A Model for Fission Product Distribution in CANDU Fuel

Abstract

This paper describes a model to estimate the distribution of active fission products among the UO₂ grains, grain-boundaries, and the free void spaces in CANDU fuel elements during normal operation. This distribution is required for the calculation of the potential release of activity from failed fuel sheaths during a loss-of-coolant accident. The activity residing in the free spaces ("free" inventory) is available for release upon sheath rupture, whereas relatively high fuel temperatures and/or thermal shock are required to release the activity in the grain boundaries or grains.

A preliminary comparison of the model with the data from in-reactor weep-gas experiments performed in Canada yields generally good agreement, with overprediction rather than underprediction of radiologically important isotopes, such as ¹³¹I. The model also appears to generally agree with the "free" inventory release calculated using ANS-5.4.