THE MOX FUEL TESTS IN THE CABRI REP-Na PROGRAMME : ANALYSIS AND MAIN OUTCOMES

THE MOX FUEL TESTS IN THE CABRI REP-Na PROGRAMME : ANALYSIS AND MAIN OUTCOMES Bernard Cazalis, Joelle Papin and Francette Lemoine Institut de Protection et de S°retÈ NuclÈaire (IPSN) DRS/SEMAR, CE-Cadarache 13108 Saint Paul lez Durance (France) International Topical Meeting on Light Water Fuel Performance, Parkcity- USA/2-4 avril 2000

A first investigation of MOX fuel behaviour under RIA conditions has been performed through the three MOX fuel tests of the CABRI REP-Na programme. From the preliminary analysis of the presently available results, no evidence for local thermal effects resulting from the heterogeneous fuel structure can be derived. The very high clad straining of the 2-cycles rod (8 % maximum value at the pellet edge in REP Na9) is attributed to the combined effect of thermal expansion and intra-granular gasinduced swelling mechanism linked to the high energy deposition. On the other hand, the results of all the tests clearly underline an enhanced fission gas contribution with regard to the UO2 fuel behaviour. A significant increase of fission gas release is found with the MOX fuel compared to UO2 fuel at similar burn-up ; such effect is explained considering the presence of a high quantity of gases in inter-granular and porosity bubbles associated with the UPuO2 agglomerates behaviour under irradiation and is much increased at high burn-up. As a consequence of a rapid power transient with fuel heat-up and gas overpressure leading to grain boundary separation, a larger amount of gases can be available for clad loading under gas pressure. Moreover, in contrast to the CABRI UO2 fuel rod failures, which occurred when the clad mechanical properties were degraded by the presence of hydride accumulations, the possibility of rod failure with a sound cladding at a low corrosion level was revealed by the REP Na7 test result with MOX fuel (at 55 GWd/t) with, however, a mean fuel enthalpy at failure time (120 cal/g) higher than the expected maximum enthalpy level in reactor conditions. Consistently with the high fission gas release results, the obtained failure can be explained by a high contribution of the fission gas pressure on the clad loading and suggests a high burn-up effect with MOX fuel.