l Yield behavior of porous nuclear fuel (UO2)

Uranium dioxide (UO2) is one of the most common nuclear fuels. During burn-up, the fuel undergoes substantial microstructural changes including the formation of pressurized pores, thus becoming a porous material. These pores reduce the elastic modulus and alter the yield behavior of the material. In this work, a finite-element-based homogenization technique has been used to map the yield surface of UO2 with pressurized pores. Two scenarios are considered; in the first, the fuel matrix is a ductile material with a Von-mises type behavior, while in the second, the matrix is quasi brittle, which is simulated using the concrete damaged plasticity (CDP) model available in ABAQUS. For both of the scenarios, it is found that the yield strength decreases with an increase in porosity for a given internal pore pressure. For a given porosity, the yield surface shifts towards the negative hydrostatic axis in the Haigh-Westergard stress space with an increase in pore pressure. When the matrix is quasi brittle, the decrease in tensile hydrostatic strength is less than the increase in compressive hydrostatic strength, whereas in the case of a ductile matrix, the changes in the hydrostatic strengths are same. Furthermore, the shape of the yield surface changes from one deviatoric plane to another in both scenarios. Analytical equations, which are functions of pore pressure and porosity, are developed to describe the yield surface of porous UO2 while accounting for the changes in shape of the yield surface from one deviatoric plane to another. These yield functions can be used to predict the failure of porous UO2 fuel.