Failure Criteria for Zircaloy-Sheathed Fuel Elements Under LOCA Conditions

Abstract

During a loss-of-coolant accident (LOU), several factors can have a significant affect on the performance (sheath deformation and failure) of CANDU fuel: 1. the amount of Zircaloy oxidation, 2. the variety of metallurgical structures of differing mechanical properties, ranging from as-received to beta-annealed Zircaloy, that exist on the fuel sheath, 3. attachment of spacer and bearing pads onto the sheath by a beryllium-braze process, 4. circumferential temperature gradients, and 5. the amount of stored energy deposited into the fuel. New fuel element failure criteria are proposed which are based on the above physical processes. A comprehensive database of over 200 laboratory tests on empty Zircaloy fuel sheathe and six in-reactor tests on CANDU-type fuel elements has been assembled for the purpose of assessing these failure criteria. Comparisons are presented between the simple "5% sheath strain to failure" criterion, which has been used in safety analysis for CANDU reactor licencing requirements, and the physically based failure criteria.