Changes of the surface-to-volume ratio and diffusion coefficient of fission gas in fuel pellets during irradiation

Abstract Short-lived fission gas release from fuel pellets during irradiation was investigated based on the experimental results of the gas-flow rigs irradiated in the Halden Heavy Water Reactor (HBWR). The release-to-birth ( R / B ) rates of short-lived fission gas were measured by means of gas-flow measurement during the irradiation experiments. Surface-to-volume ( S / V ) ratios of fuel pellets and diffusion coefficients of short-lived fission gas release were evaluated from the obtained ( R / B ) values. The increase of ( S / V ) ratio agreed well with the point where the fuel temperature exceeded the threshold of 1% fission gas release. This indicates that the interlinkage of fission gas bubbles occurred there. The evaluated diffusion coefficients scattered in the range between 10 −23 and 10 −17 m 2 /s, and the effects of fuel type (UO 2 or MOX) were not clearly observed. In addition, it is likely that the restructuring effect of fuel pellet on the diffusion coefficients of short-lived fission gas at least in the fuel pellet matrix is negligible in high burnup region where the rim structure forms in the fuel pellet.