脉冲源法对次临界系统的适用性和限制条件

doi:10.19596/j.cnki.1001-246x.8865

摘要/Abstract

摘要：

构造了不含反射层和含轻材料(低原子序数材料)反射层高浓铀次临界模型, 利用蒙特卡罗程序模拟脉冲中子源作用下中子和γ射线输运过程, 通过调整活性区、反射层尺寸和密度, 调整活性区和整个系统的有效增殖因子k_eff值, 获得不同条件下活性区中子数(对不含反射层系统, 活性区为系统本身)、泄漏γ射线计数率及γ计数率和活性区中子数比值随时间变化关系。研究了脉冲源法对不含反射层和含轻材料反射层次临界系统的适用性和限制条件。研究结果表明: 脉冲源法仅适用于活性区近临界(活性区的k_eff值接近1)的次临界系统, 活性区越接近临界, 泄漏γ计数率越能准确反映活性区裂变衰减特性。给出了高浓铀次临界模型中确保脉冲源法适用的活性区k_eff限值, 并讨论了脉冲源法应用于动态次临界系统的可能性。

关键词: 脉冲源法, 次临界系统, 泄漏γ计数率, 瞬发中子衰减时间常数, 裂变衰减特性

Abstract:

Highly enriched uranium subcritical assemblies with and without low atomic number (low-Z) reflectors are modeled. The neutron and photon transport process in the assemblies initiated by a pulsed neutron source is simulated using the Monte Carlo code. When the size or mass density of the fissile region and the reflector changes, the effective multiplication factor k_eff of the fissile region or the whole system changes accordingly (for assemblies without reflectors, the fissile region refers to the whole system). For each case, the time dependence of leakage γ count rate, neutron population in the fissile region and their ratio is obtained and analyzed to investigate the applicability and limitation of the pulsed neutron source (PNS) method for subcritical systems with and without reflectors. It is indicated that the PNS method is suitable for systems of which the fissile region is near-critical, i.e., the k_eff value of the fissile region is close to 1. The closer the fissile region is to the critical state, the more accurately the leakage γ count rate can reflect the fissile region's fission decay properties during the specific time window. For the highly enriched uranium assemblies described in this paper, the k_eff threshold of the fissile region ensuring the applicability of the PNS method is given. In addition, the possibility of applying the PNS method under dynamic conditions are also discussed according to the calculation results.

Key words: pulsed neutron source method, subcritical system, leakage γ count rate, prompt neutron decay constant, fission decay properties

高屹, 郑妍, 徐琪, 张晓立. 脉冲源法对次临界系统的适用性和限制条件[J]. 计算物理, 2025, 42(2): 224-231.

Yi GAO, Yan ZHENG, Qi XU, Xiaoli ZHANG. Applicability and Limitation of Pulsed Neutron Source Method for Subcritical Assembly[J]. Chinese Journal of Computational Physics, 2025, 42(2): 224-231.

图/表 13

参考文献 13

1	SIMMONS B E, KING J S. A pulsed neutron technique for reactivity determination[J]. Nuclear Science and Engineering, 1958, 3(5): 595- 608. DOI
2	USA Los Alamos National Laboratory. The bomb without the boom: A new subcritical measurement tool will help scientists protest and preserve the nation, s stockpile[EB/OL]. https://lanl.gov/discover/publications/1663/%202017-October/%20bomb-without-boom.%20php.
3	BELLER T, BOSWELL M, CUTLER T, et al. Primary assessment assessment technologies technologies FY2017 milestone milestone reportreport: NDSE development development using DPF[EB/OL]. (2018-10-10)[2023-08-29]. https://www.osti.gov/biblio/1477614/.
4	GOMEZ J A, KOEHLER P E, CUTLER T E, et al. Results of three neutron diagnosed subcritical experiments[J]. Nuclear Science and Engineering, 2019, 193(5): 537- 548. DOI
5	National Nuclear Security Administration. Stockpile stewardship and management plan: FY 2018[R]. Washington: National Nuclear Security Administration, 2017.
6	高屹, 佘若谷, 徐琪. 脉冲源法测量次临界系统裂变衰减特性的理论分析和数值模拟[J]. 计算物理, 2022, 39(3): 261- 267. DOI
7	王一, 潘流俊, 王瑞宏. 求解α本征值的k-α迭代方法误差估计[J]. 计算物理, 2019, 36(1): 39- 46. DOI
8	GAO Yi, ZHANG Xiaoli, XU Qi, et al. Investigation of the pulsed neutron source method measuring the fission decay properties of a subcritical assembly with low-Z reflectors[J]. Progress in Nuclear Energy, 2023, 162, 104784. DOI
9	春雷. 核武器概论[M]. 北京: 原子能出版社, 2000.
10	ZOIA A, BRUN E, MALVAGI F. Alpha eigenvalue calculations with Tripoli-4^®[J]. Annals of Nuclear Energy, 2014, 63, 276- 284. DOI
11	BELL G I, GLASSTONE S. Nuclear reactor theory[M]. New York: Van US Atomic Energy Commission, 1970.
12	CULLEN D E, CLOUSE C J, PROCASSINI R, et al. Static and dynamic criticality: Are they different?: UCRL-TR-201506[R]. Livermore: Lawrence Livermore National Laboratory, 2003.
13	杜金峰. 中子输运方程中的α本征值计算[J]. 计算物理, 2003, 20(6): 509- 513. DOI

η	t/ns	0.5	0.6	0.7	0.8	0.9	1.0	1.01	1.03
(C/N)/ns^-1	50	0.020 05	0.021 92	0.023 57	0.025 04	0.026 30	0.027 49	0.027 58	0.027 76
(C/N)/ns^-1	200	0.013 90	0.016 00	0.018 46	0.020 98	0.024 67	0.026 69	0.026 70	0.027 06
Δ(C/N)/(C/N)		-30.67%	-27.01%	-21.68%	-16.21%	-6.20%	-2.91%	-3.19%	-2.52%

η	t/ns	1.05	1.07	1.10	1.13	1.15	1.17	1.19	1.20
(C/N)/ns^-1	50	0.027 95	0.027 95	0.028 38	0.028 64	0.028 82	0.028 98	0.029 13	0.029 16
(C/N)/ns^-1	200	0.027 54	0.027 63	0.028 26	0.028 58	0.028 71	0.028 98	0.029 07	0.029 05
Δ(C/N)/(C/N)		-1.47%	-1.14%	-0.42%	-0.21%	-0.38%	0.00%	-0.21%	-0.38%

η	t/ns	0.5	0.6	0.7	0.8	0.9	1.0	1.01	1.03
(C/N)/ns^-1	50	0.020 05	0.021 92	0.023 57	0.025 04	0.026 30	0.027 49	0.027 58	0.027 76
(C/N)/ns^-1	200	0.013 90	0.016 00	0.018 46	0.020 98	0.024 67	0.026 69	0.026 70	0.027 06
Δ(C/N)/(C/N)		-30.67%	-27.01%	-21.68%	-16.21%	-6.20%	-2.91%	-3.19%	-2.52%

η	t/ns	1.05	1.07	1.10	1.13	1.15	1.17	1.19	1.20
(C/N)/ns^-1	50	0.027 95	0.027 95	0.028 38	0.028 64	0.028 82	0.028 98	0.029 13	0.029 16
(C/N)/ns^-1	200	0.027 54	0.027 63	0.028 26	0.028 58	0.028 71	0.028 98	0.029 07	0.029 05
Δ(C/N)/(C/N)		-1.47%	-1.14%	-0.42%	-0.21%	-0.38%	0.00%	-0.21%	-0.38%

η	0.9	1.0	1.01	1.03	1.05	1.07	1.10	1.13	1.15	1.17	1.19	1.20
α_0n/μs^-1	-34.760	-29.871	-29.200	-28.009	-26.774	-25.294	-23.232	-20.886	-19.285	-17.660	-16.025	-14.936
α_0γ/μs^-1	-35.259	-30.088	-29.397	-28.176	-26.914	-25.407	-23.316	-20.951	-19.340	-17.703	-16.061	-14.970
(α_0γ-α_0n)/α_0n	1.44%	0.73%	0.67%	0.60%	0.52%	0.45%	0.36%	0.31%	0.29%	0.24%	0.22%	0.23%

η	0.9	1.0	1.01	1.03	1.05	1.07	1.10	1.13	1.15	1.17	1.19	1.20
α_0n/μs^-1	-34.760	-29.871	-29.200	-28.009	-26.774	-25.294	-23.232	-20.886	-19.285	-17.660	-16.025	-14.936
α_0γ/μs^-1	-35.259	-30.088	-29.397	-28.176	-26.914	-25.407	-23.316	-20.951	-19.340	-17.703	-16.061	-14.970
(α_0γ-α_0n)/α_0n	1.44%	0.73%	0.67%	0.60%	0.52%	0.45%	0.36%	0.31%	0.29%	0.24%	0.22%	0.23%

η	t/ns	0.5	0.6	0.7	0.8	0.9	1.0	1.01	1.03	1.05
C/N_f	50	0.020 91	0.023 00	0.024 91	0.026 62	0.028 15	0.029 50	0.029 59	0.029 86	0.030 11
C/N_f	200	0.014 98	0.016 87	0.018 90	0.021 07	0.023 66	0.026 38	0.026 57	0.027 08	0.027 58
Δ(C/N_f)/(C/N_f)		-28.36%	-26.65%	-24.13%	-20.85%	-15.95%	-10.58%	-10.21%	-9.31%	-8.40%

η	t/ns	1.07	1.09	1.10	1.11	1.13	1.15	1.17	1.19	1.20
C/N_f	50	0.030 29	0.030 49	0.030 66	0.030 73	0.030 97	0.031 11	0.031 27	0.031 47	0.031 46
C/N_f	200	0.028 06	0.028 60	0.028 93	0.029 17	0.029 61	0.029 93	0.030 32	0.030 62	0.030 84
Δ(C/N_f)/(C/N_f)		-7.36%	-6.20%	-5.64%	-5.08%	-4.39%	-3.79%	-3.04%	-2.70%	-1.97%