate in the native oxides or corrosion
5.5.6 Mechanismsandkineticsofwaterinteractionwiththe
products on the fuel, cladding, or container materials. Small
fuel and container components,
quantities can also be retained by hygroscopic species in pool
5.5.7 Natureandquantityofadheredsludgeonfuelpinsand
water.Thedehydrationofhydroxidesorhydratesoccursbythe
assembly structures, and
reformation of water molecules, which are released when the
5.5.8 Maximum allowable amount of water (including both
thermal energy or energy from ionizing radiation equals or
free and bound water) remaining in the container after drying
exceeds the bonding energy of the hydrated compound. A
is completed.
number of uranium oxide hydrates may be formed as a result
of uranium or uranium oxide contact with water. Chemisorbed
5.6 Categorization of SNF for Drying Evaluation—For
purposes of drying treatments and evaluation of drying, only water may also be found in cladding and container materials.
Aluminum metal in water forms a number of surface hydrox-
two categories of SNF need to be considered depending on
whether or not the fuel is exposed. The categories are: ides such asAl (OH) (orAl O ·3H O) which begin dehydrat-
2 3 2
ing near 100°C to the formAlO(OH) (orAl O ·H O) which is
5.6.1 Intact or non-breached fuel, and
2 3 2
stable to >340°C. Zirconium cladding may also form the
5.6.2 Breached or failed fuel.
2 4
hydrated oxides ZrO(OH) or Zr(OH) during irradiation. The
5.7 Forms of Residual Water in SNF Containers—After
water content of hydrated zirconium oxides is small, and the
drying, residual water in a variety of forms may remain on the
water will not be released below 500°C (5). Carbonaceous
fuel, fuel cladding, or internal components of the container.
deposits with varying thicknesses and morphologies can also
These forms include unbound water vapor, liquid water, ice
form on fuel cladding in graphite moderated, gas-cooled
formed during drying, physisorbed water, and chemisorbed
reactors, with thicknesses up to approximately 400 µm. See
water.
Appendix X2 for other hydroxides and hydrates formed from
5.7.1 Unbound Water—Unbound water may be present in
water contact with typical fuel and container materials.
containersofSNFtransferredfromawaterstoragepool.Water
5.8 Sources of Water—It is important to understand and
retentiondependsontheconditionofthefuelanditsgeometry,
consider the source and location of the water in developing an
the container design, and the drying process. Sources of
effective drying technique. Appendix X2 cites recent literature
unboundwateraftervacuumdryingmayincludetrappedwater
forexamplesinestimationofwatersources(freeandbound)in
and water in capillaries.
commercial SNF-in-canister configurations post-dried
5.7.2 Ice—Ice formation can be a cause for water retention
condition, and the impact of those water.
in SNF containers that have undergone vacuum drying. In
vacuum drying the gas pressure is reduced below the vapor 5.8.1 General Service Environment for Water Reactor
pressureofthewatertoevaporatetheliquidphase.Theratioof Fuel—Water surrounds most SNF assemblies until they are
the heat of vaporization of water (539.6 cal/g) to the specific placed in a dry storage environment. The fuel is irradiated in
heat (1 cal/g K) corresponds to a large temperature change; water, stored in water pools, and transferred to dry storage
C1553 − 21
containers while the fuel and the container are both under associated with the thimble tube dashpots is at the bottom of
water.Thewatermayclingtothesurfacesitcontacts,seepinto the assembly which in most drying scenarios is the cooler
cracks and crevices, and pool in low places in the storage region during the drying process. A typical breached or failed
container. Locations for water that should be considered light water reactor rod is characterized by a combination of
include: primary and secondary defects. The primary defect is the
(1)Regions beneath the assemblies, original penetration, and secondary defects may be located at
(2)Dash pots in pressurized water reactor guide tubes, some distance from it. The secondary defects are normally
(3)Water rods in boiling water reactor fuel, attributed to local hydride blistering (8). The defects are holes
(4)Crevices in grid spacers, baskets, and assemblies, and of different sizes that allow water to penetrate and fill the free
(5)Neutron absorber. volume of the rod. The size and location of the defects may
retard water removal. Advanced gas-cooled reactor fuel may
Additionally, potential impacts of the drying operation itself
retain water in the end caps if retained vertically. Failed fuel
should be considered. For example, drying operations could
pins with annular fuel pellets may accumulate a substantial
cause blistering and delamination in the neutron absorber if
quantity of water within the center bore.
water is trapped in the structures.
5.8.3.2 Clad Metallic U Fuels—CladmetallicUandmostU
5.8.2 CRUD and Sludge:
alloy fuels will not allow water inside intact cladding.Vacuum
5.8.2.1 CRUD on Commercial SNF—CRUD deposits on
drying of such fuels has been performed for intact Zircaloy-
commercial SNF may include corrosion products from reactor
clad fuels from Hanford K-basin (9). Drying tests on unirradi-
coolantsystemmaterialsorothermaterials/chemicalsfromthe
ated mock-ups have been performed to demonstrate drying
system inventory. The amount and type of the deposits are
capabilityforMagnoxelementsfromSellafieldwaterpoolsas
dependent on the reactor type, operating fuel duty, and water
a contingency for dry storage (10). However, water ingress
chemistry. Characteristic CRUD area density for pressurized
through even the smallest pinholes may have a noticeable
water reactor fuel is <5 mg/cm with an inhomogeneous
effectonmetallicUfuel.Evenatpooltemperatures,watermay
distribution over the fuel surface, typically deposited on the
oxidizeUmetalsufficientlytoruptureor“unzip”fuelcladding
upper/hotter portions of the fuel rod as a layer averaging less
(11). If the oxidation processes cause the internal environment
than 25 µm (0.001 in.) but potentially reaching 100 µm (0.004
to become sufficiently anoxic, hydrogen will be produced, and
in.) in thickness (6). CRUD deposits on boiling water reactor
the U metal will react to form UH . Exposed surfaces of UH
3 3
fuel average 25 to 76 µm (0.001 to 0.003 in.) in thickness and
may react vigorously with residual moisture or air (12).
mayreachathicknessof250µm(0.010in.) (6).Dependingon
5.8.3.3 Mixed Carbide Fuels—Mixed carbide fuels encap-
CRUD type and fuel pool chemistry, CRUD levels may be
sulated in pyrolytic carbon, graphite, or both, are designed for
reduced during pool storage.The contribution of CRUD to the
gas-cooledreactorsandshouldnotbeexposedtowater.Ifsuch
water content on the surface of commercial SNF is typically
fuels become soaked with water for any reason (dry storage
small.
mishaps,incursionofwaterintodrywells,etc.),dryingmaybe
5.8.2.2 Sludge in SNF Operations—Sludgemayaccumulate
quite difficult due to absorption of water in the pores of the
in SNF water storage systems from three primary sources: (1)
graphite or carbon. An aqueous solution can penetrate the
corrosion of the SNF and other materials in the storage pool,
graphitematrixofanHTGRfuelelementthroughitsopenpore
(2) dirt and dust entering from loading doors, ventilating
system, and under normal conditions a spherical element takes
systems, etc., (3) biological materials that enter and grow in
up about 8 mL of solution (13).
storage pools. The sources of sludge are similar in that they
5.8.3.4 Miscellaneous Research and Production Reactor
may hold significant quantities of water and could get trans-
Fuels—A wide variety of research reactor fuels have been
ferred with the fuel into dry storage containers unless the fuel
irradiated. The response of these fuels to water will depend on
is appropriately cleaned.Analyses of sludge accumulated from
the fuel composition, cladding alloy, and cladding integrity.
wetstorageofdamagedmetallicuraniumfuels (7)showedthat
Research reactor fuels generally have low decay heat output,
avarietyofaluminum,iron,anduraniumhydrousoxidesmade
which may dictate the use of specialized heating processes to
up over 90% of the dry weight of the sludge.
achieve adequate dryness. Dry storage temperatures and radia-
5.8.3 Water Associated with Specific Fuel Types:
tion levels may be so low that water radiolysis and secondary
5.8.3.1 Commercial SNF—Light water reactor fuel without
oxidation reactions are insignificant. However, many of the
any through-cladding defects will not allow water inside fuel
research and production reactor fuels have been damaged
rods. However, even very small pinholes or cracks may result
during storage and, therefore, may be difficult to dry. Each
inwaterpenetratingthecladdingduringreactoroperationsand
group of these should be evaluated separately because of the
poolstorage,andbeingheldinthefuel-to-claddinggapandthe
wide variations in type and condition.
rod plenum after drying. Pressurized water reactor fuel may
also retain water in guide tubes if (1) the dashpot drain hole is 5.9 SNF Exposure Environments—The dryness required for
blocked or partially blocked with sludge or CRUD, (2)ifthe agivenfuelisoftenrelatedtothedurationofexposureandthe
discharge point is elevated above the tube bottom or (3)in radiation, temperature, and water chemistry to which it was
some designs if there are spaces such as in the tube-in-tube exposed during reactor operation and storage. Specific fuels
design. Adequate removal of the residual water will depend typicallyhaveanenvironmentalexposurehistorythatprovides
primarily on the temperature–pressure conditions at the spe- input into probable drying requirements. The drying process
cific location within the fuel assembly. For example, the water should reliably establish amounts of residual water such that
C1553 − 21
the remaining water is insufficient to cause detrimental chemi- integrity in many fuels. One possible approach to determining
cal reactions during dry storage. The potential for water the necessary dry storage conditions may include demonstrat-
adsorption by hygroscopic species derives from pool water ing that, because of prior damage to the fuel, any anticipated
chemistry leading to formation of high strength corrosive in-storage degradation would not compromise subsequent
dropletsoncladdingorstructuralcomponentswhichshouldbe disposition options.
considered where pool water contains significant quantities of
5.9.2.3 Two primary types of dry storage systems are
potentially aggressive species.
currently in use for research reactor SNF: Underground dry
5.9.1 Commercial Reactor Fuels: wellstorageandventedstorage.Undergroundwellstorageand
interior facility storage typically operate at temperatures be-
5.9.1.1 Commercial nuclear fuel is irradiated in a water
tween ambient and 60°C, and the SNF is not sealed in a
environment at elevated temperature and pressure. If a breach
container because confinement is provided by the well or the
of the cladding develops while fuel is in-core, the internal gas
facility itself. Exterior cask storage systems may be very
will be released and water may enter into the fuel rod. Upon
similar to those used for commercial SNF even though the
removalfromthereactor,thefuelisstoredinawaterpoolwith
decay heat is insufficient to heat the cask significantly. The
the water temperature typically less than 40°C. The water
experience and expertise gained in operating the current dry
pressure acting on the fuel depends on the depth of the fuel in
storagesystemsforproductionreactorSNFshouldbecarefully
the pool water. Both the reactor and pool typically have tightly
considered if the research reactor SNF is to be transferred to
controlled water chemistries that may prevent or at least
alternative dry storage systems for storage or disposition. The
minimize fuel cladding damage.
Irradiated Fuel Storage Facility at the Idaho Nuclear Technol-
5.9.1.2 The heat generated by the SNF during storage drops
ogy and Engineering Center uses a forced ventilation system
off predictably as the fission products decay. After a suitable
with high-efficiency particulate air filtration for dry storage of
cooling time that is dependent on the fuel burnup, decay heat
research reactor fuel in unsealed canisters (17).
output,systemdesign,andapplicableregulations (14),theSNF
5.9.2.4 Residual water in vented dry storage systems can
maybemovedoutofpoolstorage(wetstorage)andplacedinto
evaporate or radiolyze over long times, so water can escape
a dry storage system.
fromthesystem.However,canisterscontainingcoolfuelsmay
5.9.1.3 The thermal performance of a cask or package can
alsoaspiratewaterfromtheexternalatmosphere.Waterevapo-
be modeled to determine the expected temperature profile as a
ration and aspiration during “dry” storage may significantly
function of time (15). Design or regulatory requirements may
change the overall chemisorbed water content of the SNF,
establish short-term temperature limits for maintaining clad-
especially if it is badly damaged. Characterization of SNF
ding integrity impacted by creep or by embrittlement, for
behavior in such vented systems may provide insight into the
example. The limits may depend on burnup, cladding design
probable behavior of SNF in alternative dry storage systems.
and fuel pressurization. Limits from 250 to 570°C (16) have
been suggested. The evaluation of the limits should consider
5.10 Potential Effects of Residual Water on SNF and
how cladding integrity is affected by hydride dissolution,
Containers—Residual water in SNF can be released to the
reprecipitation, and reorientation, creep, delayed hydride
container environment by direct, thermally-induced vaporiza-
cracking, and thermal annealing of radiation damage. The
tion of physisorbed and free water, decomposition of the
impact of the hydrogen concentration and morphology on the
chemically bonded species, and radiolytic decomposition. The
cladding properties, such as the ductility transition
released water and decomposition products may cause
temperature, will affect the temperature limits.
corrosion,pressurization,andpossiblyembrittlement,although
5.9.2 Research and Non-commercial Reactor Fuels: such degradation is not generally anticipated to have a signifi-
cant impact on the condition of fuel and storage system (18).
5.9.2.1 Research reactors have irradiation temperatures and
5.10.1 Radiolysis:
pressuresthatvarywidelybutaretypicallylowerthanthoseof
a commercial power plant. Fuel lifetimes are also quite 5.10.1.1 Radiolysis occurs as a result of gamma, beta,
variableinproductionandtestreactors.Researchreactorsmay neutron, or alpha particle interaction with residual water or
operatewithlittleornochangeinfuelsformanyyears,andthe oxyhydroxides. Radiolysis within a sealed spent fuel package
fuel may be exposed to stagnant water or a humid air releases free oxygen and hydrogen which may promote corro-
environment between operating cycles. Production reactors sion or produce a flammable atmosphere (19, 20).The specific
may provide the opposite extreme as refueling scheduled to concentration of radiolysis products depends on temperature,
provide the optimum isotope abundances, and the total fuel time, the presence of hydrated oxides, and the cover gas
irradiation time may be less than a year.
including the amounts of residual air and water; recent experi-
mentalresultsshowtheprofoundeffectsoftheseparameterson
5.9.2.2 Conditions necessary for successful dry storage of
radiolytic yield (21, 22). One calculation for an SNF container
research reactor SNF will depend on the total irradiation, fuel
with one litre of water ( 20) showed that the concentration of
type,anddecayheatoutput.Theeliminationofreprocessingin
hydrogen remained well below the flammability limit for
the U.S. essentially resulted in placing the vast majority of
hydrogen/air mixtures after 300 years of storage.
research and production reactor SNF into extended pool
storage and a few dry storage systems. The primary consider- 5.10.1.2 Neutron radiolysis is important during reactor op-
ations involved with movement of these fuels into interim dry eration but diminishes rapidly after fuel removal from the
storage include the lack of significant decay heat, the wide active core, and is insignificant by the end of pool (wet)
range of fuel cladding materials, and the lack of cladding storage.
C1553 − 21
5.10.1.3 Gamma interactions with water and hydroxyl (28). Hydrogen entry into fuel or container materials may also
groups may affect both the fuel and other hydrated compounds be driven by galvanic corrosion. High-pressure hydrogen
inside the cask. Gamma radiolysis of hydrated uranium oxides effectsdata,ingeneral,shouldnotbeusedtopredicttheimpact
will occur in fields of 1000 Gy/h (20). Hydrogen production ofhydrogenonSNFstoragecontainers(seeNote1).Austenitic
from dry (no free of physisorbed water) oxyhydroxides of stainless steels and low-strength ferritic and ferritic-pearlitic
aluminum has been reported (23). Gamma activity in SNF steels are relatively insensitive to low-pressure hydrogen
decreases over time, and the levels of hydrogen, oxygen, and exposures.
nitric acid developed during storage are generally considered
NOTE 1—Unpublished SRS data on testing of austenitic stainless steel
inconsequential even after 300 years (20). (See also Appendix
tritiumshippingcontainersusedintermittentlyfor15yearstoholdtritium
X2.)
at 1 psig indicated that tritium did diffuse into the steel structure, but to a
depth less than that required to cause fracture unless the material was
5.10.1.4 Beta radiolysis of water occurs only in close
highly stressed.
proximity to the decay event because of the limited travel of
the beta particle. However, if hydrated corrosion products are
5.10.2.3 The hydrogen concentration in a sealed container
uniformly distributed in sludges or if sludges are in contact
depends on the extent of reaction of water with fuel and
with fuel surfaces, the contribution by the beta emitting
container materials.Assuming that free and physisorbed water
isotopes to water radiolysis could be significant.
are removed by drying, the issue of adequate dryness may
relate directly to:
5.10.1.5 Alpha radiolysis occurs only when the alpha emit-
(1)The mass of hydrogen in the chemisorbed water within
ter is in direct contact with the hydrated species. Therefore,
the system,
alpha radiolysis is generally limited to hydrated fuel com-
(2)Thepotentialforthermalorradiolyticdecompositionof
poundsorfuel-bearingsludgeswithinthecontainer.Theactual
the compounds holding the water, hydrogen, or both,
rates for alpha radiolysis are not well known and additional
(3)The rate of hydrogen generation by corrosion from
work is needed (24).
chemisorbed water that is released from the compounds,
5.10.2 Hydrogen—Fuel, Cladding, and Packaging Reac-
(4)The hydrogen diffusion, venting, corrosion, gettering,
tions:
and recombination rates during interacting with the system,
5.10.2.1 Hydrogen is generated by the radiolytic decompo-
(5)Free volume within the container,
sition of water and by most metal corrosion reactions. In order
(6)The rate of hydrogen reaction with metals such as
to ensure that a flammable mixture is not present in the event
zirconium-based alloys, and
a welded canister needs to be opened, the hydrogen content in
(7)Mass of metal that may absorb hydrogen.
SNF containers is usually limited to below 4volume%, the
5.10.2.4 Hydrogen gettering may be an effective technique
lower flammability limit for hydrogen in air (20). An alterna-
to mitigate hydrogen buildup in storage containers if the
tive limit for flammability control in a container is oxygen
radiation levels are low and the hydrogen is not radiolyzed
control below 4 volume% in a mixture of any balance gas
from the getter material. However, effective gettering may
composed of hydrogen and a non-reactive gas (20, 25).
require high temperatures, so getters may have limited utility
Hydrogen generation rates can be predicted with reasonable
for long-term storage.
accuracy from the temperature, radiation levels, types of
5.10.3 Water Corrosion Reactions:
materialspresent,andwatercontent (20, 26).SeeAppendixX2
5.10.3.1 The quantities of residual water expected after
for a further discussion on hydrogen generation from residual
drying are typically small relative to the substantial internal
free and bound waters.
surface area of typical dry storage containers and the mass of
5.10.2.2 Hydrogen should also be considered for SNF
the fuel and cladding, so water corrosion damage to the
container materials over long storage times, although one
structural materials and SNF should not be significant in
calculation has shown (20) that after 300 years of storage in a
establishing adequate dryness. However, there are potential
container with one litre of residual water, the hydrogen
exceptionstotheanticipatedlackofcorrosiondamage,includ-
concentrationreachesonly2.3%.Hydrogentendstocollectin
ing:
steels at locations of high stress and surface discontinuity, and
(1)Small containers of badly damaged fuel materials
it may embrittle certain steels, especially high strength ferritic
previously exposed to water,
and martensitic steels. The effects of hydrogen in steels are
(2)Fuels that may contain large quantities of water that
fairly well established, and numerousASTM test methods are
cannot be removed with drying processes,
available for evaluating hydrogen effects (27). Hydrogen may
(3)Fuels that would be expected to release aggressive
alsobeabsorbedbyzirconium-alloycladdingandmakeitmore
fission products and reach a temperature sufficient to allow
susceptible to fracture, although compared to hydrogen picked
corrosion cracking of container welds, and
up in reactor operation, any additional hydrogen absorbed in
(4)Fuels with significant chloride contamination.
the cladding will be small in comparison and have a small
5.10.4 Fission Product Reactions:
effect. In general, these effects increase with increases in the
hydrogenconcentrationorinthestrengthofametal.Hydrogen 5.10.4.1 Some fission products could be released from fuel
content increases with increasing hydrogen fugacity, which is during storage. These could react with residual water and
generally greater at a surface during corrosion by aqueous increasethecorrosivenessofthestorageenvironment.Cesium,
environments than during exposure in gaseous atmospheres rubidium, and iodine are the fission products of primary
andthusisminimalindrystoragesystemscomparedtoin-core concern. Krypton and Xenon may add to internal container
C1553 − 21
pressures, and decay of krypton to rubidium may help spread at temperatures above 600°C, and silica, methane, hydrogen,
rubidium throughout the container. Cesium and rubidium may carbon dioxide, and carbon monoxide below 600°C (39, 40).
react with residual water to form caustic hydroxides that could
However,thereactionisextremelyslowattemperaturesbelow
leadtocausticcrackingofstainlesssteelweldmentsatelevated
500°C (26) and is therefore not considered to be important.
dry storage temperatures (>110°C). Iodine would be expected
5.10.7 Water-Oxide (Fuel) Reactions and Consequences:
to behave similarly to chlorine in attacking stainless steel
5.10.7.1 Cladding damage may lead to water ingress into
packaging components if sufficient residual tensile stress and
the fueled rods and subsequent water retention. Residual water
ion concentrations are present. Fission product interactions are
may oxidize the fuel pellets toward a low density UO hydrate
notexpectedtopresentmajorproblems,buttheyshouldnotbe
andmaysubsequentlyruptureor“unzip”thefuelcladding(see
overlooked when dryness criteria are established.
AnnexA1forfueloxidereactiondata).Thecladdingruptureis
5.10.5 Galvanic Coupling with Aluminum Clad Fuel:
a direct result of the volume expansion from hydrated com-
5.10.5.1 Internal water corrosion is a primary concern for
pound formation. For example, the hydrated compound UO ·
the storage of aluminum components if residual water is
2H O has a volume 2.6 times that of the starting UO .
present. Large quantities of stainless steel are typically present 2 2
EvaluationsofthereactionprocessindicatethatUO beginsto
in storage containers, and galvanic coupling between the 2
form hydrated phases within six weeks if exposed to moisture
stainless steel and aluminum can occur if sufficient electrolyte
at fuel storage temperatures (41). Additionally, sintered UO
is present. Galvanic coupling will result in accelerated corro-
forms metaschoepite when reacted with deionized water. The
sion of the aluminum components. This process is especially
peroxide phases, studtite and meta-studtite, may form if
important with relatively cold aluminum clad fuel in vented
storage systems where water ingress is possible. Consider- radiolysis of residual water produces hydrogen peroxide (42).
ations for magnesium-clad fuels are similar. The formation of low-density hydrated compounds and
...

This document is not an ASTM standard and is intended only to provide the user of an ASTM standard an indication of what changes have been made to the previous version. Because
it may not be technically possible to adequately depict all changes accurately, ASTM recommends that users consult prior editions as appropriate. In all cases only the current version
of the standard as published by ASTM is to be considered the official document.
Designation: C1553 − 16 C1553 − 21
Standard Guide for
Drying Behavior of Spent Nuclear Fuel
This standard is issued under the fixed designation C1553; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope
1.1 This guide discusses three steps in preparing spent nuclear fuel (SNF) for placement in a sealed dry storage system: (1)
evaluating the needs for drying the SNF after removal from a water storage pool and prior to placement in dry storage, (2) drying
the SNF, and (3) demonstrating that adequate dryness has been achieved.
1.1.1 The scope of SNF includes nuclear fuel of any design (fuel core, clad materials, and geometric configuration) discharged
from power reactors and research reactors and its condition as impacted by reactor operation, handling, and water storage.
1.1.2 The guide addresses drying methods and their limitations when applied to the drying of SNF that has been stored in water
pools. The guide discusses sources and forms of water that may remain in the SNF, the container, or both after the drying process
has been completed. It also discusses the important and potential effects of the drying process and any residual water on fuel
integrity and container materials during the dry storage period. The effects of residual water are discussed mechanistically as a
function of the container thermal and radiological environment to provide guidance on situations that may require extraordinary
drying methods, specialized handling, or other treatments.
1.1.3 The basic issues in drying are: (1) to determine how dry the SNF must be in order to prevent problems with fuel
retrievability, container pressurization, or container corrosion during storage, handling, and transfer, and (2) to demonstrate that
adequate dryness has been achieved. Achieving adequate dryness may be straightforward for undamagedintact commercial fuel but
complex for any SNF where cladding damage has occurred the cladding is breached prior to or during placement and storage at
the spent fuel pools. Challenges in achieving adequate dryness may also result from the presence of sludge, CRUD, and any other
hydrated compounds. These may be transferred with the SNF to the storage container and may hold water and resist drying.
1.1.4 Units are given in both SI and non-SI units as is industry standard. In some cases, mathematical equivalents are given in
parentheses.
1.2 This standard only discusses SNF drying and does not purport to address all of the handling and safety concerns, if any,
associated with the drying process(es). its use. It is the responsibility of the user of this standard to establish appropriate safety
safety, health, and healthenvironmental practices and to meet regulatory requirements prior to and during use of the
standard.determine the applicability of regulatory limitations prior to use.
1.3 This international standard was developed in accordance with internationally recognized principles on standardization
established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued
by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
This guide is under the jurisdiction of ASTM Committee C26 on Nuclear Fuel Cycle and is the direct responsibility of Subcommittee C26.13 on Spent Fuel and High
Level Waste.
Current edition approved July 1, 2016Oct. 1, 2021. Published November 2016November 2021. Originally approved in 2008. Last previous edition approved in 20082016
as C1553 – 08.C1553 – 16. DOI: 10.1520/C1553-16.10.1520/C1553-21.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
C1553 − 21
2. Referenced Documents
2.1 ASTM Standards:
C859 Terminology Relating to Nuclear Materials
C1174 Guide for Evaluation of Long-Term Behavior of Materials Used in Engineered Barrier Systems (EBS) for Geological
Disposal of High-Level Radioactive Waste
C1562 Guide for Evaluation of Materials Used in Extended Service of Interim Spent Nuclear Fuel Dry Storage Systems
2.2 ANSI/ANS Standards:
ANSI/ANS 8.1-1998 Nuclear Criticality Safety in Operations with Fissionable Materials Outside Reactors
ANSI/ANS-8.7-1998 Nuclear Criticality Safety in the Storage of Fissile Materials
ANSI/ANS-57.9 American National Standard Design Criteria for Independent Spent Fuel Storage Installation (Dry Type)
2.3 Government Documents: The The U.S. government documents listed in 2.3 or referenced in this standard guide are included
as examples of local regulations and regulatory guidance that, depending on the location of the dry storage site, may be applicable.
Users of this standard should adhere to the applicable regulatory documents and regulations and should consider applicable
regulatory guidance.
Title 10 on Energy, Code of Federal Regulations, Part 60, 10 CFR 60, U.S. Code of Federal Regulations, Disposal of High Level
radioactive Wastes in Geologic Repositories
Title 10 on Energy, Code of Federal Regulations, Part 63, 10 CFR 63, U.S. Code of Federal Regulations, Disposal of High-Level
Radioactive Wastes in Geologic Repository at Yucca Mountain, Nevada
Title 10 on Energy, Code of Federal Regulations, Part 71, 10 CFR 71, U.S. Code of Federal Regulations, Packaging and
Transport of Radioactive Materials
Title 10 on Energy, Code of Federal Regulations, Part 72, 10 CFR 72, U.S. Code of Federal Regulations, Licensing
Requirements for the Independent Storage of Spent Nuclear Fuel and High-Level Radioactive Waste
Title 10 on Energy, Code of Federal Regulations, Part 961, 10 CFR 961 U.S. Code of Federal Regulations, Standard Contract
for Disposal of Spent Nuclear Fuel and/or High-Level Radioactive Waste SFST-IST-1, Damaged Fuel
3. Terminology
3.1 Definitions—For definitions of terms used in this guide but not defined herein, refer to Terminology C859 or Practice C1174.
3.2 Definitions of Terms Specific to This Standard:Standard—Various terms are used internationally for the broad set of
definitions related to failed fuel (ref. IAEA Nuclear Energy Series, No NF-T-3.6). In this drying guide, only two fuel conditions
that can impact drying behavior are considered (1) intact or non-breached fuel; and (2) breached or failed fuel. Refer to
SFST-ISG-1 for additional definition details.
3.2.1 breached spent fuel rod, or failed fuel, n—spent fuel rod with cladding defects that permit the release of gas from the interior
of the fuel rod. Arod; a breached spent fuel rod may also have be such that the cladding defects sufficiently largeare sufficient to
permit the release of fuel particulate. A breach may be limited to a pinhole leak or hairline crack, or may be a gross
breach.particulate; water could enter a breached spent fuel rod of any severity, and thus may adversely impact the ability to dry
the fuel to remove this water.
3.2.2 CRUD, n—in nuclear waste management, deposits on fuel surfaces from corrosion products that circulate in the reactor
coolant. Compositions of the deposits reflect materials exposed to coolant and activation products formed during irradiation. Term
was originally an acronym for “Chalk River Unidentified Deposits.”
3.2.2.1 Discussion—
Compositions of the deposits reflect materials exposed to coolant and activation products formed during irradiation.
3.2.2.2 Discussion—
The term CRUD was originally an acronym for “Chalk River Unidentified Deposits.”
3.2.3 damaged SNF, n—in nuclear waste management, any fuel rod of fuel assembly that cannot fulfill its fuel-specific or
system-related functions.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM Standards
volume information, refer to the standard’s Document Summary page on the ASTM website.
Available from American National Standards Institute (ANSI), 25 W. 43rd St., 4th Floor, New York, NY 10036, http://www.ansi.org.
The Code of Federal Regulations is available at https://www.gpo.gov/fdsys/browse/collectionCfr.action?collectionCode=CFR. SFST-ISG-1 is available at http://
www.nrc.gov/reading-rm/doc-collections/isg/spent-fuel.html.from U.S. Government Printing Office, Superintendent of Documents, 732 N. Capitol St., NW, Washington, DC
20401-0001, http://www.access.gpo.gov.
Unpublished SRS data on testing of austenitic stainless steel tritium shipping containers used intermittently for 15 years to hold tritium at 1 psig indicated that tritium
did diffuse into the steel structure, but to a depth less than that required to cause fracture unless the material was highly stressed.
C1553 − 21
3.2.3 disposal, n—in nuclear waste management, the emplacement of radioactive materials and wastes in a geologic repository
with the intent of leaving them there permanently.
3.2.4 getter, n—in nuclear waste management, a material (typically a solid) used to chemically react with certain gases (for
example, H , O , H O vapor) to form a solid compound of low vapor pressure.
2 2 2
3.2.4.1 Discussion—
Some fuel rod designs include an internal getter to remove residual hydrogen/moisture from the internal rod atmosphere.
3.2.6 grossly breached spent fuel rod, n—a subset of breached rods. A breach in spent fuel cladding that is larger than a pinhole
leak or a hairline crack and that may permit fuel particulate release.
3.2.5 independent spent fuel storage installation (ISFSI), n—a system designed and constructed for the interim storage of spent
nuclear fuel and other radioactive materials associated with spent fuel storage.
3.2.6 intact SNF, n—any fuel that can fulfill all fuel-specific and system-related functions, and that is not breached. Note that all
intact SNF is undamaged, but not all undamaged SNF is intact, since in most situations, breached spent fuel rods that are not
grossly breached will be considered undamaged.
3.2.7 packaging, or SNF storage container, n—in nuclear waste management, an assembly of components used to ensure
compliance with the applicable requirements for independent storage of spent nuclear fuel and high-level radioactive waste or for
transportation of radioactive materials.
3.2.10 pinhole leaks or hairline cracks, n—minor cladding defects that will not permit significant release of particulate matter from
the spent fuel rod, and therefore present a minimal concern during fuel handling and retrieval operations. (See discussion of gross
defects for size concerns.)
3.2.8 repository, geologic repository, n—in nuclear waste management, a disposal site, a permanent location for radioactive
wastes.
3.2.9 spent nuclear fuel (SNF), n—nuclear fuel that has been irradiated in a nuclear reactor and contains fission products,
activation products, actinides, and unreacted fissionable fuel.
3.2.10 sludge, n—in nuclear waste management, a slurry or sediment containing nuclear waste materials; a residue, generally
radioactive, that has usually been formed from processing operations, corrosion, or other similar reactions.
3.2.14 undamaged SNF, n—SNF that can meet all fuel-specific and system-related functions. Undamaged fuel may be breached.
Fuel assemblies classified as undamaged SNF may have assembly defects.
3.2.11 waste package,container, n—in nuclear waste management, the waste form and any containers, shielding, packing, and
other materials immediately surrounding an individual waste container.
3.2.12 water, n—in drying of spent nuclear fuel, refers to the various forms of H O present in the fuel storage container. It is the
total amount of moisture (specified by weight, volume, or number of moles) present in a container as a combination of vapor, free
or unbound liquid H O, physisorbed H O, chemisorbed H O, and ice. The following specific terms for water are used in this
2 2 2
document:guide:
3.2.16.1 bound water, n—adsorbed surface layers of water and chemisorbed water.
3.2.12.1 chemisorbed water, n—water that is bound to other species by forces whose energy levels approximate those of a
chemical bond.
3.2.12.2 physisorbed water (adsorbed water), n—water that is physically bound (as an adsorbate, typically by weak forces) to
internal or external surfaces of solid material.material; the binding energy of the first monolayer of water on oxides (for example,
ZrO ) is strong with reduced binding energy of successive monolayers.
3.2.12.3 trapped water, n—unbound water that is physically trapped or contained by surrounding matrix, blocked vent pores,
cavities, or by the nearby formations of solids that prevent or slow the escape of water from the waste package.
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3.2.12.4 unbound/free water, n—water, in the solid, liquid, or vapor state, that is not physically or chemically bound to another
species.
4. Significance and Use
4.1 Drying of the SNF and fuel cavity of the SNF container and its internals is needed to prepare for sealed dry storage,
transportation, or permanent disposal at a repository. This guide provides technical information for use in determining the forms
of water that need to be considered when choosing a drying process. This guide provides information to aid in (a) selecting a drying
system, (b) selecting a drying method, and (c) demonstrating that adequate dryness was achieved.achieved (see Annex A2).
4.2 The considerations affecting drying processes include:
4.2.1 Water remaining on and in commercial, research, and production reactor spent nuclear fuels after removal from wet storage
may become an issue when the fuel is sealed in a dry storage system or transport cask. The movement to a dry storage environment
typically results in an increase in fuel temperature, which may be sufficient to cause the release of water from the fuel. The water
release coupled with the temperature increase in a sealed container may result in container pressurization, corrosion of fuel or
assembly structures, or both, that could affect retrieval of the fuel, and container corrosion.
4.2.2 Removal of the water associated with the SNF may be accomplished by a variety of technologies including heating,
imposing a vacuum over the system, flushing the system with dry gases, and combinations of these and other similar processes.
4.2.3 Water removal processes are time, temperature, and pressure-dependent. Residual water in some form(s) should be
anticipated.
4.2.4 Drying processes may not readily remove the water that was retained in porous materials, capillaries, sludge, CRUD,
physical features that retain water and as thin wetted surface films. Water trapped within damagedbreached SNF may be especially
difficult to remove.
4.2.5 Drying processes may be even less successful in removing bound water from the SNF and associated materials because
removal of bound water will only occur when the threshold energy required to break the specific water-material bonds is applied
to the system. For spent nuclear fuel this threshold energy may come from the combination of thermal input from decay heat and
forced gas flow and heat, externally applied heat, or from the ionizing radiation itself.
4.2.6 The adequacy of a drying procedure may be evaluated by measuring the response of the system after the drying operation
is completed. For example, if a vacuum drying technology is used for water removal, a specific vacuum could be applied to the
system, the vacuum pumps turned off, and the time dependence of pressure rebound measured. The rebound response could then
be associated with the residual water, especially unbound water, in the system.
4.2.7 Residual water associated with the SNF, CRUD, and sludge inside a sealed package may become available to react with the
internal environment, the fuel, and the package materials under dry storage conditions.
4.2.8 Thermal gradients within the container evolve with time, and as a result water vapor will tend to migrate to the cooler
portions of the package. Water may condense in these areas. Condensed water will tend to migrate to the physically lower positions
under gravity such as the container bottom.
4.2.9 Radiolytic decomposition of hydrated and other water-containing compounds may release moisture, oxygen and hydrogen
to the container.
4.2.10 Extended time at temperature, coupled with the presence of ionizing radiation, may provide the energy necessary to release
bound or trapped water to the container.
5. Evaluating theConsiderations in Drying Approach
5.1 The properAn effective approach to drying SNF is fuel and system-specific, and thus will depend on fuel type, fuel condition,
fuel basket design, and associated materials (such as the neutron absorber in the basket). There is no single correct or even preferred
approach. Intact commercial fuel may be dried by one approach, SNF with breached fuel rods by another approach, and research
and production reactor fuels by yet another approach. Furthermore, the variables that must be considered in selecting a drying
approach for one fuel type may differ significantly from those that are important for another fuel type. For example,
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hydrogen/hydridehydride behavior should be considered in fuel systems clad with zirconium-based alloys but is not important to
aluminum or stainless steel clad SNF. The properAn effective drying approach will minimize the potential for damage of the fuel
during the drying operation and subsequent dry storage. ReferenceRef. (1) provides additional information regarding vacuum
drying.
5.1.1 Some forms of fuel degradation, such as cladding pinholes or cracks, may form before or during the dry storage period
without violating design or licensing requirements. However, damage such as small cladding cracks or pinholes formed during the
dry storage period could cause the fuel to be reclassified as failed fuel for disposal. Fuel is classified at the time of loading, so the
drying process should be chosen to balance the risks caused by the presence of water in the container and the risks incurred by
removing the water.
5.2 Thermal cycling during drying of commercial light water reactor SNF may affect the hydride morphology in the
cladding.cladding (2). Heating the SNF during a drying operation may dissolve precipitated hydrides, and subsequent cooling may
result in hydride reprecipitation. The hydride orientation and therefore the properties of the fuel cladding may be affected by the
dissolution-reprecipitation process.
5.3 Research reactor and other non-commercial SNF that is not treated or reprocessed may be stored in sealed canisters within
regulated dry storage systems. Such dry storage canisters may be expected to contain the SNF through interim storage, transport,
and repository packaging. The objectives of drying processes used on this fuel are virtually identical to the drying objectives for
commercial fuels and are to:
5.3.1 Preclude geometric reconfiguration of the packaged fuel,
5.3.2 Prevent internal components damage to the canister from over-pressurization or corrosion, and,
5.3.3 Minimize hydrogen generation and material corrosion that present problems during storage, transport, or repository handling
operations.
5.4 The following objectives of drying processes are common to most fuels and containers:
5.4.1 Preclude geometric reconfiguration of the packaged fuel,
5.4.2 Prevent damage to the container from over-pressurization,
5.4.3 Minimize damage to the internal components of the canister from material corrosion,
5.4.4 Minimize hydrogen generation that presents problems during storage, transport, or repository handling operations, and
5.4.5 Minimize the risk of the formation of significant quantities of potentially pyrophoric hydrides of metallic fuels (particularly
uranium metal).
5.5 The selection of the drying methodology for treating fuel for interim dry storage storage, transportation, or disposition in a
geologic repository will involve many factors including the following:
5.5.1 Irradiation and storage history (for example, the decay heat output and the amount of hydrogen in the cladding),burnup),
5.5.2 Nature and degree of fuel damage (for example, quantity of breached rods or rods containing water),
5.5.3 Forms and quantify of water in the container (for example, absorbed water)water),
5.5.4 Degree to which self-heating may contribute to the drying process,
5.5.5 Impact of residual water on corrosion and degradation of the fuel and container material during drying, storage,
transportation, and disposal,
5.5.6 Mechanisms and kinetics of water interaction with the fuel and container components, and
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5.5.7 Nature and quantity of adhered sludge on fuel pins and assembly structures, and
5.5.8 Maximum allowable amount of water (including both free and bound water) remaining in the container after drying is
completed.
5.6 Categorization of SNF and Defects for Drying Evaluation—This standard guide pertains to fuels that contain any cladding
breach, such as hairline cracks or holes that may permit water ingress into a fuel element or the release of fuel materials from the
fuel element. Such breaches may cause the fuel to be categorized as failed or damaged. Failed fuel may also be categorized as in
10 CFR 961, which separates failed commercial light water reactor fuel into three classes:
Class F-1 failed fuel has visually observable failure or damage
Class F-2 failed fuel has “radioactive leakage”
Class F-3 failed fuel is badly damaged and requires “encapsulation”
Drying treatments may depend on the fuel condition. Each category of damaged or breached fuel rods could require
handling/treatment operations that differ from those used for intact fuel. It is important when considering a drying process to be
clear about the class of breached fuel rods in the fuels being dried. A given fuel element could be regraded as intact for the purposes
of storage or transport performance but as having breached fuel rods for the purposes of geologic repository disposal. The
classifications differ because the performance of damaged fuel is typically more predictable over dry storage periods than over
geologic time. SNF may be characterized as damaged or undamaged or failed through the use of one or more of the following
processes: review of operating records (core history and handling), visual examinations, ultrasonic testing, wet or dry sipping, and
eddy current testing.
Fuel type and the presence and type of defects are essential inputs for evaluating drying operations. In order to organize the
information in this guide, selected varieties of spent fuels have been placed into categories with a letter designation for
consideration. Similarly, the types of defects For purposes of drying treatments and evaluation of drying, only two categories of
SNF need to be considered are identified with a number designation. The identities of these categories are provided
below.depending on whether or not the fuel is exposed. The categories are:
5.6.1 SNF Group—The following groups are used to distinguish between SNF types: Intact or non-breached fuel, and
A Commercial oxide fuel in Zr-alloy cladding
B Carbide or oxide particle fuel with SiC coating (in graphite)
C Oxide fuel in stainless steel cladding
D U-ZrH fuel in Zr-alloy or stainless steel cladding
E Oxide fuel in Al-alloy cladding
F UAl fuel or U-ZrH fuel in Al-alloy cladding
x
G Metal fuel in Zr-alloy, Al-alloy or stainless steel cladding
H Carbide or oxide particle fuel with only pyrolytic carbon coating
I Unclad metal or oxide fuel
5.6.2 Fuel Conditions—The following Fuel Conditions are designated:Breached or failed fuel.
1 Fully intact cladding with no known penetrations
2 Cladding with small pinholes or hairline cracks that may allow entry/exit
of moisture
3 Fuel with visible or suspected sludge or coatings that may hold
significant water
4 Cladding with known large cracks or holes that will allow free entry/exit of
moisture
5 Fuel with major cladding damage from previous fuel oxidation, exposure
to water, or both
6 Rubblized fuel that has little or no intact cladding, high surface area, and
previous exposure to water (Includes fuel rods or elements that have
been sectioned for analysis or study)
5.7 Forms of Residual Water in SNF Containers—After drying, residual water in a variety of forms may remain on the fuel, fuel
cladding, or internal components of the container. These forms include unbound water vapor, liquid water, ice formed during
drying, physisorbed water, and chemisorbed water.
5.7.1 Unbound Water—Unbound water may be present in containers of SNF transferred from a water storage pool. Water retention
depends on the condition of the fuel, fuel and its geometry, the container design, and the drying process. Sources of unbound water
after vacuum drying may include trapped water and water in capillaries.
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5.7.2 Ice—Ice formation can be a cause for water retention in SNF containers that have undergone vacuum drying. In vacuum
drying the gas pressure is reduced below the vapor pressure of the water to evaporate the liquid phase. The ratio of the heat of
vaporization of water (539.6 cal/g) to the specific heat (1 cal/g K) corresponds to a large temperature change; consequently, liquid
water may undergo a considerable temperature drop during drying. Since the heat of fusion of water (79.7 cal/g) is relatively small,
the energy removed from the liquid by evaporation can cause the remaining water to freeze. Additionally, ice can form during water
expansion from cracks. Measures may be necessary to prevent the water from freezing in the container or in the vacuum lines.
Drying procedures with thermal homogenization steps such as a helium backfill or use of other hot inert gases usually prevent ice
formation. It is also important to route vacuum lines to avoid low spots. Throttling of vacuum pumps to slow the rate of vacuum
drying may also prevent ice formation. (Seeformation (see Annex A2.)).
5.7.3 Physisorbed Water—Physisorbed water is found on all external surfaces of the SNF (for example, cladding and assembly
hardware) and the container internals (for example container walls, baskets, etc.). Typical water concentrations are about 0.03 to
0.05 g/mThe mass of a single-molecule thick (monolayer) physisorbed water layer is reported to be between 0.187 and 0.3 mg/m
per(3). monolayer. The The layers of physisorbed water may be partial, or multiple layers, depending on the relative humidity.
Additionally, the binding force holding the water to the surface is weak and the water layer can be removed at relatively low
temperatures (<50°C) by vacuum drying orvaries depending on the number of monolayers with the outer layers being less bound
than the first monolayer forced(4). gas dehydration. Cracks, open pores, and corrosion products may hinder evaporation and hold
also increases the amount of physisorbed water by virtue of additional surfaces.surfaces Corrosion (4products ).on exposed U
plate fuel are notable for retaining physisorbed water.
5.7.4 Chemisorbed Water—Chemisorbed water may exist in a hydroxide or hydrate in the native oxides or corrosion products on
the fuel, cladding, or container materials. Small quantities can also be retained by hygroscopic species in pool water. The
dehydration of hydroxides or hydrates occurs by the reformation of water molecules, which are released when the thermal energy
or energy from ionizing radiation equals or exceeds the bonding energy of the hydrated compound. A number of uranium oxide
hydrates may be formed as a result of uranium or uranium oxide contact with water. Chemisorbed water may also be found in
cladding and container materials. Aluminum metal in water forms a number of surface hydroxides such as Al (OH) (or
Al O ·3H O) which begin dehydrating near 100°C to the form AlO(OH) (or Al O ·H O) which is stable to >340°C. Zirconium
2 3 2 2 3 2
2 4
cladding may also form the hydrated oxides ZrO(OH) or Zr(OH) during irradiation. The water content of hydrated zirconium
oxides is small, and the water will not be released below 500°C500 °C (25). Carbonaceous deposits with varying thicknesses and
morphologies can also form on fuel cladding in graphite moderated, gas-cooled reactors, with thicknesses up to approximately 400
μm. See Annex A1Appendix X2 for other hydroxides and hydrates formed from water contact with typical fuel and container
materials.
5.8 Sources of Water: Water—It is important to understand and consider the source and location of the water in developing an
effective drying technique. Appendix X2 cites recent literature for examples in estimation of water sources (free and bound) in
commercial SNF-in-canister configurations post-dried condition, and the impact of those water.
5.8.1 General Service Environment for Water Reactor Fuel—Water surrounds most SNF assemblies until they are placed in a dry
storage environment. The fuel is irradiated in water, stored in water pools, and transferred to dry storage containers while the fuel
and the container are both under water. The water may cling to the surfaces it contacts, seep into cracks and crevices, and pool
in low places in the storage container. Locations for water that should be considered include:
(1) Regions beneath the assemblies,
(2) Dash pots in pressurized water reactor guide tubes,
(3) Water rods in boiling water reactor fuel, and
(4) Crevices in grid spacers, baskets, and assemblies.assemblies, and
(5) Neutron absorber.
Additionally, potential impacts of the drying operation itself should be considered. For example, drying operations could cause
blistering and delamination in the neutron absorber if water is trapped in the structures.
5.8.2 CRUD and Sludge:
5.8.2.1 CRUD on Commercial SNF—CRUD deposits on commercial SNF may include corrosion products from reactor coolant
system materials or other materials/chemicals from the system inventory. The amount and type of the deposits are dependent on
the reactor type, operating fuel duty, and water chemistry. Characteristic CRUD area density for pressurized water reactor fuel is
<5 mg/cm with an inhomogeneous distribution over the fuel surface, typically deposited on the upper/hotter portions of the fuel
rod as a layer averaging less than 25 μm (0.001 in.) but potentially reaching 100 μm (0.004 in.) in thickness (36). CRUD deposits
on boiling water reactor fuel average 25 to 76 μm (0.001 to 0.003 in.) in thickness and may reach a thickness of 250 μm (0.010
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in.) (36). Depending on CRUD type and fuel pool chemistry, CRUD levels may be reduced during pool storage. The contribution
of CRUD to the water content on the surface of commercial SNF is typically small.
5.8.2.2 Sludge in SNF Operations—Sludge may accumulate in SNF water storage systems from twothree primary sources: (1)
corrosion of the SNF and other materials in the storage pool, (2) dirt and dust entering from loading doors, ventilating systems,
etc. Bothetc., (3) biological materials that enter and grow in storage pools. The sources of sludge are similar in that they may hold
significant quantities of water and could get transferred with the fuel into dry storage containers unless the fuel is appropriately
cleaned. Sludge is probably not a concern for fuels in SNF Groups A–D that have Fuel Conditions 1 or 2 and are from
well-controlled wet storage systems. However, the quantity and characteristics of the sludge carryover and its impact on drying
should be evaluated for fuels in SNF Groups A–D that have Fuel Conditions 3–6 and for SNF Groups E–I, especially those with
Fuel Conditions 3–6. Analyses of sludge accumulated from wet storage of damaged metallic uranium fuels (47) showed that a
variety of aluminum, iron, and uranium hydrous oxides made up over 90 % of the dry weight of the sludge.
5.8.3 Water Associated with Specific Fuel Types:
5.8.3.1 Commercial SNF, SNF Group A, Fuel Conditions 1–4—SNF—Light water reactor fuel without any through-cladding
defects (Fuel Condition 1) will not allow water inside fuel rods. However, even very small pinholes or cracks (Fuel Condition 2)
may result in water penetrating the cladding during reactor operations and pool storage, and being held in the fuel-to-cladding gap
and the rod plenum after drying. Similar water retention may also occur for Fuel Condition 4. Pressurized water reactor fuel in
Fuel Condition 3 may also retain water in guide tubes if (a1) the dashpot drain hole is blocked or partially blocked with sludge
or CRUD, (b2) if the discharge point is elevated above the tube bottom or (c3) in some designs if there are spaces such as in the
tube-in-tube design. Adequate removal of the residual water will depend primarily on the temperature–pressure conditions at the
specific location within the fuel assembly. For example, the water associated with the thimble tube dashpots is at the bottom of
the assembly which in most drying scenarios is the cooler region during the drying process. A typical breached or failed light water
reactor rod in Fuel Condition 4 is characterized by a combination of primary and secondary defects. The primary defect is the
original penetration, and secondary defects may be located at some distance from it. The secondary defects are normally attributed
to local hydride blistering (58). The defects are holes of different sizes that allow water to penetrate and fill the free volume of the
rod. The size and location of the defects may retard water removal. Advanced gas-cooled reactor fuel may retain water in the end
caps if retained vertically. Failed fuel pins with annular fuel pellets may accumulate a substantial quantity of water within the center
bore.
5.8.3.2 Clad Metallic U Fuels (SNF Groups F and G), Fuel Conditions 1–4—Fuels—Clad metallic U and most U allowalloy fuels
will not allow water inside intact (Fuel Condition 1) cladding. Vacuum drying of such fuels has been performed for intact
Zircaloy-clad fuels from Hanford K-basin (69). Drying tests on unirradiated mock-ups have been performed to demonstrate drying
capability for Magnox elements from Sella field water basinspools as a contingency for dry storage (710). However, water ingress
through even the smallest pinholes (Fuel Condition 2 or 4) may have a noticeable effect on metallic U fuel. Even at basinpool
temperatures, water may oxidize U metal sufficiently to rupture or “unzip” fuel cladding (811). If the oxidation processes cause
the internal environment to become sufficiently anoxic, hydrogen will be produced, and the U metal will react to form UH .
Exposed surfaces of UH may react vigorously with residual moisture or air (912).
5.8.3.3 Mixed Carbide Fuels, SNF Groups B and H—Fuels—Mixed carbide fuels encapsulated in pyrolytic carbon, graphite, or
both, are designed for gas-cooled reactors and should not be exposed to water. If such fuels become soaked with water for any
reason (dry storage mishaps, incursion of water into dry wells, etc.), drying may be quite difficult due to absorption of water in
the pores of the graphite or carbon. An aqueous solution can penetrate the graphite matrix of an HTGR fuel element through its
open pore system, and under normal conditions a spherical element takes up about 8 mL of solution (1013).
5.8.3.4 Miscellaneous Research and Production Reactor Fuels, SNF Groups C, D, and E—Fuels—A wide variety of research
reactor fuels have been irradiated. The response of these fuels to water will depend on the fuel composition, cladding alloy, and
cladding integrity. Research reactor fuels generally have low decay heat output, which may dictate the use of specialized heating
processes to achieve adequate dryness. Dry storage temperatures and radiation levels may be so low that water radiolysis and
secondary oxidation reactions are insignificant. However, many of the research and production reactor fuels have been damaged
during storage and, therefore, may be difficult to dry. Each group of these should be evaluated separately because of the wide
variations in type and condition.
5.9 SNF Exposure Environments—The dryness required for a given fuel is often related to the duration of exposure and the
radiation, temperature, and water chemistry to which it was exposed during reactor operation and storage. Specific fuels typically
have an environmental exposure history that provides input into probable drying requirements. The drying process should reliably
establish water vapor pressures levelsamounts of residual water such that the remaining water is insufficient to cause detrimental
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chemical reactions during dry storage. The potential for water adsorption by hygroscopic species derives from pool water
chemistry leading to formation of high strength corrosive droplets on cladding or structural components which should be
considered where pool water contains significant quantities of potentially aggressive species.
5.9.1 Commercial Reactor Fuels:
5.9.1.1 Commercial nuclear fuel is irradiated in a water environment at elevated temperature and pressure. If a breach of the
cladding develops while fuel is in-core, the internal gas will be released and water may enter into the fuel rod. Upon removal from
the reactor, the fuel is stored in a water basinpool with the water temperature typically less than 40°C. The water pressure acting
on the fuel depends on the depth of the fuel in the basinpool water. Both the reactor and basinpool typically have tightly controlled
water chemistries that may prevent or at least minimize fuel cladding damage.
5.9.1.2 The heat generated by the SNF during storage drops off predictably as the fission products decay. After a suitable cooling
time that is dependent on the fuel burnup, decay heat output, system design, and applicable regulations (1114), the SNF may be
moved out of basinpool storage (wet storage) and placed into a dry storage system.
5.9.1.3 The thermal performance of a cask or package can be modeled to determine the expected temperature profile as a function
of time (1215). Design or regulatory requirements may establish short-term temperature limits for maintaining cladding integrity
impacted by creep or by embrittlement, for example. The limits may depend on burnup, cladding design and fuel pressurization.
Limits from 250 to 570°C (1316) have been suggested. The evaluation of the limits should consider how cladding integrity is
affected by hydride dissolution, reprecipitation, and reorientation, creep, delayed,delayed hydride cracking, and thermal annealing
of radiation damage. The impact of the hydrogen concentration and morphology on the cladding properties, such as the ductility
transition temperature, will affect the temperature limits.
5.9.2 Research and Non-commercial Reactor Fuels:
5.9.2.1 Research reactors have irradiation temperatures and pressures that vary widely but are typically lower than those of a
commercial power plant. Fuel lifetimes are also quite variable in production and test reactors. Research reactors may operate with
little or no change in fuels for many years, and the fuel may be exposed to stagnant water or a humid air environment between
operating cycles. Production reactors may provide the opposite extreme as refueling scheduled to provide the optimum isotope
abundances, and the total fuel irradiation time may be less than a year.
5.9.2.2 Conditions necessary for successful dry storage of research reactor SNF will depend on the total irradiation, fuel type, and
decay heat output. The elimination of reprocessing in the U.S. essentially resulted in placing the vast majority of research and
production reactor SNF into extended basinpool storage and a few dry storage systems. The primary considerations involved with
movement of these fuels into interim dry storage include the lack of significant decay heat, the wide range of fuel cladding
materials, and the lack of cladding integrity in many fuels. One possible approach to determining the necessary dry storage
conditions may include demonstrating that, because of prior damage to the fuel, any anticipated in-storage degradation would not
compromise subsequent disposition options.
5.9.2.3 Two primary types of dry storage systems are currently in use for research reactor SNF: Underground dry well storage and
vented storage. Underground well storage and interior facility storage typically operate at temperatures between ambient and 60°C,
and the SNF is not sealed in a container because confinement is provided by the well or the facility itself. Exterior cask storage
systems may be very similar to those used for commercial SNF even though the decay heat is insufficient to heat the cask
significantly. The experience and expertise gained in operating the current dry storage systems for production reactor SNF should
be carefully considered if the research reactor SNF is to be transferred to alternative dry storage systems for storage or disposition.
The Irradiated Fuel Storage Facility at the Idaho Nuclear Technology and Engineering Center uses a forced ventilation system with
high-efficiency particulate air filtration for dry storage of research reactor fuel in unsealed canisters (1417).
5.9.2.4 Residual water in vented dry storage systems can evaporate or radiolyze over long times, so water can escape from the
system. However, canisters containing cool fuels may also aspirate water from the external atmosphere. Water evaporation and
aspiration during “dry” storage may significantly change the overall chemisorbed water content of the SNF, especially if it is badly
damaged. Characterization of SNF behavior in such vented systems may provide insight into the probable behavior of SNF in
alternative dry storage systems.
5.10 Potential Effects of Residual Water on SNF and Containers—Residual water in SNF can be released to the container
environment by direct, thermally induced thermally-induced vaporization of physisorbed and free water, decomposition of the
chemically bonded species, vaporization of physisorbed and free water, and and radiolytic decomposition. The released water and
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decomposition products may cause corrosion, pressurization, and possibly embrittlement, although such degradation is not
generally anticipated to have a significant impact on the condition of fuel and storage system (1518).
5.10.1 Radiolysis:
5.10.1.1 Radiolysis occurs as a result of gamma, beta, neutron, or alpha particle interaction with residual water or oxyhydroxides.
Radiolysis within a sealed spent fuel package releases free oxygen and hydrogen which may promote corrosion or produce a
flammable atmosphere (1619, 1720). These radiolysis products, in the absence of residual air, recombine and persist at very low
concentrations; however, as little as 0.1 % residual air can impede recombination and increase the concentration of radiolysis
products. The specific concentration of radiolysis products dependsThe specific concentration of radiolysis products depends on
temperature, time, the presence of hydrated oxides, and the cover gas including the amounts of residual air and water; recent
experimental results show the profound effects of these parameters on radiolytic yield temperature,(21, 22time,). and the amounts
of residual air and water. One calculation for an SNF container with one litre of water ( 1720) showed that the concentration of
hydrogen remained well below the flammability limit for hydrogen/air mixtures after 300 years of storage.
5.10.1.2 Neutron radiolysis is important during reactor operation but diminishes rapidly after fuel removal from the active core,
and is insignificant by the end of pool (basin or wet) (wet) storage.
5.10.1.3 Gamma interactions with water and hydroxyl groups may affect both the fuel and other hydrated compounds inside the
cask. Gamma radiolysis of hydrated uranium oxides will occur in fields of 1000 Gy/h (1720). Hydrogen production from dry (no
free of physisorbed water) oxyhydroxides of aluminum has been reported (1823).
...