ASTM C1500-08(2017)
(Test Method)Standard Test Method for Nondestructive Assay of Plutonium by Passive Neutron Multiplicity Counting
Standard Test Method for Nondestructive Assay of Plutonium by Passive Neutron Multiplicity Counting
SIGNIFICANCE AND USE
5.1 This test method is useful for determining the plutonium content of items such as impure Pu oxide, mixed Pu/U oxide, oxidized Pu metal, Pu scrap and waste, Pu process residues, and weapons components.
5.2 Measurements made with this test method may be suitable for safeguards or waste characterization requirements such as:
5.2.1 Nuclear materials accountability,
5.2.2 Inventory verification (7),
5.2.3 Confirmation of nuclear materials content (8),
5.2.4 Resolution of shipper/receiver differences (9),
5.2.5 Excess weapons materials inspections (10, 11),
5.2.6 Safeguards termination on waste (12, 13),
5.2.7 Determination of fissile equivalent content (14).
5.3 A significant feature of neutron multiplicity counting is its ability to capture more information than neutron coincidence counting because of the availability of a third measured parameter, leading to reduced measurement bias for most material categories for which suitable precision can be attained. This feature also makes it possible to assay some in-plant materials that are not amenable to conventional coincidence counting, including moist or impure plutonium oxide, oxidized metal, and some categories of scrap, waste, and residues (10).
5.4 Calibration for many material types does not require representative standards. Thus, the technique can be used for inventory verification without calibration standards (7), although measurement bias may be lower if representative standards were available.
5.4.1 The repeatability of the measurement results due to counting statistics is related to the quantity of nuclear material, interfering neutrons, and the count time of the measurement (15) .
5.4.2 For certain materials such as small Pu, items of less than 1 g, some Pu-bearing waste, or very impure Pu process residues where the (α,n) reaction rate overwhelms the triples signal, multiplicity information may not be useful because of the poor counting statistics of the triple coinci...
SCOPE
1.1 This test method describes the nondestructive assay of plutonium in forms such as metal, oxide, scrap, residue, or waste using passive neutron multiplicity counting. This test method provides results that are usually more accurate than conventional neutron coincidence counting. The method can be applied to a large variety of plutonium items in various containers including cans, 208-L drums, or 1900-L Standard Waste Boxes. It has been used to assay items whose plutonium content ranges from 1 g to 1000s of g.
1.2 There are several electronics or mathematical approaches available for multiplicity analysis, including the multiplicity shift register, the Euratom Time Correlation Analyzer, and the List Mode Module, as described briefly in Ref. (1).2
1.3 This test method is primarily intended to address the assay of 240Pu-effective by moments-based multiplicity analysis using shift register electronics (1, 2, 3) and high efficiency neutron counters specifically designed for multiplicity analysis.
1.4 This test method requires knowledge of the relative abundances of the plutonium isotopes to determine the total plutonium mass (See Test Method C1030).
1.5 This test method may also be applied to modified neutron coincidence counters (4) which were not specifically designed as multiplicity counters (that is, HLNCC, AWCC, etc), with a corresponding degradation of results.
1.6 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.
1.7 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.
General Information
Relations
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: C1500 − 08 (Reapproved 2017)
Standard Test Method for
Nondestructive Assay of Plutonium by Passive Neutron
Multiplicity Counting
This standard is issued under the fixed designation C1500; 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 priate safety, health, and environmental practices and deter-
mine the applicability of regulatory limitations prior to use.
1.1 This test method describes the nondestructive assay of
plutonium in forms such as metal, oxide, scrap, residue, or
2. Referenced Documents
waste using passive neutron multiplicity counting. This test
2.1 ASTM Standards:
method provides results that are usually more accurate than
C1030 Test Method for Determination of Plutonium Isotopic
conventional neutron coincidence counting. The method can be
Composition by Gamma-Ray Spectrometry
applied to a large variety of plutonium items in various
C1207 Test Method for Nondestructive Assay of Plutonium
containers including cans, 208-L drums, or 1900-L Standard
in Scrap and Waste by Passive Neutron Coincidence
Waste Boxes. It has been used to assay items whose plutonium
Counting
content ranges from 1 g to 1000s of g.
C1458 Test Method for Nondestructive Assay of Plutonium,
1.2 There are several electronics or mathematical ap- 241
Tritium and Am by Calorimetric Assay
proaches available for multiplicity analysis, including the
C1490 Guide for the Selection, Training and Qualification of
multiplicity shift register, the Euratom Time Correlation
Nondestructive Assay (NDA) Personnel
Analyzer, and the List Mode Module, as described briefly in
C1592 Guide for Making Quality Nondestructive Assay
Ref. (1).
Measurements
1.3 This test method is primarily intended to address the C1673 Terminology of C26.10 Nondestructive Assay Meth-
assay of Pu-effective by moments-based multiplicity analy- ods
sis using shift register electronics (1, 2, 3) and high efficiency
3. Terminology
neutron counters specifically designed for multiplicity analysis.
3.1 Definitions:
1.4 This test method requires knowledge of the relative
3.1.1 Terms shall be defined in accordance with Terminol-
abundances of the plutonium isotopes to determine the total
ogy C1673 except for the following:
plutonium mass (See Test Method C1030).
3.1.2 gate fractions, n—the fraction of the total coincidence
1.5 This test method may also be applied to modified
events that occur within the coincidence gate.
neutron coincidence counters (4) which were not specifically
3.1.2.1 doubles gate fraction (f ), n—the fraction of the
d
designed as multiplicity counters (that is, HLNCC, AWCC,
theoretical double coincidences that can be detected within the
etc), with a corresponding degradation of results.
coincidence gate (see Eq 1).
1.6 The values stated in SI units are to be regarded as
3.1.2.2 triples gate fraction (f ), n—the fraction of the
t
standard. No other units of measurement are included in this
theoretical triple coincidences that can be detected within the
standard.
coincidence gate (see Eq 2).
1.7 This standard does not purport to address all of the
3.1.3 factorial moment of order, n—this is a derived quantity
safety concerns, if any, associated with its use. It is the
calculated by summing the neutron multiplicity distribution
responsibility of the user of this standard to establish appro-
weighted by ν!/(ν – n)! where n is the order of the moment.
3.1.4 induced fission neutron multiplicities (ν , ν , ν ),
i1 i2 i3
n—the factorial moments of the induced fission neutron mul-
This test method is under the jurisdiction of ASTM Committee C26 on Nuclear
tiplicity distribution. Typically multiplicity analysis will utilize
Fuel Cycle and is the direct responsibility of Subcommittee C26.10 on Non
Destructive Assay.
Current edition approved Jan. 1, 2017. Published January 2017. Originally
approved in 2002. Last previous edition approved in 2008 as C1500 – 08. DOI: For referenced ASTM standards, visit the ASTM website, www.astm.org, or
10.1520/C1500-08R17. contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
The boldface numbers in parentheses refer to the list of references at the end of Standards volume information, refer to the standard’s Document Summary page on
this standard. the ASTM website.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
C1500 − 08 (2017)
the data from fast neutron-induced fission of Pu to calculate 5.2 Measurements made with this test method may be
these moments (5, 6). suitable for safeguards or waste characterization requirements
such as:
4. Summary of Test Method
5.2.1 Nuclear materials accountability,
4.1 The item is placed in the sample chamber or “well” of 5.2.2 Inventory verification (7),
5.2.3 Confirmation of nuclear materials content (8),
the multiplicity counter, and the emitted neutrons are detected
5.2.4 Resolution of shipper/receiver differences (9),
by the He tubes that surround the well.
5.2.5 Excess weapons materials inspections (10, 11),
4.2 The detected neutron multiplicity distribution is pro-
5.2.6 Safeguards termination on waste (12, 13),
cessed by the multiplicity shift register electronics package to
5.2.7 Determination of fissile equivalent content (14).
obtain the number of neutrons of each multiplicity in the (R +
A) and (A) gates. Gates are pictorially depicted in Fig. 1. 5.3 A significant feature of neutron multiplicity counting is
its ability to capture more information than neutron coinci-
4.3 The first three moments of the (R + A) and (A)
dence counting because of the availability of a third measured
multiplicity distributions are computed to obtain the singles (or
parameter, leading to reduced measurement bias for most
totals), the doubles (or reals), and the triples. Using these three
material categories for which suitable precision can be at-
calculated values, it is possible to solve for 3 unknown item
240 tained. This feature also makes it possible to assay some
properties, the Pu-effective mass, the self-multiplication,
in-plant materials that are not amenable to conventional
and the α ratio. Details of the calculations may be found in
coincidence counting, including moist or impure plutonium
Annex A1.
oxide, oxidized metal, and some categories of scrap, waste, and
4.4 The total plutonium mass is then determined from the
residues (10).
known plutonium isotopic ratios and the Pu-effective mass.
5.4 Calibration for many material types does not require
4.5 Corrections are routinely made for neutron background,
representative standards. Thus, the technique can be used for
cosmic ray effects, small changes in detector efficiency with
inventory verification without calibration standards (7), al-
time, and electronic deadtimes.
though measurement bias may be lower if representative
4.6 Optional algorithms are available to correct for the standards were available.
biases caused by spatial variations in self-multiplication or 5.4.1 The repeatability of the measurement results due to
changes in the neutron die-away time. counting statistics is related to the quantity of nuclear material,
interfering neutrons, and the count time of the measurement
4.7 Multiplicity counters should be carefully designed by
(15).
Monte Carlo techniques to minimize variations in detection
5.4.2 For certain materials such as small Pu, items of less
efficiency caused by spatial effects and energy spectrum
than 1 g, some Pu-bearing waste, or very impure Pu process
effects. Corrections are not routinely made for neutron detec-
residues where the (α,n) reaction rate overwhelms the triples
tion efficiency variations across the item, energy spectrum
signal, multiplicity information may not be useful because of
effects on detection efficiency, or neutron capture in the item.
the poor counting statistics of the triple coincidences within
practical counting times (12).
5. Significance and Use
5.1 This test method is useful for determining the plutonium 5.5 For pure Pu metal, pure oxide, or other well-
content of items such as impure Pu oxide, mixed Pu/U oxide, characterized materials, the additional multiplicity information
oxidized Pu metal, Pu scrap and waste, Pu process residues, is not needed, and conventional coincidence counting will
and weapons components. provide better repeatability because the low counting statistics
FIG. 1 (a) Simplified probability distribution showing the approximately exponential decay, as a function of time, for detecting a second
neutron from a single fission event. The probability of detecting a random neutron is constant with time. (b) Typical coincidence timing
parameters.
C1500 − 08 (2017)
of the triple coincidences are not used. Conventional coinci- hardware. The relative effect is greatest on the triples, and next
dence information can be obtained either by changing to greatest on the doubles. Cosmic ray effects increase in signifi-
coincidence analyzer mode, or analyzing the multiplicity data cance for assay items containing large quantities of high atomic
in coincidence mode. number matrix constituents and small gram quantities of
plutonium. Multiplicity data analysis software packages should
5.6 The mathematical analysis of neutron multiplicity data
include correction algorithms for count bursts caused by
is based on several assumptions that are detailed in Annex A1.
cosmic rays.
The mathematical model considered is a point in space, with
assumptions that neutron detection efficiency, die-away time, 6.7 Other spontaneous fission nuclides (for example, curium
and multiplication are constant across the entire item (16, 17).
or californium) will increase the coincident neutron count
As the measurement deviates from these assumptions, the rates, causing a positive bias in the plutonium assay that
biases will increase.
multiplicity counting does not correct for. The triples/doubles
5.6.1 Bias in passive neutron multiplicity measurements is
ratio can sometimes be used as a warning flag.
related to deviations from the “point model” such as variations
6.8 Total counting rates should be limited to about 900 kHz
in detection efficiency, matrix composition, or distribution of
to limit the triples deadtime correction to about 50 % and to
nuclear material in the item’s interior.
ensure that less than 25 % of the shift register steps are
5.6.2 Heterogeneity in the distribution of nuclear material,
occupied. Otherwise incorrect assay results may be obtained
neutron moderators, and neutron absorbers may introduce
due to inadequate electronic deadtime corrections.
biases that affect the accuracy of the results. Measurements
6.9 Unless instrument design takes high gamma-ray field
made on items with homogeneous contents will be more
into account, high gamma-ray exposure levels from the item
accurate than those made on items with inhomogeneous
may interfere with the neutron measurement through pile-up
contents.
effects if the dose is higher than about 1 R/h at the He tubes.
6. Interferences
6.1 For measurements of items containing one or more 7. Apparatus
lumps that are each several hundred grams or more of
7.1 Multiplicity Counters:
plutonium metal, multiplication effects are not adequately
7.1.1 Neutron multiplicity counters are similar in design and
corrected by the point model analysis (18). Variable-
construction to conventional neutron coincidence counters, as
multiplication bias corrections must be applied.
described in Test Method C1207. Both are thermal neutron
6.2 For items with high (α,n) reaction rates, the additional
detector systems that utilize polyethylene-moderated He pro-
uncorrelated neutrons will significantly increase the accidental
portional counters. However, multiplicity counters are de-
coincidence rate. The practical application of multiplicity
signed to maximize neutron counting efficiency and minimize
counting is usually limited to items where the ratio of (α,n) to
neutron die-away time, with detection efficiencies that are
spontaneous fission neutrons (α) is low, that is, less than 10 (7).
much less dependent on neutron energy. Cylindrical multiplic-
ity well counters typically have 3 to 5 rings of He tubes and
6.3 For measurement of large items with high (α,n) reaction
absolute neutron detection efficiencies of 40 to 60 %, whereas
rates, the neutrons from (α,n) reactions can introduce biases if
conventional coincidence counters typically have 1 or 2 rings
their energy spectra are different from the spontaneous fission
of He tubes and efficiencies of 15 to 25 %. A multiplicity
energy spectrum. The ratio of the singles in the inner and outer
counter for the assay of cans of plutonium is illustrated in Fig.
rings can provide a warning flag for this effect (19).
2 (20).
6.3.1 High mass, high α items will produce large count rates
7.1.2 Multiplicity counters are designed to keep the radial
with large accidental coincidence rates. Sometimes this pre-
and axial efficiency profile of the sample cavity as flat as
vents obtaining a meaningful result.
possible (within several percent) to minimize the effects of
6.4 Neutron moderation by low atomic mass materials in the
item placement or item size in the cavity. Provision for
item affects neutron detection efficiency, neutron multiplication
reproducible item positioning in the cavity is still recom-
in the item, and neutron absorption by poisons. For nominal
mended for best results.
levels of neutron moderation, the multiplicity analysis will
7.1.3 Multiplicity counters are designed with a nearly flat
automatically correct the assay for changes in multiplication.
neutron detection efficiency as a function of the neutron energy
The presence of neutron poisons or other absorbers in the
spectrum, largely through the use of multiple rings of He
measurement item will introduce bias. Determination of the
tubes placed at different depths in the polyethylene moderator
correction factors required for these items will have to be
material.
individually determined.
7.1.4 Multiplicity counters usually have a thick external
6.5 It is important to keep neutron background levels from
layer of polyethylene shielding to reduce the contribution of
external sources as low and constant as practical for measure-
background neutrons from external sources.
ment of low Pu mass items. High backgr
...
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