Grain-boundary gas release and swelling in high burn-up uranium dioxide

Abstract

Miniature UO 2-stainless steel fuel pins were irradiated in DIDO to 0.8–5.0% burn-up. Centre fuel temperatures were calculated from chemical burn-up and heat transfer data and were calibrated from grain growth observations in unirradiated material. On this basis centre temperatures did not exceed 1630 °C. Fission gas release was compatible with diffusional release and “knock-out” models up to about 2% burn-up. Above 3% burn-up a large increase in gas release occurred in both diffusional and non-diffusional regimes. Apparent diffusion coefficients were strongly sensitive to the initial fuel density at the lower burn-up, but at the higher burn-up they were relatively independent of this parameter. With increasing burn-up, fission gas bubbles accumulate at grain boundaries and later smaller fission gas bubbles form within the grains. Thermal stress cracking was entirely transgranular up to 2 % burn-up ; at 3% burn-up some intergranular cracking was seen. and at 5% burn-up extensive areas of predominantly circumferential grain boundary cracking appeared. The marked increase in gas release above 3% burn-up is believed to arise in part from the interconnection of grain boundary gas bubbles and in part from the fracture under thermal stress of grain boundaries weakened by gas bubbles. At high burn-ups the specific surface areas calculated from gas release values approach the grain boundary surface area; it is suggested that the increase in the apparent diffusion coefficient with burn-up is essentially an increase in the specific surface area and not in the true diffusion coefficient. The disappearance of small residual pores even at low temperatures is believed to indicate an irradiationenhanced sintering process and is associated with some densification of the fuel. At higher temperatures grain boundary porosity is stabilised, probably by fission gases. An incubation period of 2% burn-up was observed before diametral expansions of the cladding became significant. This is usually ascribed to the initial accommodation of swelling in internal voidage. However, even in pins of 97% smeared density the incubation period is not greatly lower than that reported for fuel of only 80% density. It is concluded that the voidage in the high density pins is being used more efficiently than that in the low density fuel pins.