Radiation-Induced Creep of UO2

In-reactor creep tests were performed on stoichiometric UO₂ helices enriched with 1.82%²³⁵U at fission rates of 2×10¹² to 1.5×10¹³ fissions/cm³-s and outer-fiber shear stresses of 2890 and 2090 psi. Depending on the test procedure, the initial creep transient either decelerated continuously or was of the sigmoidal type; this difference is rationalized in terms of the radiation-produced substructure. For constant substructure, however, strain rate was linearly proportional to fission rate. Strain recovery was also observed when the specimens were unloaded and then irradiated. Postirradiation creep tests conducted at zero fission rate showed that fission events are required for either creep or strain recovery to occur. Incremental-temperature-change experiments and normalized strain-rate-vs-temperature plots verified that the deformation was athermal at T ≤200°C. Postirradiation examination showed that 0.01% strain was recovered after a specimen was fractured and irradiated, but no measurable swelling, changes in density, grain size, or stoichiometry had occurred. No evidence of microfracture was observed on ceramographic sections. A model for radiation-induced creep is proposed in which Frank loops and dislocations absorb interstitials produced locally by the fission spike and move with the aid of the localized thermal energy of the spike.