Behaviour of CANDU Fuel During a High-Temperature Transient

Abstract

CANDU UO₂-fuel/Zircaloy-sheath assemblies may undergo a high-temperature transient during a postulated loss-of-coolant accident. The presence of steam under such conditions causes an exothermic Zircaloy/steam reaction and thereby hydrogen production. Two high-temperature test facilities, one with a vertical and the other with a horizontal reaction tube, have been built at the Whiteshell Nuclear Research Establishment (WNRE) to study the high-temperature transient behaviour of UO₂-fuel/Zircaloy-sheath assemblies in inert gas, steam or steam/inert gas mixtures at temperatures up to 2000 °C. Hydrogen detection systems capable of determining the rate of fuel sheath/steam oxidation reactions have also been developed. Preliminary high-temperature transient tests conducted in steam using a single pellet/fuel sheath assembly, in which the melt relocated into a gap between the fuel pellet and a thoria catch crucible, seem to indicate that the kinetics of the relocation phenomena can be determined in these facilities by monitoring the hydrogen evolution rate.