Synthesis of the results obtained on the advanced UO2 microstructures irradiated in the TANOX device

In order to achieve higher burn-ups in PWR, new fuels are required with improved gas retention capacities. In this context, the Advanced UO2 Microstructures program, first presented in Tokyo (TCM 1996), aimed at fabricating, irradiating and testing new UO2 fuels intended for high-burnup and for PCMI effect reduction. A large number of UO2 fuels were fabricated, which allowed to evaluate the influence of numerous parameters: grain size, stoichiometry, additives with various concentration and chemical form. Small fuel rods were irradiated in the TANOX device, in the Siloe reactor, to a burn-up of about 10 GWd/tU. The temperature at the centre of the annular pellets was controlled and maintained under 700 deg. C in order to avoid activation of thermal gas release. After irradiation, the rod was punctured; the clad was cut to take the pellets out. The fission gas retention capacities of the different kinds of fuels were assessed by post-irradiation annealing tests. The anneals were performed in a high frequency furnace at 1700 deg. C; 85Kr released from the fuel was measured by gamma spectrometry. Micrographic examinations were performed on each kind of pellets before and after annealing tests. The results show that the fuels with best fission gas retention are characterized by both a large grain size (over 50 μm) and intragranular precipitates or defects, which are favourable to bubble nucleation and pinning. This type of microstructure can be achieved by adding Cr2O3 at a percentage over the solubility limit. Cr2O3 then stimulates grain growth and also forms oxide precipitates. Gas retention can be even larger with an improved intergranular retention capacity, which is obtained by adding SiO2 to Cr2O3. Hyperstoichiometric (UO2,01 or UO2,02) large grain fuel also showed an improved fission gas retention capacity. On the basis of these results, several microstructures were selected for a further study of gas retention at high burn-up (65 GWd/tU). A new irradiation program of doped UO2 fuels started in the TANOXOS device, in the Osiris reactor. (author)