OBSERVATIONS OF ATHERMAL GAS-RELEASE FROM WATER-REACTOR FUEL AT EXTENDED BURNUP

An experimental fuel, which was transient-tested in the Third Riso Fis sion Gas Project, released gas at an unusually low power of about 11 k W/m during the ramp. The percentage of gas released, 6-7% in the instr umented tests, was small compared with the 35-41% release obtained whe n the same fuel was transient-tested to 40 kW/m. The evidence suggests that the gas that was released had left the UO2 matrix during the bas e irradiation and had been stored for some considerable time in large closed pores in the fuel. It is assumed that release occurred when som e of these pores were vented by cracks initiated in response to the hi gh ramp rate. The main conclusion is that in low density fuel, fission gas may collect in the fabrication pores during steady-state irradiat ion and be released during load following if cracking occurs.