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

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 m2/s, and the effects of fuel type (UO2 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.