In-reactor measurement of cladding strain: fuel density and relocation effects

Measurement of fuel element diameters while the fuel is operating at power, in-reactor, has provided evidence of in-reactor fuel densification and relocation. The design and operation of the in-reactor diameter measuring rig used for these measurements are described. Diameter measurements were obtained from two fresh Zircaloy-clad UO2 elements containing fuel of 10.64 and 10.82 Mg/m3 density, respectively, at linear power outputs up to 61 kW/m. Similar measurements were also obtained from a 10.64 Mg/m3 density element after low power irradiation at 26 kW/m to a burnup of 75 MW X h/kg uranium. Results indicate that higher starting fuel density and prior irradiation both reduce the amount and rate of in-reactor fuel densification observed. Diameter measurements following reactor shutdowns, particularly on the higher burnup element, also indicate that fuel relocation can overcome diameter decreases due to fuel densification and restore pellet-clad interaction