Distribution characteristics of fission gas along the axial direction for an irradiated fuel rod of a pressurized water reactor (PWR)

Abstract

In this work, the fission gas distribution of an irradiated oxide fuel was investigated by measuring the fission gas release (FGR) and retained gas from and in an oxide fuel along the axial height of the fuel rod. In addition to the fission gas measurements, other destructive and non-destructive post irradiation examination tests such as gamma scanning, eddy current testing, density and oxide layer thickness measurements, and ceramography/metallography were conducted to review their impact on the FGR and gas retention. A lead rod was extracted from a lead test assembly, which was irradiated to 56.9 GWd/tU during three irradiation cycles for a total exposure of 1,406 effective full power days. Considering the water-side oxide layer thickness and local burnup of the fuel rod, the rod was sectioned into four positions along the axial height and four samples were prepared from these positions. A sample fragment of around 0.1–0.2 g was individually melted to measure its retained krypton and xenon concentration. According to the measurement results, the retained krypton and xenon concentration ranges were 0.114–0.139 cc/gU and 1.073–1.338 cc/gU, respectively, and their retention percentage after normalization of the sample local burnup showed a decreasing trend as the axial height of the fuel rod raised. The water-side oxide layer thickness scope of the tested fuels measured by an observation of the optical microscope images was 13–42 μm, and the thickness was increased along the axial height of the rod. The amount of released krypton and xenon into the rod-free volume was measured as 5.7 and 54.6 cc, respectively, by a rod puncturing/collection procedure, which corresponds to a 1.94 % fractional fission gas release referring to fission gas generation by a code calculation.