Grain size effects on radiogenic Helium gas in the nuclear fuel UO2

Due to the extreme environment in which it is operated, nuclear fuel shows changes of its microstructure and thermophysical properties . In particular it develops a rim structure also named high burnup structure characterized by the subdivision of the original micrometer sized grains into 100 nm grains. In this work, the synthesis of dense UO2 and ThO2 with grains size down to 100 nm was designed. Such material would allow studying through single effect studies the impact of the high burnup structure on the fuel behavior in and out of normal operation. UO2 doped with 238Pu was also synthesised to study the accelerated effect of alpha-decays on fuel microstructure and thermophysical properties since alpha activity will be dominating in spent nuclear fuels for millenaries. Self-irradiated UO2 cumulating up to 0.41 dpa, the same reached by a LWR spent fuel after few centuries, was characterized periodically with a broad set of techniques. XRD showed a saturation of the lattice parameter increase around 0.3 %, while thermal desorption spectroscopy proved that the totality of the radiogenic He is still retained. SEM observations highlighted the integrity of the fuel is preserved, while TEM evidenced the ingrowth of dislocation loops and He bubbles within the matrix. Thermal conductivity as measured by laser flash had already decreased by 40 % at 0.03 dpa, and no defects annealing was detected by differential scanning calorimetry at the temperatures foreseen for spent fuel storage. Raman spectra were acquired for the first time on (U,Pu)O2 as a function of the self-irradiation dose. This study allowed assessing some aspects of the behaviour of LWR spent nuclear fuel during the first 300 years of storage time.