<p>In a classic reactor system, the neutron population is large and the neutron chains overlap. The number of neutrons can be described by the average value. In a weak neutron source system, the neutron chains rarely overlap. The random fluctuation of the neutron number is obvious, and the average number of neutrons is insufficient to describe the system. It is necessary to describe this system using stochastic kinetics. Quantities of interest in the field of stochastic kinetics are Probability of Initiation (POI) and Probability of Survival (POS). Deterministic equations for describing stochastic kinetics were first developed by Bell. The equation is difficult to solve analytically because it is a differential-integral equation. Due to the randomness of the stochastic kinetics system, the Monte Carlo method has natural advantages to describe the system. The Reactor Monte Carlo (RMC) program developed by the Reactor Engineering Analysis Laboratory (REAL) of Tsinghua University already has the function of calculating the POI in a static system. In this study, the calculation function of POS in dynamic systems is developed in RMC. The Monte Carlo direct simulation method is used for neutron transport in the program. The number of neutrons produced by a neutron fission reaction is described by the fission multiplicity, not the average number of fission neutrons. The reactivity of the dynamic system changes with time through the linear interpolation method. The reactivity is changed by geometric and material parameters, corresponding to the movement of the control rod and the change of boron concentration in the actual reactor. The total number of simulated particles was controlled by the threshold and comb method. The benchmark is calculated proposed by Los Alamos National Laboratory (LANL) in the United States, and the reactivity of the benchmark varies from subcritical to supercritical over time. The results of POI and POS are obtained at specific moments (such as 100sh, 200sh, 500sh, etc). The calculation results of RMC are compared and verified with the Monte Carlo program MCATK of LANL. The calculation results of RMC and MCATK for POI are completely consistent. The calculation results of the two programs for POS are in good agreement except for 600sh. Then the reason for the different calculation results of POS at time 600sh was analyzed, and found that the threshold setting in the comb method has a greater impact on the calculation results of POS. Finally, the influence of different neutron chain numbers and time steps on the results is studied. The calculation result of POS converges only when the total number of neutron chains is sufficient, and the size of the time step has a great impact on POS. This research is helpful for the further study of stochastic kinetics application.</p>

Calculation of Probability of Survival (Pos) in Dynamic Systems Based on Rmc Code

Paper Proceedings: https://asmedigitalcollection.asme.org/ICONE/ICONE28/volume/85246

2021 28th International Conference on Nuclear Engineering (ICONE28)

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