Fuel Thermal/Mechanical Behaviour Under Loss of Coolant Accident Conditions as Predicted by the FACTAR Code

Abstract

FACTAR (Fuel And Channel Temperature And Response) is a computer code developed to simulate the transient thermal and mechanical behaviour of 37-element or 28-element fie bundles within a single CANDU fuel channel for moderate (i.e., sheath temperatures less than the melting point of Zircaloy) loss-of-coolant accident (LOCA) conditions including transition and large break LOCAs with emergency coolant injection assumed available. FACTAR's predictions o fuel temperature and sheath failure times are used for subsequent assessment of fission product releases and fuel string expansion. In this paper, model capabilities and calculated quantities of the code are summarised. The results from overly severe test cases are presented in order to clearly demonstrate the effect on calculated fuel channel behaviour of a mechanistic assessment of fuel-to-sheath heat transfer, and the impact of using a diffusion-limited model for Zircaloy/steam reaction (i.e. FROM) as opposed to a reaction rate correlation, coupled with the assumption of unlimited steam supply.