Accuracy of Monte Carlo Criticality Calculations During BR2 Operation

The objective of this paper is validation of a combined MCNP&ORIGENS 3D model for reactivity predictions of the entire BR2 core during reactor operation. MCNP is used for evaluation of the effective multiplication factor keff and 3D space dependent specific power distribution. The 1D code ORIGENS is used for calculation of isotopic fuel depletion versus burnup and preparation of a database (DB) with depleted fuel compositions. The approach taken is to evaluate the 3D power distribution at each time step and along with DB to predict the 3D isotopic fuel depletion at the next step and to deduce the corresponding shim rods positions of the reactor operation. The reactor has a complex operation, with important shutdowns between cycles, and its reactivity is strongly influenced by poisons 3He and 6Li from the beryllium reflector, and burnable absorbers 149Sm and 10B in the fresh UAlx fuel. Our computational predictions for the shim rods position at various restarts are within +/-30 mm. The computational time for a Monte Carlo simulation by MCNP of one depletion time step for the 3D full scale heterogeneous geometry reactor model, containing ~ 4000 cells with varied fuel depletion, is 10000 histories/minute on a single PC PENTIUM-4/2GHz.