Swelling Rate and Diameter Change of Annular Pellet Irradiated in HANARO

In-reactor behavior of annular UO2 fuel may differ from conventional solid UO2 fuel because it is expected to operate at low temperature, and because the geometrical shape of the two kinds of fuel rods is not identical. Since the temperature of an annular fuel rod is lower than solid fuel by several hundred degrees, densification and swelling generally known as athermal phenomena could be affected by low temperature. In contrast with the solid fuel having only one gap, annular fuel has both inner and outer radial gaps which can accommodate the change in pellet volume. When pellet densification occurs at the beginning of life (BOL), outer gap would obviously increase due to the decreased outer pellet diameter. On the other hand, it is unclear whether inner gap would either increase or decrease. KAERI performed out-of pile re-sintering test for annual fuel pellets with four densities. Test result shows that both the outer and inner diameter of the annular fuel pellet decreased as a result of re-sintering. This implies that, while the pellet densification would lead to the outer gap increase, it would reduce the inner gap at the early stage of irradiation. Therefore, in order to investigate the in reactor behavior of the annular UO2 pellet, the irradiation test was carried out in the HANARO and post irradiation test was carried out in KAERI's PIE facility