An Improved Model for the Release of Fission Gas in CANDU Fuel

Abstract

Previous studies have indicated the desirability of increasing the burnup of CANDU fuel, from 7-9 GWd/t to about 21 GWd/t. At extended burnups, one issue in fuel integrity is fission gas release. Earlier studies showed that at extended burnups, fuel performance codes ELESIM [1] and ELESTRES [2] underpredict the release of fission gas by about a factor of two. An improved model is proposed in this paper. Via parametric studies that compared measurements to predictions, ad-hoc modifications have been determined for the following sub-models affecting gas release: thermal conductivity of UO,; diffusivity; and size/density of intergranular bubbles. The levels of the modifications are consistent with experimental data. The new model improves the predictions of ELESTRES for fission gas release when compared against measurements from 98 irradiations in research as well as in commercial reactors. On average, the predictions now differ from the measurements by 1.3%. For reference, the experimental scatter in a typical irradiation is 23-4%.