In order to examine the fission gas release behavior under restraint condit
ions, an out-of-pile high pressure annealing technique was applied to UO2 f
uels irradiated to 37 GWd/t in a BWR. The irradiated fuels were annealed at
temperatures of 1,500-1,800 degrees C and external pressures of 10-150MPa.
The fractional release of Kr-85 was evaluated by beta-activities in small
amounts of gas that were intermittently collected from the high pressure fu
rnace. The fission gas release was almost completely suppressed by strong e
xternal restraint pressures of 100-150 MPa in the temperature range of 1,50
0-1,800 degrees C, and the critical pressure for the onset of burst release
due to rapid bubble growth and tunnel formation was found to be in the ran
ge of 40-60 MPa at 1,500 degrees C. This type of burst release was very sim
ilar to that observed during the power-down stage in the irradiation tests
of high burnup fuels.