Helium Behavior in Oxide Nuclear Fuels: First Principles Modeling

TL;DR

This study uses first principles DFT calculations to investigate helium behavior in UO2, PuO2, and MOX nuclear fuels, revealing how helium incorporation energies vary with Pu concentration and the effects on material properties.

Contribution

It provides the first detailed DFT analysis of helium in MOX fuels, including the impact of Pu concentration and electron correlation effects on helium incorporation.

Findings

PuO2 remains semiconducting with He in octahedral sites

He incorporation energy increases with Pu concentration

UO2 requires lattice distortion for He incorporation

Abstract

UO2 and (U,Pu)O2 solid solutions (the so-called MOX) nowadays are used as commercial nuclear fuels in many countries. One of the safety issues during the storage of these fuels is related to their self-irradiation that produces and accumulates point defects and helium therein. We present density functional theory (DFT) calculations for UO2, PuO2 and MOX containing He atoms in octahedral interstitial positions. In particular, we calculated basic MOX properties and He incorporation energies as functions of Pu concentration within the spin-polarized, generalized gradient approximation (GGA) DFT calculations. We also included the on-site electron correlation corrections using the Hubbard model (in the framework of the so-called DFT+U approach). We found that PuO2 remains semiconducting with He in the octahedral position while UO2 requires a specific lattice distortion. Both materials reveal a positive energy for He incorporation, which, therefore, is an exothermic process. The He incorporation energy increases with the Pu concentration in the MOX fuel.