Fission Gas Release Behavior from High Burnup UO 2 Fuels under Rapid Heating Conditions

Fission gas release (FGR) behavior under rapid heating conditions of high burnup UO2 fuels with developed rim structure has been examined using two different out-of-pile heating techniques with no restraint pressure. The burnups of the fuel specimens were 36–86 GWd/tU. The bare fuel specimens were heated up to 600-1,800°C at heating rates of 1.7 to 4,600°C/s. The FGR process strongly depended on fuel burnup (extent of rim structure formation) and heating conditions (heating rate, terminal temperature). At lower heating rates below about 10°C/s, growth and interlinkage of fission gas bubbles controlled FGR only at higher temperatures above the threshold temperatures of 1,250--1,450°C, depending on fuel burnup. At higher heating rates above at least 90°C/s, instantaneous FGRs, which originated from the occurrence of microcracks and fuel fragmentation induced by the overpressurization of rim bubbles, arose at higher temperatures above 700°C, only for the high burnup fuels of 74 and 86 GWd/tU with developed rim structure. Almost no FGR was found for non rim-structured fuels, even at the higher terminal temperatures of 1,500--1,800°C. From these and our previous out-of-pile heating results, a map of heating condition dependent FGR under unrestrained conditions is proposed as a function of heating rate and terminal temperature.