Recalculation of small-zone sodium-void reactivities in assemblies BZA and BZB using improved methods

The sodium-void reactivity measurements in the BIZET assemblies BZA/1, BZA/1b and BZB/2 were evaluated using coarse-group cross-sections collapsed by bilinear weighting. The sodium-void reactivities were calculated by both first-order and exact perturbation theory, the direct and adjoint neutron fluxes resulting from diffusion theory calculations. The coarse-group results are closer to the appropriate fine-group results and to the experimental values than preliminary results from an evaluation using coarse-group cross-sections collapsed by real flux weighting. The evaluation leaves two problems: the overestimation of axial neutron leakage and the different results for equivalent pin and plate cells