Fission gas release from rock-like fuels, PuO(sub 2)-ZrO(sub 2)(Y)(l brace)or ThO(sub 2)(r brace)-Al(sub 2)O(sub 3)-MgO at burn-up of 20 MWd/kg

摘要

Two types of fuels, that is, a 20w/o PuO(sub 2) incorporated into ThO(sub 2)-Al(sub 2)O(sub 3)-MgAl(sub 2)O(sub 4) and a 23w/o PuO(sub 2) incorporated into ZrO(sub 2)(Y)-Al(sub 2)O(sub 3)-MgAl(sub 2)O(sub 4) were fabricated into a disk form (outer diameter by 3mm x thickness by 1mm) by JAERI's self-established development technique. The two were provided for steady-state irradiation test at the Japan Research Reactor 3M (JRR-3M) up to the average burn-up of 20 MWd/kg (27 MWd/kg in peak) to understanding fuel behaviour. Post-irradiation examination (PIE) was carried out and the followings were revealed. (1) Despite of low irradiation temperature <1000degC, there occurred significant fission gas release (FGR) which could not be explained by diffusion mechanism. A possible explanation obtained from fuel microstructural study is direct escape of FP gas from fuel matrix to gas gap via open pores. (2) A nuclide Cs migrated from fuel matrix to plenum region. Its amount was roughly 20% of totally produced. This was partially due to slight temperature gradient across disk fuel and partly due to low Cs retentiveness in fabricated fuel matrix. A magnitude of Cs-137 migration was smaller in ThO(sub 2)-Al(sub 2)O(sub 3)-MgAl(sub 2)O(sub 4) disk fuel than that in ZrO(sub 2)(Y)-Al(sub 2)O(sub 3)-MgAl(sub 2)O(sub 4) disk fuel. (3) Gas bubble swelling rate estimated by total porosity increase was 14% for ZrO(sub 2)(Y)-Al(sub 2)O(sub 3)-MgAl(sub 2)O(sub 4) disk fuel and 11% for ThO(sub 2)-Al(sub 2)O(sub 3)-MgAl(sub 2)O(sub 4) disk fuel each per 10 MWd/kg. (4) Slight bonding between disk fuels and spacers (Nb-1w/oZr) and that between disk fuels and sheath (Nb-1w/oZr) occurred. The mechanism was attributed to mutual diffusion mainly between Al compound from fuel and Nb one from spacer or sheath. (author)