FUEL FAILURE AND FISSION-GAS RELEASE IN HIGH BURNUP PWR FUELS UNDER RIA CONDITIONS

To study the fuel behavior and to evaluate the fuel enthalpy threshold of fuel rod failure under reactivity initiated accident (RIA) conditi ons, a series of experiments using pulse irradiation capability of the Nuclear Safety Research Reactor (NSRR) has been performed. During the experiments with 50 MWd/kg U PWR fuel rods (HBO test series; an acron ym for high burnup fuels irradiated in Ohi unit 1 reactor), significan t cladding failure occurred. The energy deposition level at the instan t of the fuel failure in the test is 60 cal/g fuel, and is considerabl y lower than those expected and pre-evaluated. The result suggests tha t mechanical interaction between the fuel pellets and the cladding tub e with decreased integrity due to hydrogen embrittlement causes fuel f ailure at the low energy deposition lever. After the pulse irradiation , the fuel pellets were found as fragmented debris in the coolant wate r, and most of these were finely fragmented. This paper describes seve ral key observations in the NSRR experiments, which include cladding f ailure at the lower enthalpy lever, possible post-failure events and l arge fission gas release. (C) 1997 Elsevier Science B.V.