1.1
本试验方法涵盖使用主动、差分衰减技术(DDT)和被动中子符合计数对铀或钚或两者进行无损检测(NDA)的系统。将主动和被动测量的结果结合起来,以确定废品或废物桶中裂变和自发裂变材料的总量。假设中子慢化和吸收的影响是整个转鼓体积的平均值,并且不存在显著的核材料团块,则对测量结果进行校正。这些系统最广泛地用于测定低水平和超铀废物,但也可用于测量废料。本试验方法中给出的示例针对第二种试验方法-
洛斯阿拉莫斯国家实验室(LANL)被动-主动中子分析系统。
1.1.1
在主动模式下,系统测量裂变同位素,如
235
U和
239
聚氨基甲酸酯。将来自脉冲14 MeV中子发生器的中子加热,以在分析项目中诱发裂变。在发生器脉冲之间,系统检测裂变材料发出的瞬发裂变中子。脉冲之间检测到的中子数与裂变材料的质量成正比。这种方法称为差分冲模技术。
1.1.2
在被动模式下,系统检测自发裂变同位素发射的时间一致中子。测量的主要同位素为
238
聚氨基甲酸酯,
240
Pu,和
242
聚氨基甲酸酯;然而,该系统可以自适应地用于其他系统-
裂变同位素,例如千克数量的
238
U、 检测到的重合中子数与自发裂变材料的质量成正比。
1.2
主动模式用于在以下范围内测定裂变材料。
1.2.1
对于仅含铀的项目,滴滴涕可以测量
235
铀含量在约0.02至100 g以上的范围内。小质量含铀项目通常使用主动模式测量,只有大质量项目在被动模式下测量。
1.2.2
对于仅含钚的物品,滴滴涕法测量
239
Pu含量在约0.01至20 g之间。
1.3
被动模式能够在0.05至15g的标称范围内测定自发裂变核
240
聚氨酯等效物。
1.
4.
本试验方法需要了解钚或铀同位素的相对丰度,以确定钚或铀的总质量。
1.5
当废物体不符合校准规范和本试验方法中提出的关于均匀基质、均匀源分布和无核材料团块的要求的测量假设时,本试验方法将给出有偏差的结果,其影响测量。
1.6
208 L桶的完整主动和被动分析通常为10分钟或更少,但任何一种模式都可以扩展以满足数据质量目标。
1.7
报告了对该试验方法的一些改进
(
1.
,
2.
,
3.
,
4.
)
.
2.
尽管这些改进仍在继续,但本试验方法中不包括对这些改进的讨论。
1.8
以国际单位制表示的数值应视为标准值。本标准不包括其他计量单位。
1.9
本标准可能涉及危险材料、操作和设备。本标准并非旨在解决与其使用相关的所有安全问题(如有)。本标准的用户有责任在使用前制定适当的安全、健康和环境实践,并确定监管限制的适用性。
第节给出了具体的预防说明
8.
.
1.10
本国际标准是根据世界贸易组织技术性贸易壁垒(TBT)委员会发布的《关于制定国际标准、指南和建议的原则的决定》中确立的国际公认标准化原则制定的。
====意义和用途======
5.1
该测试方法可用于量化裂变物质(例如,
233
U
235
U
239
Pu和
241
Pu)和自发裂变核(例如,
238
聚氨基甲酸酯,
240
聚氨基甲酸酯,
242
聚氨基甲酸酯,
244
厘米
248
Cm,以及
252
Cf)在废物桶和废料桶中。如果已知每种放射性核素的相对丰度,则可以计算放射性材料的总元素质量。
5.1.1
通常,该测试方法用于测量一种裂变同位素(例如,
235
U或
239
Pu)。
5.2
该试验方法可用于在目前要求的100 nCi/g浓度水平下分离低水平和超铀废物,以满足DOE废物隔离试验工厂(WIPP)废物验收标准
(
5.
,
8.
,
9
)
.
5.3
该测试方法可用于废物表征,以证明符合废物、处置和环境法规中规定的放射性水平(见NRC监管指南,DOE第435号令)。
1、10 CFR第71部分、40 CFR第191部分和DOE/WIPP-069)。
5.3.1
在主动模式下,DDT系统可以测量
235
U含量在<0.02到>100 g的范围内,并且
239
Pu含量,名义上介于<0.01和>20g之间。
5.3.2
在被动模式下,滴滴涕系统能够在0.05到15克的标称范围内测定自发裂变的核
240
Pu或同等产品
(
5.
,
10
,
11
,
12
,
13
)
.
5.4
本试验方法应与废物管理计划结合使用,该计划根据以下部分或全部标准将分析项目的内容分为材料类别:废物的体积密度、钚或铀的化学形式和基质、(α,n)中子强度、氢(慢化剂)和吸收剂含量、裂变物质的厚度,以及分析项目容器的大小和组成。
每个矩阵可能需要一组不同的校准标准,并且可能具有不同的质量校准极限。对分析质量的影响(即最小化精度和偏差)在很大程度上取决于遵守该废物管理计划的程度。
5.5
测量结果的偏差与填充高度、基质的均匀性和成分、核材料的数量和分布以及项目尺寸有关。测量结果的精度与核材料的数量、背景和测量的计数时间有关。
5.5.1
对于特定于基体和宽范围的校准,本试验方法假设校准材料与待测量项目相匹配,包括基体的均匀性和成分、中子慢化剂和吸收剂含量,以及核材料的数量、分布和形式,其影响测量的程度。
5.5.2
该测试方法的算法假设同质性。核材料、中子慢化剂和中子吸收剂分布的不均匀性有可能导致有偏差的结果
(
14
)
.
5.5.3
本试验方法假设放射性同位素在整个容器中的分布是均匀的,并且不存在核材料团块。
5.6
应用本试验方法得到的可靠结果要求对废物进行包装,以满足第节的条件
5.5
可以满足。在某些情况下,特定于现场的要求将决定包装要求,并可能对测量结果产生不利影响。
5.7
主动模式和被动模式都提供钚的分析值。在校准过程中,操作员应确定两种操作模式的适用质量范围。
1.1
This test method covers a system that performs nondestructive assay (NDA) of uranium or plutonium, or both, using the active, differential die-away technique (DDT), and passive neutron coincidence counting. Results from the active and passive measurements are combined to determine the total amount of fissile and spontaneously-fissioning material in drums of scrap or waste. Corrections are made to the measurements for the effects of neutron moderation and absorption, assuming that the effects are averaged over the volume of the drum and that no significant lumps of nuclear material are present. These systems are most widely used to assay low-level and transuranic waste, but may also be used for the measurement of scrap materials. The examples given within this test method are specific to the second-generation Los Alamos National Laboratory (LANL) passive-active neutron assay system.
1.1.1
In the active mode, the system measures fissile isotopes such as
235
U and
239
Pu. The neutrons from a pulsed, 14-MeV neutron generator are thermalized to induce fission in the assay item. Between generator pulses, the system detects prompt-fission neutrons emitted from the fissile material. The number of detected neutrons between pulses is proportional to the mass of fissile material. This method is called the differential die-away technique.
1.1.2
In the passive mode, the system detects time-coincident neutrons emitted from spontaneously fissioning isotopes. The primary isotopes measured are
238
Pu,
240
Pu, and
242
Pu; however, the system may be adapted for use on other spontaneously-fissioning isotopes as well, such as kilogram quantities of
238
U. The number of coincident neutrons detected is proportional to the mass of spontaneously-fissioning material.
1.2
The active mode is used to assay fissile material in the following ranges.
1.2.1
For uranium-only bearing items, the DDT can measure the
235
U content in the range from about 0.02 to over 100 g. Small mass uranium-bearing items are typically measured using the active mode and only large mass items are measured in passive mode.
1.2.2
For plutonium-only bearing items, the DDT method measures the
239
Pu content in the range between about 0.01 and 20 g.
1.3
The passive mode is capable of assaying spontaneously-fissioning nuclei, over a nominal range from 0.05 to 15 g
240
Pu equivalent.
1.4
This test method requires knowledge of the relative abundances of the plutonium or uranium isotopes to determine the total plutonium or uranium mass.
1.5
This test method will give biased results when the waste form does not meet the calibration specifications and the measurement assumptions presented in this test method regarding the requirements for a homogeneous matrix, uniform source distribution, and the absence of nuclear material lumps, to the extent that they effect the measurement.
1.6
The complete active and passive assay of a 208 L drum is nominally 10 min or less but either mode can be extended to meet data quality objectives.
1.7
Some improvements to this test method have been reported
(
1
,
2
,
3
,
4
)
.
2
Discussions of these improvements are not included in this test method although improvements continue to occur.
1.8
The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.
1.9
This standard may involve hazardous materials, operations, and equipment. 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, health, and environmental practices and determine the applicability of regulatory limitations prior to use.
Specific precautionary statements are given in Section
8
.
1.10
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.
====== Significance And Use ======
5.1
This test method is useful for quantifying fissile (for example,
233
U,
235
U,
239
Pu and
241
Pu) and spontaneously-fissioning nuclei (for example,
238
Pu,
240
Pu,
242
Pu,
244
Cm,
248
Cm, and
252
Cf) in waste and scrap drums. Total elemental mass of the radioactive materials can be calculated if the relative abundances of each radionuclide are known.
5.1.1
Typically, this test method is used to measure one fissile isotope (for example,
235
U or
239
Pu).
5.2
This test method can be used to segregate low level and transuranic waste at the 100 nCi/g concentration level currently required to meet the DOE Waste Isolation Pilot Plant (WIPP) waste acceptance criterion
(
5
,
8
,
9
)
.
5.3
This test method can be used for waste characterization to demonstrate compliance with the radioactivity levels specified in waste, disposal, and environmental regulations (See NRC regulatory guides, DOE Order 435.1, 10 CFR Part 71, 40 CFR Part 191, and DOE /WIPP-069).
5.3.1
In the active mode, the DDT system can measure the
235
U content in the range from <0.02 to >100 g and the
239
Pu content, nominally between <0.01 and >20 g.
5.3.2
In the passive mode, the DDT system is capable of assaying spontaneously-fissioning nuclei, over a nominal range from 0.05 to 15 g of
240
Pu, or equivalent
(
5
,
10
,
11
,
12
,
13
)
.
5.4
This test method should be used in conjunction with a waste management plan that segregates the contents of assay items into material categories according to some or all of the following criteria: bulk density of the waste, chemical forms of the plutonium or uranium and matrix, (α, n) neutron intensity, hydrogen (moderator) and absorber content, thickness of fissile mass(es), and the assay item container size and composition. Each matrix may require a different set of calibration standards and may have different mass calibration limits. The effect on the quality of the assay (that is, minimizing precision and bias) can significantly depend on the degree of adherence to this waste management plan.
5.5
The bias of the measurement results is related to the fill height, the homogeneity and composition of the matrix, the quantity and distribution of the nuclear material, and the item size. The precision of the measurement results is related to the quantity of the nuclear material, the background, and the count time of the measurement.
5.5.1
For both matrix-specific and wide-range calibrations, this test method assumes the calibration material matches the items to be measured with respect to homogeneity and composition of the matrix, the neutron moderator and absorber content, and the quantity, distribution, and form of nuclear material, to the extent they affect the measurement.
5.5.2
The algorithms for this test method assume homogeneity. Heterogeneity in the distribution of nuclear material, neutron moderators, and neutron absorbers has the potential to cause biased results
(
14
)
.
5.5.3
This test method assumes that the distribution of the contributing radioisotopes is uniform throughout the container and that lumps of nuclear material are not present.
5.6
Reliable results from the application of this test method require waste to be packaged so the conditions of Section
5.5
can be met. In some cases, site-specific requirements will dictate the packaging requirements with possible detrimental effects to the measurement results.
5.7
Both the active mode and the passive mode provide assay values for plutonium. During the calibration process, the operator should determine the applicable mass ranges for both modes of operation.