A Model for Predicting Coolant Activity Behaviour for Fuel-Failure Monitoring Analysis

Abstract

A mathematical treatment has been developed to predict the release of volatile fission products from operating defected nuclear fuel elements. Diffusion theory is used to account for fission product migration in the fuel matrix and a source release into the fuel-to-clad gap. Precursor diffusion is also considered for the isotopes of I-132 and Xe-135, which have relatively long lived precursors. The transport and release of fission products from the gap is treated as a first order rate process as characterized by a gap escape-rate coefficient. The fission product activity in the gap and coolant follows from a mass balance considering losses due to radioactive decay, neutron transmutation and coolant purification. The activity in both the fuel-to-clad gap and coolant as a function of time can therefore be predicted during all reactor operations including reactor shutdown, startup and bundle-shifting maneuvers. The model has been implemented as the STAR (Steady state and Transient Activity Release) code for use on personal computers with a finite-element solution of the mass transport equations using FEMLAB. The model parameters are derived from in-reactor experiments conducted with defected fuel elements containing natural and artificial failures at the Chalk River Laboratories. The STAR code has also been successfully validated against an analytical solution and benchmarked against several defect occurrences in the Bruce Nuclear Generating Station.