Fission gas release behaviour of a 103 GWd/tHM fuel disc during a 1200 °C annealing test

Abstract

Within the Nuclear Fuel Industry Research (NFIR) program, several fuel variants, in the form of thin circular discs, were irradiated in the Halden Boiling Water Reactor (HBWR) to a range of burn-ups ∼100GWd/tHM. The design of the assembly was similar to that used in other HBWR programs: the assembly contained several rods with fuel discs sandwiched between Mo discs, which limited temperature gradients within the fuel discs. One such rod contained standard grain UO2 discs (3D grain size=18μm) reaching a burn-up of 103GWd/tHM. After the irradiation, the gas release upon rod puncturing was measured to be 2.9%.Detailed characterizations of one of these irradiated UO2 discs, using electron probe microanalysis (EPMA), scanning electron microscopy (SEM) and secondary ion mass spectrometry (SIMS), were performed in a CEA Cadarache hot laboratory. Examination revealed the high burn-up structure (HBS) formation throughout the whole of the disc, also the fission gas distribution within this HBS, with a very high proportion of the gas in the HBS bubbles. A sibling disc was submitted to a temperature transient up to 1200°C in the out-of-pile (OOP) annealing test device “Merarg” at a relatively low temperature ramp rate (0.2°C/s). In addition to the total gas release during this annealing test, the release peaks throughout the temperature range were monitored. The fuel was then characterized with the same microanalysis techniques as before the annealing test to investigate the effects of this test on the microstructure of the fuel and on the fission gases.It provided valuable insights into fission gas localization and the release behaviour in UO2 fuel with high burn-up structure (HBS).