Theoretical Analysis and Numerical Simulation of PNS Method Measuring Fission Decay Properties of a Subcritical Facility

doi:10.19596/j.cnki.1001-246x.8413

Abstract

Abstract:

For problems of point-kinetic equation in explaining theory of the pulsed neutron source (PNS) method, evolution of neutron population and leakage γ-ray count rate of a subcritical system was analyzed with neutron transport theory. It is indicated that in a certain time range after the pulsed neutron source, the count rate of γ-rays leaked from the system is approximately proportional to the neutron population. And both of them change with time according to an exponential decay law, from which fission decay properties of the system can be deduced and the prompt neutron decay constant α₀ can be calculated. To verify the theoretical analysis, a Godiva-like bare uranium-sphere subcritical facility with a pulsed neutron source at the center was modeled. Neutron and γ-ray transport process was simulated with a Monte Carlo code. Evolutions of neutron population, γ-ray count rate and ratio of them were obtained. They are consistent with theoretical analysis. With PNS method, prompt neutron decay constant α₀ is calculated. It is consistent with the α-k power iteration result, which indicates that the neutron population and leakage γ-ray count rate reflect intrinsic fission decay properties of the system accurately. In addition, a criteria of fitting time window for the PNS method is presented. Factors influencing the ratio of γ-ray count rate and neutron population are discussed.

Key words: pulsed neutron source method, fission decay, prompt neutron decay constant, subcritical system, neutron diagnosed subcritical experiment

Yi GAO, Ruogu SHE, Qi XU. Theoretical Analysis and Numerical Simulation of PNS Method Measuring Fission Decay Properties of a Subcritical Facility[J]. Chinese Journal of Computational Physics, 2022, 39(3): 261-267.

Figures/Tables 6

Fig.1 Layout of a Godiva-like subcritical system

Fig.2 Neutron population and leakage γ-ray count rate (a) E0=14.1 MeV; (b) E0= 2.45 MeV

Fig.3 Natural logarithm of neutron population and leakage γ-ray count rate

Fig.4 Ratio of leakage γ-ray count rate to neutron population

Fig.5 Relation between νγ and R0

Table 1 Calculation results of α0

源中子能量E₀/MeV	拟合数据	α₀/μs^-1	ε/μs^-1	R_adj²
14.1	总中子数	- 6.01	4.469 37 × 10^-5	1
14.1	γ计数率	- 6.01	3.793 01 × 10^-4	0.999 96
2.45	总中子数	- 6.01	7.860 01 × 10^-5	1
2.45	γ计数率	-6.01	5.517 24 × 10^-4	0.999 92

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