A Monte Carlo method for calculation on the dynamic behaviour of nuclear reactors

In safety calculations of nuclear reactors, dynamic calculations are of great interest. This type of calculation is mainly done by making an approximation and than using a deterministic code to solve the simplified problem. In this paper a full Monte Carlo method is proposed to perform these calculations on seconds to minutes scale without approximations. First the sampling of precursor decay is needed. The decay of precursors plays a crucial role in the dynamics of nuclear reactors, but the lifetime of a precursor can go up to 102 s. This is much larger than the order of the lifetime of a prompt neutron (10-4 s) or the order of a prompt neutron chain (10-2 s). Therefore precursors in this simulation are forced to decay according to a modified pdf. This ensures a small variance in the delayed neutron distribution in time. The simulation scheme has also been adapted for dynamic Monte Carlo. The simulation is now divided in time steps and the ancestor of a particle needs to be tracked throughout the simulation for proper variance estimation. After each time step the system properties can be adjusted. Finally the dynamic Monte Carlo method is used to calculate the power production in a simple system. The results agree with a point kinetics calculation of the same problem. (author)