Helium release from the uranium–plutonium mixed oxide (MOX) fuel irradiated to high burn-up in a fast breeder reactor (FBR)

The helium releases were investigated in FBR fuel pins irradiated to high burn-up. The released amounts increased with the increase of burn-up, but no burn-up dependence of their release fraction to total amounts generated was seen. The contents of 241Am in fuel pellets affected the released amounts. The effect of released helium on the internal pressure in fuel pins was smaller in FBR fuel pins than in LWR fuel pins, but released helium in fuel pins containing increased amounts of 241Am, which are anticipated in future FBRs, is expected to affect the internal pressure.