NuFuel & MMSNF 2015

First Workshop on Research into Nuclear Fuel in Europe
and Materials Modeling and Simulation for Nuclear Fuels Workshop
Karlsruhe, Germany, November 16th to 18th, 2015

Updated: Tue 08 Dec 2015, 14:27

Poster 5.6: Atomistic modeling of Xe adsorption on UO2 surfaces

Jack Arayro1, Fabienne Ribeiro1 and Guy Tr├ęglia2
  • 1: Institut de Radioprotection et de Sûreté Nucléaire/PSN-RES/SEMIA/LPTM, France
  • 2: Centre Interdisciplinaire de Nanoscience de Marseille, CNRS, France

Abstract

Uranium dioxide UO2 is used as a standard fuel in pressurized water reactors (PWR). During fission reactions in uranium intragranular bubbles of xenon are generated. The presence of these bubbles modifies the thermomechanical properties of the fuel. The need to characterize these effects led to an extensive work both from experimental and theoretical points of view.

It is known from the literature that these bubbles are microfaceted, with (111) and (100) surfaces. We then study here simplified systems of xenon on semi infinite UO2 surfaces. In a first step, we assess the relative stability of UO2 surfaces according to their orientation and then to their polarity, by mixing thermostatistical relaxation and analytic formulations within a simple electrostatic model. The main result is that, whereas the (111) surface appears stable by construction and does not involved major reorganization, the polar (100) one is only stabilized through drastic rearrangement of the surface region.

In a second step, we proceed to xenon adsorption on these relaxed surfaces through Monte Carlo simulations in the grand canonical ensemble within semi-empirical potentials. The most striking feature revealed by the simulation is the existence of a phase transition from a dilute xenon phase towards a dense one. Structure analysis of the dense phase indicates a coexistence of the FCC, BCC and HCP structures. Otherwise, the xenon density is found to increase with the temperature for a given chemical potential.

In a third step, the pressure inside the xenon bubble and in the UO2 matrix has been investigated. In the former case, we show that whatever the xenon structure may be, the pressure increases with the xenon density, but not with the temperature for a fixed density. Concerning the UO2 matrix, we present pressure profiles before and after xenon adsorption. The next step will be to introduce these results in a micromechanical model, which will allow to derive a thermomechanical behavior law for the porous UO2.