Post irradiation examination of two MOX fuel rods re-irradiated in IFA-648/629.3

To study the thermomechanical behaviour, fission gas release, and fuel structural characteristics of high burnup MIMAS-MOX fuel two experimental rods, rods 5 and 6, were refabricated from fuel rods irradiated for four cycles in Gravelines-4 PWR, instrumented, pressurised with helium and in 1999 loaded into the burnup extension rig IFA 648.1 in the HBWR. In 2000, the rig was unloaded, and rod 6 was punctured, reinstrumented and repressurised with helium. The rods were then re-irradiated in the ramp rig IFA 629.3 to a final discharge burn up of .60 MWd/kg MOX. The rods were subjected to post irradiation examination (PIE). PIE showed that both high burnup rods remained intact after the stepwise power ramp up to 25 kW/m. Fission gas release and internal rod pressure, found from puncturing test and fission gas analysis, were consistent with in-pile measurement (rod 6). Fission gas release in rod 5 (not re-instrumented between IFA-648 and 629.3) was significantly higher than in rod 6. During ramping in IFA-629.3 the measured fuel temperature was higher in rod 5 than in rod 6. The burnup of rod 5 was also somewhat larger. Fission gas analysis showed an unusual high Xe/Kr ratio of .19.5 for both rods. Both fuel cross-sections showed an inhomogeneous microstructure with large Pu-islands (up to 300 μm) in the UO2 matrix. The Puislands showed signs of a higher burn-up than the matrix (indicated by larger MFPs and larger porosity). Both fuel rods showed a rim structure and pellet clad mechanical interaction (PCMI) with chemical bonding between cladding and fuel. The waterside oxide layer of rod 5 (35-40μ) is somewhat thinner than that found on the rod 6. Permanent diameter increase was approximately 50 μ on rod 5 and 25 μ on rod 6. (Author)