Harper-Dorn and power law creep in uranium dioxide

Abstract

The creep behavior of uranium dioxide is reviewed based on the results of seven separate investigations. The data fall into two regimes: a power-law region at high stresses where the stress exponent, n, is about five and a Newtonian-viscous region at low stresses where the stress exponent is unity. It is shown that the data in the region where n = 1 cannot be explained by a Coble diffusional creep mechanism. This result negates the recent conclusion of Knorr, Cannon, and Coble who claim that “it is conclusively shown that Coble creep is the dominant mechanism” in the viscous region. It is demonstrated, rather, that Harper-Dorn creep describes the creep behavior of polycrystalline UO2 above 0.6 Tm and in single crystalline material in the region where n =1. Harper-Dorn creep in uranium dioxide is analyzed using an internal stress model developed to describe pure metal behavior. The value of the modulus-compensated internal stress, which is a measure of the random dislocation density, is shown to be higher for both polycrystalline and single crystalline uranium dioxide than it is for pure metals.