Burnup dependence of melting temperature of FBR mixed oxide fuels irradiated to high burnup

Highlights: → Melting temperatures for irradiated MOX fuels were measured by a new method. → In the new method, a rhenium inner capsule is used to stop fuel-tungsten reactions. → Melting temperatures of irradiated MOX fuels linearly-decrease depending on burnup. → The decrease mainly results from solid solution of fission products in fuels. - Abstract: The melting temperatures of FBR MOX fuels with Pu content of 28-30 wt.% irradiated to from 22.5 to 112.5 MWd kg-1 were measured using a rhenium inner capsule to hold the specimens. The rhenium inner capsule could prevent chemical reactions between fuels and tungsten materials which decrease the melting temperature. The melting temperatures were about 30 K higher than the previous data using tungsten capsules. The melting temperature decreases in a linear manner with burnup due to solid solution of fission products in fuels. However, the slopes of the lines plotting melting temperature versus burnup are almost similar to the previous data.