Thermophysical and anion diffusion properties of (U x ,Th1-x )O2.

Michael W D Cooper,Paul C M Fossati,Robin W Grimes,Samuel T Murphy,Michael J D Rushton

doi:10.1098/rspa.2014.0427

Michael W D Cooper, Paul C M Fossati + Show 3 more

Open Access

https://doi.org/10.1098/rspa.2014.0427

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Abstract

Using molecular dynamics, the thermophysical properties of the (Ux,Th1−x)O2 system have been investigated between 300 and 3600 K. The thermal dependence of lattice parameter, linear thermal expansion coefficient, enthalpy and specific heat at constant pressure is explained in terms of defect formation and diffusivity on the oxygen sublattice. Vegard's law is approximately observed for solid solution thermal expansion below 2000 K. Different deviations from Vegard's law above this temperature occur owing to the different temperatures at which the solid solutions undergo the superionic transition (2500–3300 K). Similarly, a spike in the specific heat, associated with the superionic transition, occurs at lower temperatures in solid solutions that have a high U content. Correspondingly, oxygen diffusivity is higher in pure UO2 than in pure ThO2. Furthermore, at temperatures below the superionic transition, oxygen mobility is notably higher in solid solutions than in the end members. Enhanced diffusivity is promoted by lower oxygen-defect enthalpies in (Ux,Th1−x)O2 solid solutions. Unlike in UO2 and ThO2, there is considerable variety of oxygen vacancy and oxygen interstitial sites in solid solutions generating a wide range of property values. Trends in the defect enthalpies are discussed in terms of composition and the lattice parameter of (Ux,Th1−x)O2.

Highlights

UO2 has been studied extensively as the main component of conventional nuclear fuel. It is blended with other actinide oxides, such as ThO2 [1] and PuO2 [2,3], forming mixed oxide (MOX) fuel
Using molecular dynamics (MD), the superionic transition in (Ux,Th1−x)O2 is investigated for compositions where x equals 0.00, 0.25, 0.50, 0.75 and 1.00
It is shown that reduced oxygen-defect enthalpies in the three solid solution compositions studied here contribute to enhanced oxygen diffusivity below the superionic transition

Summary

Introduction

UO2 has been studied extensively as the main component of conventional nuclear fuel. It is blended with other actinide oxides, such as ThO2 [1] and PuO2 [2,3], forming mixed oxide (MOX) fuel. The ability of an interatomic potential to accurately reproduce the thermophysical properties of UO2, such as lattice parameter, elastic constants, thermal conductivity and specific heat over a wide range of temperatures, has often been used as a key discriminator for the suitability of a potential set [17,18,19,20]. A significant improvement in the ability of empirical interatomic potentials to reproduce the bulk modulus over a large range of temperatures has been achieved This potential set employs the same description of oxygen–oxygen interactions throughout, enabling the simulation of actinide oxide solid solutions. It accurately reproduces the melting points of UO2 and ThO2 well, making it suitable for investigating (Ux,Th1−x)O2 solid solutions. The influence of solid solution composition on oxygen-defect formation, oxygen diffusivity and the superionic transition is reported

Methodology

Conclusions