High burnup effects on fuel behaviour under accident conditions: the testsCABRI REP-Na

A large, performance based, knowledge and experience in the field of nuclea r fuel behaviour is available for nominal operation conditions. The databas e is continuously completed and precursor assembly irradiations are perform ed for testing of new materials and innovative designs. This procedure prod uces data and arguments to extend licencing limits in the permanent researc h for economic competitiveness. A similar effort must be devoted to the est ablishment of a database for fuel behaviour under off-normal and accident c onditions. In particular, special attention must be given to the so-called design-basis-accident (DBA) conditions. Safety criteria are formulated for these situations and must be respected without consideration of the occurre nce probability and the risk associated to the accident situation. The intr oduction of MOX fuel into the cores of light water reactors and the steadil y increasing goal burnup of the fuel call for research work, both experimen tal and analytical, in the field of fuel response to DBA conditions. In 199 2, a significant programme step, CABRI REP-Na, has been launched by the Fre nch Nuclear Safety and Protection Institute (IPSN) in the held of the react ivity initiated accident (RIA). After performing the nine experiments of th e initial test matrix it can be concluded that important new findings have been evidenced. High burnup clad corrosion and the associated degradation o f the mechanical properties of the ZIRCALOY4 clad is one of the key phenome na of the fuel behaviour under accident conditions. Equally important is th e evidence that transient, dynamic fission gas effects resulting from the c lose to adiabatic heating introduces a new explosive loading mechanism whic h may lead to clad rupture under RIA conditions, especially in the case of heterogeneous MOX fuel. (C) 1999 Elsevier Science B.V. All rights reserved.