Modelling the Activity of 99Tc in the Primary Heat Transport System of a CANDU Reactor

Abstract

A physical model has been developed to describe the coolant activity behaviour of 99Tc, during constant and reactor shutdown operations. This analysis accounts for the fission production of technetium and molybdenum, in which their chemical form and volatility is determined by a thermodynamic treatment using a Gibbs-energy minimization. The release kinetics are calculated according to the ratecontrolling step of diffusional transport in the fuel matrix and vaporization from the fuel-grain smface. Based on several in-reactor tests with defective fuel elements, and as supported by the thermodynamic analysis, the model accounts for the washout of molybdenum from the defective fuel on reactor shutdown. The model also considers the recoil release of both 99Mo and 99Tc from uranium contamination, as well as a corrosion source due to activation of 98Mo. The model has provided an estimate of the activity ratio 99Tc/37Cs in the ion-exchange columns of the Darlington Nuclear Generating Station, i.e., 6 x 10^-6 (following -200 d of steady reactor operation) and 4 x 10^-6 (with reactor shutdown). These results are consistent with that measured by the Battelle Pacific Northwest Laboratories with a mixed-bed resin-sampling device installed in a number of Pressurized Water Reactor and Boiling Water Reactor plants.