Effect of the loss of coolant on the resonance escape probability

Abstract

In the analysis of the loss of coolant accident in natural uranium reactors of the Atucha or Candu type, the influence of the coolant voiding on the resonance escape probability appears to be of great importance. In this paper we study this effect on the resonance absorption, analyzing its behaviour vs coolant density. We use the codes COCAP and WIMS, in which the shadowing among pins in the cluster, or the Dancoff effect, is taken into account with different models. In COCAP, the Dancoff factor calculation based on Carlvik's method shows a remarkable dependency on the coolant density, and its values at very low densities lead to an overestimation of the cell reactivity. It is shown that the inclusion of a Bell factor in the definition of the cluster effective surface improves the agreement with experimental results.