Fission gases and helium gas behavior in irradiated mixed oxide fuel pin

Behavior of helium and fission gases in irradiated mixed oxide (MOX) fuels was investigated by pin puncture and heating tests by quantitatively measuring amounts of helium and fission gases in a fuel pin irradiated in JOYO to ∼50 MW d kg−1 as a whole pin average burnup. While the fission gas releases were 47% and 48% for Kr and Xe respectively, all helium generated during irradiation was released (∼100%), and the helium released during the heating test was derived from α-decay after irradiation. The release profile during the heating test indicated that helium gas release onset temperature was below 1173 K at an isothermal condition, but during irradiation, the helium release behavior could be understood by taking its high diffusion coefficient into consideration. The different release behavior of helium and fission is mainly explained by their different mobility in the fuel.