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.