Fission gas release from externally restrained uranium dioxide fuel

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.