Fission product releases from thoria and urania fuels under reactor accident conditions: Hot cell experiment #6 (HCE6)

•Under inert conditions, releases of noble gases from ThO2 were lower than from UO2.•A sheathed sample of ThO2 fuel had delayed release of noble gases relative to ThO2 fragments.•In a steam environment, releases of Kr, Xe, I, and Cs were similar for ThO2 and UO2 samples.•No significant releases of Nb, Ru, Ba, or Pa. The accident-conditions fission-product release experiment HCE6 was performed on samples of thoria (ThO2) fuel and urania (UO2) fuel irradiated at water reactor operating conditions in the pressurized water loop in the NRU reactor. The tests in the experiment were intended to compare fission-product releases from ThO2 fuel and UO2 fuel under similar circumstances, and to explore the behaviour of fission products in ThO2 fuel under oxidizing conditions. Under inert conditions, or temperatures of ~1720 K, the releases from ThO2 were significantly lower than from UO2. A 1920 K test on a sheathed sample of ThO2 exhibited a delay of ~3000 s in noble gas releases relative to an inert-environment ThO2 fragment test, possibly due to a decrease in oxygen potential by contact with Zircaloy at high temperature. In a steam environment, the releases of Kr, Xe, I, and Cs, were quite similar for the ThO2 and UO2 samples, and the release timing illustrated similar effects of Zircaloy oxidation on the release of these fission products. In an air-steam mixed environment, lower releases of noble gases and volatile fission products from the ThO2 sample were observed, probably due to the shorter exposure to oxidizing environment and greater amount of Zircaloy present. None of the tests showed significant releases of 95Nb, 103Ru, 140Ba, or 233Pa.