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In the field of nuclear reactor physics, transient phenomena are usuallystudied using deterministic or hybrids methods. These methods require manyapproximations, such as: geometry, time and energy discretizations, materialhomogenization and assumption of diffusion conditions, among others. In thiscontext, Monte Carlo simulations are specially adequate to study theseproblems. Challenges presented when using Monte Carlo simulations in space-timekinetics in fissile systems are the different time-scales involved in promptand delayed neutron emission, which implies that results obtained have a largevariance associated if an analog Monte Carlo simulation is utilized.Furthermore, in both deterministic and Monte Carlo simulations delayed neutronprecursors are grouped in a $6$- or $8$- group structure, but nowadays there isnot a solid reason to keep this aggregation. In this work, and for the firsttime, individual precursor data is implemented in a Monte Carlo simulation,explicitly including the time dependence related to the $\beta$-delayed neutronemission. This was accomplished by modifying the open source Monte Carlo codeOpenMC. In the modified code -- Time Dependent OpenMC or OpenMC(TD) -- timedependency related to delayed neutron emission originated from $\beta$-decaywas addressed. Continuous energy neutron cross-sections data used comes fromJEFF-$3$.$1$.$1$ library. Individual precursor data was taken fromJEFF-$3$.$1$.$1$ (cumulative yields) and ENDF-B/VIII.$0$ (delayed neutronemission probabilities and delayed neutron energy spectra). OpenMC(TD) wastested in: i) a monoenergetic system; ii) an energy dependent unmoderatedsystem 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.

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