First evidence of metallisation circle in a Chromium doped UO2 pellet submitted to a thermal gradient

In order to better understand the formation of metallic fission product precipitates at the centre of power ramped UO2 fuel, we performed a separate effects experiment. An annular pellet made of chromia doped UO2 with an initially uniform distribution of chromium oxide precipitates, was subjected to a temperature gradient. The chromia precipitates became metallic in the hottest part of the pellet but stayed in an oxide phase in the coldest part. The border between oxide and metallic precipitate, named metallisation circle, moved from the pellet inner surface towards mid-pellet as a function of heating time. After one hour of heating, stationary state under thermal gradient was reached. The position of the metallisation circle in stationary conditions allowed an assessment of the oxygen potential profile inside the pellet.