Experimental characterization and modelling of UO2 behavior at high temperatures and high strain rates

This work presents an experimental characterization of uranium dioxide (UO2) in compression under Reactivity Initiated Accident (RIA) conditions. Pellet samples were tested at four temperatures (1100, 1350, 1550 and 1700°C) and at a strain rate varying over 4 decades (10-4-10-3-10-2-10-1/s). The experimental results show that the stress-strain curves cannot be fitted with a unique power law as it is the case at smaller strain rates (10-9-10-5/s). A strain-hardening also appears in most of the tests. The microstructural observations show a pronounced evolution of the porosity at the pellet center during the tests. A hyperbolic sine model which accounts for volume variations (pore compressibility) was therefore proposed to describe the behavior of UO2 on a large range of temperatures (1100-1700°C) and strain rates (10-9-10-1/s). The Finite Element simulations of the compression tests lead to results (maximum stress, axial and hoop strain distribution, porosity distribution) in good agreement with the measurements. The model was then assessed on a database of more than two hundred creep tests.