[SY-B2] Intragranular bubble impact on nuclear fuel thermomechanical properties
UO2 is used as a standard fuel in pressurized water reactors. During fission reactions bubbles of xenon are generated. The presence of these bubbles modifies the thermo-mechanical properties of the fuel. The need to characterize these effects led to an extensive work both from experimental and theoretical points of view. Our contribution belongs to the later type.
First, the variation of the thermomechanical properties of UO2 versus porosity is studied through atomistic simulations with semi-empirical potentials. A good agreement is found between the elastic properties calculated in the present simulations, and those coming from micro-mechanical modelling and experimental ones. Concerning thermal properties, an analytical model taking into account the nanoporosities is derived. This study emphasizes the importance of bubble surface effects of intragranular bubbles on the thermomechanical behavior of the matrix.
Second, to clarify this effect, we study simplified systems of xenon on UO2 surfaces. We first determine the surface relative stability according to their orientations and then to their polarities, by combining thermostatistical relaxation and analytic formulations within a simple electrostatic model. The main result is that, whereas the (111) surface appears stable with only minor reorganization, the polar (100) one is only stabilized through drastic rearrangement of the surface region. Xenon adsorption on these relaxed surfaces is then realized through Grand Canonical Monte Carlo simulation.
Finally, the pressure inside the Xenon and in the UO2 matrix is investigated. For the Xenon, we show that whatever its cristallographic structure, the pressure increases with the density, but not with the temperature for a fixed density. In the latter case, we present stress profiles through the UO2 matrix before and after xenon adsorption. The next step will be to introduce these results in a micromechanical model in order to derive a thermomechanical behavior law for the porous UO2.
First, the variation of the thermomechanical properties of UO2 versus porosity is studied through atomistic simulations with semi-empirical potentials. A good agreement is found between the elastic properties calculated in the present simulations, and those coming from micro-mechanical modelling and experimental ones. Concerning thermal properties, an analytical model taking into account the nanoporosities is derived. This study emphasizes the importance of bubble surface effects of intragranular bubbles on the thermomechanical behavior of the matrix.
Second, to clarify this effect, we study simplified systems of xenon on UO2 surfaces. We first determine the surface relative stability according to their orientations and then to their polarities, by combining thermostatistical relaxation and analytic formulations within a simple electrostatic model. The main result is that, whereas the (111) surface appears stable with only minor reorganization, the polar (100) one is only stabilized through drastic rearrangement of the surface region. Xenon adsorption on these relaxed surfaces is then realized through Grand Canonical Monte Carlo simulation.
Finally, the pressure inside the Xenon and in the UO2 matrix is investigated. For the Xenon, we show that whatever its cristallographic structure, the pressure increases with the density, but not with the temperature for a fixed density. In the latter case, we present stress profiles through the UO2 matrix before and after xenon adsorption. The next step will be to introduce these results in a micromechanical model in order to derive a thermomechanical behavior law for the porous UO2.