Fission gas release from high burnup mixed oxide (MOX) fuel pellets

Amaya, Masaki  ; Nakamura, Jinichi ; Nagase, Fumihisa 

Fission gas release (FGR) from the high burnup MOX fuel pellets was investigated by monitoring rod-internal pressure change during re-irradiation in the Halden reactor. At the first power ramp in the re-irradiation test, the internal pressures of the test rods abruptly increased when the fuel centre temperatures exceeded 800C, which was 200C lower than the 1% FGR threshold temperature expected from its burnup dependence in the low burnup region. The size of the pellet fragment which controls FGR in the test rod was evaluated based on the measured fractional FGR, and the results indicated that grain boundary tunnels and/or micro-cracks did not significantly form even after the increase of rod inner pressure was detected. This suggests that the abrupt FGR observed in the test rods was not due to the formation of grain boundary tunnels and/or micro-cracks, and the mechanism of FGR in the test rods differed from that in the low and middle burnup region.