[en] 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)

[en] Thermal behaviour studies of gadolinia doped UO₂ fuel, as compared to UO₂ fuel, were performed in the HBWR in the test rig IFA-515. Two small diameter, hollow pellet fuel rods A1, with 11.5 wt% enriched UO₂, and A2, with 8 wt% Gadolina doped and 13 wt% enriched UO₂, irradiated from July 1994 to October 2000 to a final discharge burnup of 76 (A1) and 84 MWd/kgU (A2), were sent to Kjeller for post irradiation examination (PIE). Both rods were instrumented with Expansion Thermometers (ET). PIE was performed to obtain more information on fission gas release, fuel burnup, and structural characteristics of fuel and cladding. PIE included: axial gamma scanning - dimensional measurement - rod puncturing - fission gas analysis - burn up analysis - ceramography. The gamma activity and Cs isotope distributions measured were similar for both rods. The value measured for diametrical cladding creep-out was approximately 0.46% for rod A1 and 0.6% for rod A2. FGR was similar (-10%) for both rods, while the inventory of helium was somewhat larger in the UO₂ rod (A1). The Xe/Kr ratio was slightly higher in rod A2 with the higher burnup. Ceramography showed almost no cracks in A2, while A1 had circumferential and radial cracks. Both fuel rods revealed a closed cold gap and PCMI. Somewhat larger fuel swelling was seen in A2 as indicated by the diameter reduction of the pellet centre hole and the larger diameter. The two types of high burnup fuel showed different microstructures. A higher quantity of small roundish pores (evidence for retained fission gas) could be observed in A2. The pellet microstructure in A1 showed two radial characteristic zones and grain-growth, while A2, Gd-doped UO₂ fuel, exhibited only one zone across the radiuswith a characteristic 'rim structure'. (Author)