Thermal conductivity change in high burnup MOX fuel pellet

Nakamura, Jinichi ; Amaya, Masaki  ; Nagase, Fumihisa ; Fuketa, Toyoshi

High burnup MOX and UO test rods were prepared from the fuel rods irradiated in commercial BWRs. Each test rod was equipped with a fuel center thermocouple, and was re-irradiated in the Halden boiling water reactor (HBWR) in Norway. The burnups of MOX and UO test rods reached about 84 GWd/tHM and 72 GWd/t, respectively. Thermal conductivity change in high burnup fuel was evaluated from the comparison between the measured fuel temperature and the data calculated by using the fuel analysis code, FEMAXI-6. The comparison results suggested that the thermal conductivity of MOX fuel pellet is comparable to that of UO fuel pellet in the high burnup region around 80 GWd/t. It is probable that the impurity effect of Pu atom gradually diminishes with increasing burnup because other factors which affect pellet thermal conductivity, such as the accumulation effect of soluble fission products and irradiation-induced defects in crystal lattice, become dominant in high burnup region.