First-principles molecular dynamics study of high-temperature properties of thorium dioxide

第一原理分子動力学計算による二酸化トリウムの高温物性研究

町田 昌彦  ; 中村 博樹  

Machida, Masahiko; Nakamura, Hiroki

Thorium has been considered as an alternative nuclear fuel element to uranium. Thorium is well-known to be more abundant in nature than uranium. Furthermore, it is widely spread that thorium dioxide is chemically more stable than uranium dioxide. From an economical and secure point of view, thorium fuel can be a candidate fuel in next-generation nuclear reactors. However, the data of thermal properties of thorium dioxide are insufficient, in particular, in the high-temperature region near its melting point. In order to supplement such thermal data of the materials, classical molecular dynamics have played an important role and been applied also to thorium compounds. However, the thermal properties obtained by classical molecular dynamics often depend on the empirical parameters giving the atomic potential. Especially, it is known that the Bredig transition (rapid growth of heat capacity slightly below melting point) strongly depends on the choice of parameters of the atomic potential in the case of molecular dynamics of uranium dioxide. Thus, classical molecular dynamics is not so reliable for high-temperature behaviours, such as the Bredig transition. Therefore, we adopt first-principles molecular dynamics to avoid this parameter sensitivity. In the present paper, we evaluate thermal properties of thorium dioxide at high temperature using first-principles molecular dynamics. The calculated enthalpy agrees well with the observed data. We find that the obtained Bredig transition temperature coincides with the experimental values and that this transition is caused by the high mobility of oxygen by visualizing the oxygen motions. Consequently, we reveal that the first-principles molecular dynamics can provide reliable data of thermal properties of nuclear fuels even at high temperature.