Validation of MCNP&ORIGEN-S 3-D Computational Model for Reactivity Predictions During BR2 Operation

The Belgian Material Test Reactor (MTR) BR2 is strongly heterogeneous high flux engineering test reactor at SCK-CEN (Centre d'Etude de l'energie Nucléaire) in Mol at a thermal power 60 to 100 MW. It deploys highly enriched uranium, water cooled concentric plate fuel elements, positioned inside a beryllium reflector with complex hyperboloid arrangement of test holes. The objective of this paper is the validation of a MCNP&ORIGEN-S 3D model for reactivity predictions of the entire BR2 core during reactor operation. We employ the Monte Carlo code MCNP-4C for evaluating the effective multiplication factor keff and 3D space dependent specific power distribution. The 1D code ORIGEN-S is used for calculation of isotopic fuel depletion versus burn up 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 evaluate the 3D isotopic fuel depletion at the next step and to deduce the corresponding shim rods positions of the reactor operation. The capabilities of the both codes are fully exploited without constraints on the number of involved isotope depletion chains or increase of the computational time. The reactor has a complex operation, with important shutdowns between cycles, and its reactivity is strongly influenced by poisons, mainly 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 0.5$ (eff=0.0072).