ASTM C1807-15(2023)

Standard Guide for Nondestructive Assay of Special Nuclear Material (SNM) Holdup Using Passive Neutron Measurement Methods

SIGNIFICANCE AND USE
5.1 This guide assists in satisfying requirements in such areas as safeguards, SNM inventory control, nuclear criticality safety, waste disposal, and decontamination and decommissioning (D&D). This guide can apply to the measurement of holdup in process equipment or discrete items whose neutron production properties may be measured or estimated. These methods may meet target accuracy for items with complex distributions of SNM in the presence of moderators, absorbers, and neutron poisons; however, the results are subject to larger measurement uncertainties than measurements of less complex items.  
5.2 Quantitative Measurements—These measurements result in quantification of the mass of SNM in the holdup. They include all the corrections and descriptive information, such as isotopic composition, that are available.  
5.2.1 High-quality results require detailed knowledge of radiation sources and detectors, radiation transport, calibration, facility operations, and error analysis. Consultation with qualified NDA personnel is recommended (Guide C1490).  
5.2.2 Holdup estimates for a single piece of process equipment or piping often include some compilation of multiple measurements. The holdup estimate must appropriately combine the results of each individual measurement. In addition, uncertainty estimates for each individual measurement must be made and appropriately combined.  
5.3 Scan—Radiation scanning, typically gamma, may be used to provide a qualitative description of the extent, location, and the relative quantity of holdup. It can be used to plan or supplement the quantitative neutron measurements. Other indicators (for example, visual) may also indicate a need for a holdup measurement.  
5.4 Nuclide Mapping—To appropriately interpret the neutron data, the specific neutron yield is needed. Isotopic measurements to determine the relative isotopic composition of the holdup at specific locations may be required, depending on the facility.  
5.5 Spot Check an...
SCOPE
1.1 This guide describes passive neutron measurement methods used to nondestructively estimate the amount of neutron-emitting special nuclear material compounds remaining as holdup in nuclear facilities. Holdup occurs in all facilities in which nuclear material is processed. Material may exist, for example, in process equipment, in exhaust ventilation systems, and in building walls and floors.  
1.1.1 The most frequent uses of passive neutron holdup techniques are for the measurement of uranium or plutonium deposits in processing facilities.  
1.2 This guide includes information useful for management, planning, selection of equipment, consideration of interferences, measurement program definition, and the utilization of resources.  
1.3 Counting modes include both singles (totals) or gross counting and neutron coincidence techniques.  
1.3.1 Neutron holdup measurements of uranium are typically performed on neutrons emitted during (α, n) reactions and spontaneous fission using singles (totals) or gross counting. While the method does not preclude measurement using coincidence or multiplicity counting for uranium, measurement efficiency is generally not sufficient to permit assays in reasonable counting times.  
1.3.2 For measurement of plutonium in gloveboxes, installed measurement equipment may provide sufficient efficiency for performing counting using neutron coincidence techniques in reasonable counting times.  
1.4 The measurement of nuclear material holdup in process equipment requires a scientific knowledge of radiation sources and detectors, radiation transport, modeling methods, calibration, facility operations, and uncertainty analysis. It is subject to the constraints of the facility, management, budget, and schedule, plus health and safety requirements, as well as the laws of physics. This guide does not purport to instruct the NDA practitioner on these principles.  
1.5 The measurement process includes...

General Information

• Status: Published
• Publication Date: 30-Nov-2023
• ICS: 27.120.30 - Fissile materials and nuclear fuel technology
• Technical Committee: C26 - Nuclear Fuel Cycle
• Drafting Committee: C26.10 - Non Destructive Assay

Relations

• Replaces: ASTM C1807-15 - Standard Guide for Nondestructive Assay of Special Nuclear Material (SNM) Holdup Using Passive Neutron Measurement Methods
• Effective Date: 01-Dec-2023
• Referred By: ASTM C1592/C1592M-21 - Standard Guide for Making Quality Nondestructive Assay Measurements
• Effective Date: 01-Dec-2023

Overview

ASTM C1807-15(2023): Standard Guide for Nondestructive Assay of Special Nuclear Material (SNM) Holdup Using Passive Neutron Measurement Methods is an internationally recognized standard developed by ASTM. This guide outlines procedures and best practices for using passive neutron measurement techniques to nondestructively estimate the quantity of neutron-emitting special nuclear materials, such as uranium and plutonium, remaining as holdup within various types of nuclear process equipment and facilities.

Passive neutron measurement methods are crucial for ensuring effective safeguards, SNM inventory management, nuclear criticality safety, waste disposal, and decontamination and decommissioning (D&D) activities. The standard supports facility management in planning, executing, and documenting holdup measurement programs, especially where holdup may occur in complicated or inaccessible areas such as process piping, exhaust systems, and building structures.

Key Topics

• Measurement Methods: The guide focuses on passive neutron measurement techniques using singles (totals/gross counting) and neutron coincidence counting. It addresses their use for both qualitative and quantitative holdup assessments.
• Application Scope: Primarily used for measuring uranium or plutonium deposits in processing facilities, these techniques are adapted to accommodate complex distributions, presence of neutron moderators/absorbers, and varying facility constraints.
• Measurement Planning & Execution:
  • Selection of suitable equipment and detectors, considering efficiency, complexity, portability, and shielding requirements.
  • Calibration using reference materials or surrogate standards, with traceability to national measurement institutes.
  • Data evaluation, uncertainty estimation, and model validation using techniques like Monte Carlo simulations when required.
• Data Quality and Uncertainty:
  • Importance of rigorous calibration and error analysis.
  • Procedures for combining multiple measurements and propagating uncertainty correctly.
  • Regular control measurements and scan assessments to ensure ongoing data reliability.
• Interferences and Background Correction: Guidance for mitigating biases from sources such as cosmic-ray spallation, environmental neutron backgrounds, chemical and isotopic variations, and matrix effects.

Applications

• Safeguards and Regulatory Compliance: Provides methodologies for satisfying national and international safeguards requirements by enabling accurate SNM holdup quantification.
• Inventory Control and Material Accounting: Supports periodic and spot-check measurements required for ongoing SNM accountancy, contributing to transparency and traceability.
• Criticality Safety Assessment: Ensures detection and management of residual material to minimize risks associated with unintended nuclear chain reactions.
• Waste Management and D&D: Essential in characterizing remaining SNM during facility cleanout, waste classification, and decontamination efforts.
• Quality Assurance: Facilitates compliance with broader nuclear quality management systems by referencing related ASTM guides for personnel qualification and laboratory quality controls.

Related Standards

Professionals implementing ASTM C1807-15(2023) should be aware of and may need to reference several related standards, including:

• ASTM C1009 - Guide for Establishing and Maintaining a Quality Assurance Program for Analytical Laboratories Within the Nuclear Industry
• ASTM C1455 - Test Method for Nondestructive Assay of Special Nuclear Material Holdup Using Gamma-Ray Spectroscopic Methods
• ASTM C1490 - Guide for the Selection, Training and Qualification of Nondestructive Assay (NDA) Personnel
• ASTM C1592/C1592M - Guide for Making Quality Nondestructive Assay Measurements
• ASTM C1673 - Terminology of C26.10 Nondestructive Assay Methods
• ANSI N15.20 - Guide to Calibrating Nondestructive Assay Systems
• NRC Regulatory Guide 5.23 - In-Situ Assay of Plutonium Residual Holdup

Practical Value

Adherence to ASTM C1807-15(2023) ensures that nuclear facilities can conduct reliable, auditable, and safe nondestructive assay (NDA) measurements of special nuclear material holdup. By following this guide, organizations enhance their ability to manage nuclear materials responsibly, meet compliance obligations, and implement robust safety practices, all while supporting efficient facility operations and regulatory reporting.

Keywords: ASTM C1807, passive neutron measurement, holdup assay, special nuclear material, SNM, nondestructive assay, uranium, plutonium, nuclear safeguards, criticality safety, inventory control, nuclear decommissioning, neutron coincidence counting, calibration, measurement uncertainty, decontamination, nuclear waste management.

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: C1807 − 15 (Reapproved 2023)
Standard Guide for
Nondestructive Assay of Special Nuclear Material (SNM)
Holdup Using Passive Neutron Measurement Methods
This standard is issued under the fixed designation C1807; 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 the laws of physics. This guide does not purport to instruct the
NDA practitioner on these principles.
1.1 This guide describes passive neutron measurement
methods used to nondestructively estimate the amount of 1.5 The measurement process includes defining measure-
ment uncertainties and is sensitive to the chemical
neutron-emitting special nuclear material compounds remain-
ing as holdup in nuclear facilities. Holdup occurs in all composition, isotopic composition, distribution of the material,
various backgrounds, and interferences. The work includes
facilities in which nuclear material is processed. Material may
exist, for example, in process equipment, in exhaust ventilation investigation of material distributions within a facility, which
could include potentially large holdup surface areas. Nuclear
systems, and in building walls and floors.
material held up in pipes, ductwork, gloveboxes, and heavy
1.1.1 The most frequent uses of passive neutron holdup
equipment is usually distributed in a diffuse and irregular
techniques are for the measurement of uranium or plutonium
manner. It is difficult to define the measurement geometry,
deposits in processing facilities.
identify the form of the material, and measure it.
1.2 This guide includes information useful for management,
1.6 Units—The values stated in SI units are to be regarded
planning, selection of equipment, consideration of
as the standard. No other units of measurement are included in
interferences, measurement program definition, and the utili-
this standard.
zation of resources.
1.7 This standard does not purport to address all of the
1.3 Counting modes include both singles (totals) or gross
safety concerns, if any, associated with its use. It is the
counting and neutron coincidence techniques.
responsibility of the user of this standard to establish appro-
1.3.1 Neutron holdup measurements of uranium are typi-
priate safety, health, and environmental practices and deter-
cally performed on neutrons emitted during (α, n) reactions and
mine the applicability of regulatory limitations prior to use.
spontaneous fission using singles (totals) or gross counting.
1.8 This international standard was developed in accor-
While the method does not preclude measurement using
dance with internationally recognized principles on standard-
coincidence or multiplicity counting for uranium, measurement
ization established in the Decision on Principles for the
efficiency is generally not sufficient to permit assays in
Development of International Standards, Guides and Recom-
reasonable counting times.
mendations issued by the World Trade Organization Technical
1.3.2 For measurement of plutonium in gloveboxes, in-
Barriers to Trade (TBT) Committee.
stalled measurement equipment may provide sufficient effi-
ciency for performing counting using neutron coincidence
2. Referenced Documents
techniques in reasonable counting times.
2.1 ASTM Standards:
1.4 The measurement of nuclear material holdup in process
C1009 Guide for Establishing and Maintaining a Quality
equipment requires a scientific knowledge of radiation sources
Assurance Program for Analytical Laboratories Within the
and detectors, radiation transport, modeling methods,
Nuclear Industry
calibration, facility operations, and uncertainty analysis. It is
C1455 Test Method for Nondestructive Assay of Special
subject to the constraints of the facility, management, budget,
Nuclear Material Holdup Using Gamma-Ray Spectro-
and schedule, plus health and safety requirements, as well as
scopic Methods
C1490 Guide for the Selection, Training and Qualification of
Nondestructive Assay (NDA) Personnel
This guide is under the jurisdiction of ASTM Committee C26 on Nuclear Fuel
Cycle and is the direct responsibility of Subcommittee C26.10 on Non Destructive
Assay. For referenced ASTM standards, visit the ASTM website, www.astm.org, or
Current edition approved Dec. 1, 2023. Published December 2023. Originally contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM
approved in 2015. Last previous edition approved in 2015 as C1807 – 15. DOI: Standards volume information, refer to the standard’s Document Summary page on
10.1520/C1807-15R23. the ASTM website.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
C1807 − 15 (2023)
C1592/C1592M Guide for Making Quality Nondestructive methods (for example, sampling and X-ray fluorescence to
Assay Measurements determine chemical composition and high-resolution gamma-
C1673 Terminology of C26.10 Nondestructive Assay Meth- ray spectroscopy to determine isotopic composition). Both the
ods chemical and isotopic distribution have significant effects on
specific neutron yield.
2.2 NRC Standard:
NRC Regulatory Guide 5.23 In-Situ Assay of Plutonium
4.4 Definition of Requirements—Definition of the holdup
Residual Holdup
measurement requirements should include, as a minimum, the
2.3 ANSI Standard:
measurement objectives (that is, nuclear criticality safety,
ANSI N15.20 Guide to Calibrating Nondestructive Assay
special nuclear material (SNM) accountability, radiological
Systems
safety, or combinations thereof); time and resource constraints;
the desired measurement sensitivity, accuracy, and uncertainty;
3. Terminology
and available resources (schedule, funds, and subject matter
3.1 Definitions—Refer to Terminology C1673 for defini-
experts). Specific data quality objectives should be provided
tions used in this guide.
when available.
4.5 Information Gathering and Initial Evaluation—
4. Summary of Guide
Information shall be gathered concerning the item or items to
4.1 Introduction—Holdup measurements using neutron
be assayed, and an initial evaluation should be made of the
methods typically measure the (α, n) or spontaneous fission
measurement techniques and level of effort needed to meet the
production of neutrons, or both. Neutrons generated in items
holdup measurement requirements. Preliminary radiation mea-
that do not include significant masses of neutron moderators,
surements may be needed to define the location and extent of
such as hydrogenous materials, typically have an escape
the holdup. Additional information should be collected prior to
fraction of nearly one. The isotopic distribution and, for (α, n)
commencement of measurements. This information includes,
production, the chemical composition of the measured material
but is not limited to, the geometric configuration of the item or
affect assay results and shall be determined by process knowl-
process equipment to be assayed, location of the equipment in
edge or an alternative measurement technique. Ref (1) pro-
the facility, the presence of neutron moderators and absorbers,
vides an example of a holdup campaign using neutron mea-
neutron leakage multiplication, factors affecting specific neu-
surements.
tron yield, sources of background or interferences, facility
4.2 Choice of Measurement Method—Passive neutron mea-
processing status, radiological and industrial safety
surement methods are typically used for holdup when other considerations, plus the personnel and equipment needed to
methods of measurement (for example, gamma-ray assay) are
complete the assay. Sources of information may include a
not practical or would produce large biases. In some cases, visual survey, engineering drawings, process knowledge, pro-
neutron measurements are performed in conjunction with
cess operators, results of sampling and wet chemical analysis,
gamma-ray measurements for defense in depth or to obtain and prior assay documentation.
isotopic information, or both. Neutron measurement instru-
4.6 Measurement Plan—A measurement plan shall be de-
mentation is typically heavier, more difficult to shield, and has
veloped. The initial evaluation provides a basis for choosing
more difficult data interpretation than other NDA measurement
the quantitative method and assay model and, subsequently,
methods. Neutrons, though, are very penetrating and less
leads to the determination of the detection system and calibra-
influenced by lumps than gamma rays, and the instrumentation
tion method to be used. Appropriate reference materials and
has a very stable response. Examples of when neutron mea-
support equipment are developed or assembled for the specific
surements are preferred include containers that severely attenu-
measurement technique. The plan will include measurement
ate gamma rays of interest for the nuclides measured or when
locations and geometries or guidance for their selection. In the
sufficient nuclear material is present that self-attenuation of
plan, required documentation; operating procedures; back-
gamma rays of interest is severe (see Test Method C1455 and
ground measurement methods and frequencies; plus training,
Guide C1592/C1592M).
quality, and measurement control requirements (Guide C1009)
4.3 Specific Neutron Yield—The number of neutrons gener-
are typically outlined. Necessary procedures, including those
ated per unit time per unit mass of the nuclide(s) of interest is
for measurement control, shall be developed, documented, and
an important parameter that is affected by conditions (for
approved.
example, chemical composition and isotopic distribution) not
4.7 Calibration—Calibration and initialization of measure-
detectable by passive neutron holdup measurement methods.
ment control is completed before measurements of unknowns.
Information used to estimate specific neutron yield shall be
Calibration requires reference materials traceable to a National
determined using process knowledge or alternate analysis
Measurement Institute to establish detection efficiency and
modeling detector response to neutron sources. If modeling is
used for calibration (for example, Monte Carlo n-Partical
Available from U. S. Nuclear Regulatory Commission (NRC), One White Flint
North, 11555 Rockville Pk., Rockville, MD 20852-2738, http://www.nrc.gov.
(MCNP) modeling), detailed specifications for the detector
Available from American National Standards Institute (ANSI), 25 W. 43rd St.,
package will be required. If modeling is used, validation of the
4th Floor, New York, NY 10036, http://www.ansi.org.
calibration shall include validation of each model developed.
The boldface numbers in parentheses refer to a list of references at the end of
this standard. Familiarity with the facility on which assays will be performed
C1807 − 15 (2023)
is required to ensure that calibration is sufficiently robust to of the measurement parameters against calibration and model-
encompass all reasonable measurement situations. ing assumptions. Depending on the calibration, models, and
252 252
4.7.1 Calibration Using Cf— Cf is commonly used for measurement methods used, corrections may be necessary for
calibrating neutron detectors. Cf is convenient in that it geometric effects (differences between holdup measurement
provides a point source of neutron emissions with a strong and calibration geometries); neutron moderators, absorbers, or
signal so that calibrations can be completed using relatively poisons; scattering from nearby process equipment; the influ-
short measurement times. Corrections for the difference in ence (scattering and shielding) of and holdup in nearby process
detection efficiency between neutrons from Cf and neutrons equipment that is in the detector field of view; background; and
from assayed items may be significant because of the differ- interferences. Measurement uncertainties (random and item-
ence in average energy from the two sources. For example, the specific bias) are estimated based on uncertainties in assay
average energy of neutrons from Cf is 2.14 MeV and the parameters. A comprehensive total measurement uncertainty
average energy of neutrons from holdup is 1.2 MeV for (α, n) analysis must accompany every measurement result.
with Fluorine as a target and an alpha energy of 5.2 MeV (2). 4.9.2 Results should be evaluated against previous results or
An additional issue is that Cf standards are typically clean-out data, if either are available. This evaluation provides
certified for total neutron activity, and isotopes present in the a cross-check between measurement techniques. The results of
this evaluation can be used to provide feedback to measure-
standards produce an increasing number of neutrons as the
mass of Cf decreases relative to the mass of longer-lived ment personnel, to refine the measurement and analysis
techniques, and to evaluate the measurement uncertainty
isotopes as time passes. As the time since separation of the
Cf increases, this may become a significant source of bias against estimates. If a discrepancy is evident, an evaluation
should be made. Modeling errors or other sources of bias can
unless appropriate corrections are made.
4.7.2 Calibration Using Surrogate Materials—Surrogate be identified using this technique. Additional measurements
with subsequent evaluation may be required. This can be used
materials, typically created using the same materials that will
be subsequently measured, may also be used for calibration, as a step in a phased approach.
4.9.3 If practical, measurements should be made of clean
provided sufficient characterization is performed to establish
traceability. These sources typically produce fewer neutrons process equipment or, ideally, a plant that has not yet had
nuclear material introduced. This provides a baseline for future
per unit time than Cf and require longer measurement times
for equivalent calibration uncertainty. In addition, surrogate measurement of holdup.
materials are typically significantly larger than point sources,
4.10 Documentation—Measurement documentation should
which may complicate the process of evaluating calibration
include the plans and procedures, a description of measurement
data. Calibration using surrogate materials reduces the number
parameters considered important to the calibration and for each
of corrections (for example, for energy difference between
measurement location, the measurement techniques used, the
neutrons produced by the calibration source and measured
raw data, assumptions and correction factors used in the
materials) and may result in a lower total measurement
analysis, a thorough description of the models used, the results
uncertainty.
with estimated precision and bias, and comparison to other
4.7.3 Calibration Confirmation—A calibration confirmation
measurement techniques when available.
is needed to produce objective evidence demonstrating the
applicability and correctness of the calibration relative to the
5. Significance and Use
items in which holdup is to be measured. The recommended
5.1 This guide assists in satisfying requirements in such
method is to assemble test item(s) consisting of source/matrix
areas as safeguards, SNM inventory control, nuclear criticality
and radioactive material configuration(s) nominally represen-
safety, waste disposal, and decontamination and decommis-
tative of the items to be characterized. The test item(s) should
sioning (D&D). This guide can apply to the measurement of
contain known and, preferably, traceable quantity of radioac-
holdup in process equipment or discrete items whose neutron
tive material in a known and representative configuration. If
production properties may be measured or estimated. These
practical, the range of expected materials should be spanned.
methods may meet target accuracy for items with complex
Acceptance criteria for the calibration confirmation measure-
distributions of SNM in the presence of moderators, absorbers,
ments should be established in the measurement plan.
and neutron poisons; however, the results are subject to larger
4.8 Measurements—Perform measurements and measure-
measurement uncertainties than measurements of less complex
ment control as detailed in the measurement plan or procedure. items.
4.9 Evaluation of Measurement Data—As appropriate, cor-
5.2 Quantitative Measurements—These measurements re-
rections are estimated and made for factors that may bias the sult in quantification of the mass of SNM in the holdup. They
measurement. Examples include neutron scattering; cosmic ray
include all the corrections and descriptive information, such as
induced spallation; leakage multiplication; neutron moderators, isotopic composition, that are available.
absorbers, and poisons; and the presence of targets that produce
5.2.1 High-quality results require detailed knowledge of
(α, n) neutrons. These corrections are applied in the calculation radiation sources and detectors, radiation transport, calibration,
of the assay value. Measurement uncertainties are established
facility operations, and error analysis. Consultation with quali-
based on factors affecting the assay. fied NDA personnel is recommended (Guide C1490).
4.9.1 Converting measurement data to estimates of the 5.2.2 Holdup estimates for a single piece of process equip-
quantity of nuclear material holdup requires careful evaluation ment or piping often include some compilation of multiple
C1807 − 15 (2023)
measurements. The holdup estimate must appropriately com- should be performed in a manner and at a frequency consistent
bine the results of each individual measurement. In addition, with the signal-to-noise ratio for the desired measurement
uncertainty estimates for each individual measurement must be sensitivity.
made and appropriately combined. 6.1.4 Interfering Neutron Production—Neutrons generated
by unexpected sources may produce a bias.
5.3 Scan—Radiation scanning, typically gamma, may be
6.1.4.1 Alpha Targets—In assays of material for which (α,
used to provide a qualitative description of the extent, location,
n) production is significant, specific neutron yield is propor-
and the relative quantity of holdup. It can be used to plan or
tional to the type and concentration of alpha targets. Incorrect
supplement the quantitative neutron measurements. Other in-
assumptions of chemical composition and the presence of
dicators (for example, visual) may also indicate a need for a
alpha targets (for example, impurities such as Be or O) that are
holdup measurement.
not included in the model will result in a bias.
5.4 Nuclide Mapping—To appropriately interpret the neu-
6.1.4.2 Neutron-Producing Radionuclides—Neutron-
tron data, the specific neutron yield is needed. Isotopic mea-
producing radionuclides whose presence are not considered in
surements to determine the relative isotopic composition of the
the estimate of specific neutron yield will result in a bias.
holdup at specific locations may be required, depending on the
6.1.4.3 Gamma-ray Producing Radionuclides—Gamma-
facility.
rays can register as counts in neutron instrumentation. This
effect is noticeable in non- He neutron detection systems, and
5.5 Spot Check and Verification Measurements—Periodic
in He detection systems if the high voltage is set to a high
re-measurement of holdup at a defined point using the same
value.
technique and assumptions can be used to detect or track
6.1.5 Cosmic-Ray-Induced Spallation—Neutrons produced
relative changes in the holdup quantity at that point over time.
by interaction between cosmic-ray spallation showers and both
Either a qualitative or quantitative method can be used.
non-nuclear and nuclear materials in measured items (for
5.6 Indirect Measurements—Neutron measurements do not
example, steel container walls, lead included as part of the
identify the radionuclide that produced the neutron signal. The
container wall or as a gamma-ray shield, and uranium) can
specific neutron yield shall be determined independently of the
cause a bias. Cosmic-ray induced spallation can result in a
neutron measurement.
significant bias. Estimation of the cosmic-ray background
should be performed close in time to the assay. Cosmic ray
5.7 Modeling—Modeling is recommended as an aid in the
spallation can shift with changing conditions, such as atmo-
evaluation of complex measurement situations. Measurement
spheric pressure and rain.
data are used with a radiation transport model that includes a
6.1.6 Matrix Effects—Matrix materials that are unexpected
description of the physical location of equipment and materi-
or improperly modeled can cause a bias. Examples include
als. Because of the complexity of neutron transport
modeling for leaded glass when acrylic is substituted, model-
calculations, models are often developed using a transport code
ing a full tank and then measuring an empty tank, and not
such as MCNP. Geometric models can also be used but,
generally, do not account for phenomena such as scattering and accounting for the presence of Raschig rings. Peer review of
calculations and measurement assumptions can be used to limit
estimation of neutron escape fraction.
this type of bias.
6. Interferences
7. Apparatus
6.1 Background can cause a bias or have adverse effects on
7.1 The apparatus chosen for measurements shall have
the precision or both. Because of scattering and physical
capabilities appropriate to the requirements of the measure-
limitations (for example, weight of the shielded detector
ment being performed. For example, a scalar is sufficient for
package), background often cannot effectively be reduced to an
singles counting, while more sophisticated electronics are
insignificant level and is a significant contributor to total
required for coincidence measurements. The quality of assay
measurement uncertainty.
results is partially dependent upon the capabilities of equip-
6.1.1 If background changes with measurement position, it
ment. The user will choose a suitable trade-off between
may be necessary to develop a model of the background that
detection efficiency, background shielding capabilities, equip-
incorporates the effects of measurement parameters (for
ment complexity, and equipment portability (weight, size, and
example, physical structure, presence of concrete, personnel,
number of pieces).
height at which background is measured).
6.1.2 Unrecognized and uncompensated background varia-
7.2 Neutron Measurement Systems—A quantitative holdup
tions can cause biased results. For example, SNM in nearby
measurement may be performed using instrumentation that
items that are moved or shielding that is moved during or
offers portability and simplicity of operation. The instrumen-
between the commencement of the background and the
tation typically includes a detector package with several He
completion of the assay measurement can cause biased results.
detectors imbedded in cadmium-shielded polyethylene and
This bias depends on the signal to background ratio.
supporting electronics in a portable package. The design of the
6.1.3 Neutron production rates are often low and back- neutron measurement system is dependent on the data quality
ground rates are often large relative to the neutron flux from the objectives. In general, the size and weight of detector packages
holdup. In this case, the overall assay sensitivity will be both increase with increased requirements for detection effi-
reduced and uncertainty increased. Background measurements ciency and background shielding.
C1807 − 15 (2023)
7.2.1 Neutron measurement systems usually consist of one 8.2.3 Presence of Moderators—If the holdup includes mod-
or more detector tubes (typically He proportional counters) erators (for example, oil or water) and appropriate allowances
embedded in a moderating material (typically high-density are not made, results will be adversely affected.
polyethylene). The outer surfaces of the moderating material
8.2.4 Leakage Multiplication—The fraction of source neu-
are typically covered with cadmium to stop thermal (highly
trons that escape from the item can be significantly different
moderated) room-scattered or environmental neutrons from
from unity. Careful modeling may be required to estimate these
entering the detector package. The measurement system is
effects accurately.
normally made somewhat directional by surrounding the sides
8.2.5 Background—Lack of understanding of background
and back of the detector package with a shield that consists of
effects on the measurement or incorrect background measure-
moderating material (typically high-density polyethylene).
ments may impact the results.
8.2.5.1 It can be challenging in plant conditions to position
7.3 Detector Shielding:
the detector to account for background properly.
7.3.1 Design of a shield generally involves arriving at a
8.2.5.2 Neutrons can travel a long distance (for example,
compromise among several factors. Among these are a man-
ageable weight versus detection efficiency and adequate shield- from a nuclear material storage location nearly 800 m away
from the measured item). Because of scattering, simply point-
ing against background neutrons. Because of scattering, colli-
mation of neutrons is generally not considered practical, and ing the detector away from the background source may provide
unexpected results. An example is a measurement in which
the detector packages have a somewhat directional but very
wide field of view. background taken with the detector pointed upwards toward
open sky was 25 % higher than the measurement of the item.
7.3.2 Detector shields can affect the efficiency of the detec-
tor package by scattering fast neutrons into the detector 8.2.5.3 Neutrons from adjacent items can scatter off of the
package, and each detector-shield assembly should be indi- measured item and into the detector.
vidually calibrated.
8.2.5.4 Neutrons from an item behind the measured item
can be scattered away from the detector making the item
7.4 Detector positioning apparatus such as measuring and
measurement lower than the background measurement.
pointing devices or support stands to help attain reproducible
geometry are recommended. Detector shield assemblies are
9. Procedure
typically too heavy to be moved and positioned without the aid
of a positioning apparatus of some kind.
9.1 A holdup measurement campaign procedure generally
includes the following:
8. Hazards
9.1.1 Development (or review) of measurement strategy and
development (or review) of a detailed measurement plan,
8.1 Safety Hazards:
8.1.1 Holdup measurements sometimes need to be carried 9.1.2 Preparation for measurements,
out in areas with radiological contamination or high radiation.
9.1.3 Perform the measurements,
Proper industrial safety and health-physics practices shall be
9.1.4 Calculations and modeling (for example, specific neu-
followed.
tron yield, detection efficiency, mass, uncertainty),
8.1.2 Neutron detectors may use power-supply voltages
9.1.5 Estimation of measurement uncertainty (typically pre-
typically as high as 2 kV for He proportional counters. The
cision and bias), and,
power supply should be off before connecting or disconnecting
9.1.6 Recording of data and results (2-6).
high-voltage cables. Care should be taken to avoid damage to
9.2 Measurement Strategy/Plan Development:
cables when moving the measurement systems through the
9.2.1 Measurement Program Requirements—Before the
measured facility.
evaluation of an assay situation, specific information shall be
8.1.3 Materials are used, for example, cadmium, that are
gathered regarding what is expected of the measurement or
considered hazardous or toxic or both. Proper care in their use
measurement program. The information should provide the
and disposal are required.
boundaries for the task or project. This information typically
8.1.4 Holdup measurements often require performing as-
includes the following:
says with heavy instrumentation positioned in relatively inac-
9.2.1.1 Identification of the item or piece of equipment to be
cessible locations, as well as in elevated locations. Appropriate
measured;
industrial safety precautions shall be taken to ensure personnel
9.2.1.2 Radionuclide(s) of interest;
are not injured by falling objects, including the detector shield
assembly, or that personnel do not fall while trying to reach the
9.2.1.3 Acceptable level of measurement uncertainty;
desired location.
9.2.1.4 Acceptable lower detection limit for the assay;
9.2.1.5 Intended and potential applications for results, for
8.2 Technical Hazards:
example, criticality risk assessment, SNM accountability,
8.2.1 High-energy gamma rays can cause counts in some
health physics, or decontamination and demolition; and
circumstances. Manufacturer instructions on the setup of elec-
9.2.1.6 Administrative requirements, for example, quality
tronics shall be followed rigorously to reduce this effect.
assurance requirements, documentation, and reporting require-
8.2.2 Electronic instability can impact assay results. For
ments.
example, noise or microphonics can artificially increase mea-
sured count rates. 9.2.2 Constraints that are useful to know about:
C1807 − 15 (2023)
9.2.2.1 The time available to perform the measurement(s), composition remain constant throughout the areas to be as-
that is, how long before a report or compilation of data is sayed. This will include the radionuclides of interest as well as
required, and interfering radionuclides.
9.2.2.2 Resources available to perform the individual mea- 9.2.5.6 Scan measurements can be performed to locate areas
surement or the measurement program. that will later be measured quantitatively. The scan information
9.2.3 Personnel and Procedures—Note that there are typi- also can be used to assess the size and complexity of the task.
cally two levels of procedures: generic or all-encompassing 9.2.5.7 Locations of holdup exceeding a predetermined
such as the measurement strategy or selection of models and
activity level can be noted for later quantitative measurements.
the detailed work instructions for each data acquisition:
9.2.5.8 Removal of background sources, attenuating
9.2.3.1 Since holdup measurements are made with little or
equipment, and extraneous items can facilitate subsequent
no sample preparation and under a wide range of conditions, measurements, requiring less time and resources and providing
formal procedures might be developed for the item measure-
more accurate results.
ments. Procedures can evolve to incorporate lessons learned
9.3 Develop Detailed Measurement Plan—A critical step in
from previous experience.
the evaluation process is the determination of how the mea-
9.2.3.2 Personnel performing hol
...