Calculational and experimental investigations of void effect. A new model for leakage treatment of heterogeneous assemblies

Abstract

The growing interest in safety problems had led physicists to try to increase their knowledge of the neutron leakage phenomenon, both by calculation and by experiment The flux calculation in a heterogeneous assembly is frequently performed by collision probability method On the contrary, neutron leakage is calculated for a flux-weighted homogenized assembly, which is a good approximation for a non-voided assembly (PWR in normal conditions) In a LOCA situation, the assembly may contain void zones, and this model for leakage calculation may become insufficient We propose here a new theoretical model, taking into account the effect of heterogeneity of the assembly on neutron leakage, and which was implemented as TIBERE procedure in the multigroup transport assembly code APPOLO-2 One of the advantages of this new model is to allow a perfectly consistent definition of cell reaction and cell leakage rates used in the equivalence procedure As well as this theoretical work was made, an experimental program concerning this phenomenon was performed as a part of EPICURE experiment Comparisons of experimental and calculational results point out better agreement of the new model with the measurements