Lattice Characteristics that Influence the Change in Reactivity with Coolant Voiding in CANDU Lattices

Abstract

The reactivity change in CANDU following coolant voiding is determined by a delicate balance of reactor physics phenomena. These in turn depend, most sensitively, upon the relative volumes of coolant and moderator, the relative temperature of the coolant and moderator and the fuel enrichment. The influence of these parameters is explained in terms of classic four-factor reactor theory. A proper simulation of the void effect can only be obtained from full core calculations. However, the simpler approach of using a lattice cell code such as WIMS-CRNL does allow the magnitude of the void effect to be estimated. WIMS-CRNL results that predict the way the void effect changes with fuel enrichment and channel geometry are presented and shown to be in qualitative agreement with the simple four-factor explanation. The geometry changes include moderator volume, coolant-to-fuel volume ratio and coolant displacers. It is shown that one way that loss of coolant results in a positive void effect is due to the loss of scattering inside the fuel bundle. If this loss can be reduced, a less positive void effect would be expected. Coolant displacers could be used to replace the innermost fuel pins with a solid scattering material which is not removed when the coolant is voided. WIMS- CRNL results confirm these expectations. As all methods of changing the size of the void effect result in changes to the absolute reactivity, corresponding burnup changes are to be expected. WIMS-CRNL calculations demonstrate that, with the exception of coolant dis- placers, a significant reduction in the magnitude of the positive void coefficient is accompanied by large burnup losses. For completeness some experimental data are presented. These data indicate the way that the void effect changes with lattice pitch,the agreement between WIMS-CRNL and experimental data for the total void effect, and the effect of coolant displacers.