Thermophysical properties and oxygen transport in (Thx,Pu1−x)O2

Abstract

Using Molecular Dynamics, this paper investigates the thermophysical properties and oxygen transport of (Thx,Pu1−x)O2 (0 ≤ x ≤ 1) between 300–3500 K. In particular, the superionic transition is investigated and viewed via the thermal dependence of lattice parameter, linear thermal expansion coefficient, enthalpy and specific heat at constant pressure. Oxygen diffusivity and activation enthalpy are also investigated. Below the superionic temperature an increase of oxygen diffusivity for certain compositions of (Thx,Pu1−x)O2 compared to the pure end members is predicted. Oxygen defect formation enthalpies are also examined, as they underpin the superionic transition temperature and the increase in oxygen diffusivity. The increase in oxygen diffusivity for (Thx,Pu1−x)O2 is explained in terms of lower oxygen defect formation enthalpies for (Thx,Pu1−x)O2 than PuO2 and ThO2, while links are drawn between the superionic transition temperature and oxygen Frenkel disorder.

Oxygen diffusion in Gd-doped mixed oxides

“…The diffusivity predictions are now shown as a function of actinide composition (figure 8) and compared with previous data for undoped MOX systems (figure 9). It was predicted previously that enhanced oxygen diffusivity occurs for (U x Pu x−1 ), (U x Th x−1 ) and (Th x Pu x−1 ) at temperatures under the superionic transition [13] (i.e. diffusivity is greater than just a linear interpolation from the end members).…”

“…Turning now to the Gd doped MOX systems, each of the oxygen diffusivity graphs shown in figures 1,2 and 3 depict three regions of constant gradient, suggesting three different diffusion regimes. It has been shown previously [9,10,13] for the undoped (U…”

“…This is important as to describe oxygen transport properties the potential must describe the thermal expansion accurately [44]. Moreover, the CRG potential approach has been used previously to investigate oxygen transport in MOX [9,10,13,45].…”

“…There are~40 reactors worldwide licensed to use MOX [4]. Understanding how the compositions of these mixed oxides affect their thermophysical properties is of importance for safe and efficient reactor operation and a substantial body of experimental and modelling work is being carried out to investigate these properties [5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22].…”

“…Previous work has focused on the thermophysical properties and oxygen transport of (U, Pu)O 2 , (U, Th)O 2 and (Th, Pu)O 2 [9-14, 16, 20] and how they are changed as a consequence of mixed actinide composition. For UO 2 , PuO 2 , ThO 2 and MOX [9,10,13,18,19,34,35] there is a deviation from linear thermal expansion and a classical Debye description of the constant pressure specific heat above 1300 K. Similar effects were reported in urania-gadolinia where a dramatic increase in specific heat occurs at elevated temperatures [36]. Krishnan et al [36] suggest that the increase in heat capacity might be due "to predominant contribution of formation of Frenkel pairs of oxygen" resulting from the UO 2 being doped with aliovalent cations.…”