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

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 immensely 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. The variance of the expected values of observables, such as neutron flux, associated to the different time scales between prompt and delayed neutrons was reduced by forcing the decay of a new Monte Carlo particle-like added to the code, the precursor, within each time interval, intentionally increasing the number of delayed neutrons in the simulation. Since there is a continuous production of delayed neutrons, population control had to be enforced. This was accomplished by using the combing method at the end of each time interval. 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.