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arXiv · 2101.09338

Time-dependent Monte Carlo in fissile systems with beta-delayed neutron precursors

Jaime Romero-Barrientos
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Abstract

In the field of nuclear reactor physics, transient phenomena are usually studied using deterministic or hybrids methods. These methods require many approximations, such as: geometry, time and energy discretizations, material homogenization and assumption of diffusion conditions, among others. In this context, Monte Carlo simulations are specially adequate to study these problems. Challenges presented when using Monte Carlo simulations in space-time kinetics in fissile systems are the different time-scales involved in prompt and delayed neutron emission, which implies that results obtained have a large variance associated if an analog Monte Carlo simulation is utilized. Furthermore, in both deterministic and Monte Carlo simulations delayed neutron precursors are grouped in a $6$- or $8$- group structure, but nowadays there is not a solid reason to keep this aggregation. In this work, and for the first time, individual precursor data is implemented in a Monte Carlo simulation, explicitly including the time dependence related to the $\beta$-delayed neutron emission. This was accomplished by modifying the open source Monte Carlo code OpenMC. In the modified code -- Time Dependent OpenMC or OpenMC(TD) -- time dependency related to delayed neutron emission originated from $\beta$-decay was addressed. Continuous energy neutron cross-sections data used comes from JEFF-$3$.$1$.$1$ library. Individual precursor data was taken from JEFF-$3$.$1$.$1$ (cumulative yields) and ENDF-B/VIII.$0$ (delayed neutron emission probabilities and delayed neutron energy spectra). OpenMC(TD) was tested in: i) a monoenergetic system; ii) an energy dependent unmoderated system where the precursors were taken individually or in a group structure; and finally iii) a light-water moderated energy dependent system, using $6$-groups, $50$ and $40$ individual precursors.

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Jaime Romero-Barrientos. 2021-01-22. Time-dependent Monte Carlo in fissile systems with beta-delayed neutron precursors. https://arxiv.org/abs/2101.09338

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