Applicability and Limitation of Pulsed Neutron Source Method for Subcritical Assembly

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

Abstract

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

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.

Figures/Tables 13

References 13

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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%