Pipe and grain boundary diffusion of He in UO₂

Molecular dynamics simulations have been conducted to study the effects of dislocations and grain boundaries on He diffusion in UO2. Calculations were carried out for the {100}, {110} and {111} h110i edge dislocations, the screw h110i dislocation and Σ5, Σ13, Σ19 and Σ25 tilt grain boundaries. He diffusivity as a function of distance from the dislocation core and grain boundaries was investigated for the temperature range 2300 - 3000 K. An enhancement in diffusivity was predicted within 20 Å of the dislocations or grain boundaries. Further investigation showed that He diffusion in the edge dislocations follows anisotropic behaviour along the dislocation core, suggesting that pipe diffusion occurs. An Arrhenius plot of He diffusivity against the inverse of temperature was also presented and the activation energy calculated for each structure, as a function of distance from the dislocation or grain boundar

Melting behavior of (Th,U)O2 and (Th,Pu)O2 mixed oxides

© 2016 Elsevier B.V.The melting behaviors of pure ThO2, UO2 and PuO2 as well as (Th,U)O2 and (Th,Pu)O2 mixed oxides (MOX) have been studied using molecular dynamics (MD) simulations. The MD calculated melting temperatures (MT) of ThO2, UO2 and PuO2 using two-phase simulations, lie between 3650-3675 K, 3050–3075 K and 2800–2825 K, respectively, which match well with experiments. Variation of enthalpy increments and density with temperature, for solid and liquid phases of ThO2, PuO2 as well as the ThO2 rich part of (Th,U)O2 and (Th,Pu)O2 MOX are also reported. The MD calculated MT of (Th,U)O2 and (Th,Pu)O2 MOX show good agreement with the ideal solidus line in the high thoria section of the phase diagram, and evidence for a minima is identified around 5 atom% of ThO2 in the phase diagram of (Th,Pu)O2 MOX

Using molecular dynamics to predict the solidus and liquidus of mixed oxides (Th,U)O2, (Th,Pu)O2 and (Pu,U)O2

Molecular dynamics (MD) was used to establish a mechanistic basis for the experimentally observed reduction in liquidus and solidus temperatures below the melting point of the end-members for the mixed oxides (Th, U)O2, (Th, Pu)O2 and (Pu, U)O2. This dip is found at additions of the oxide with higher melting point to the oxide with the lower melting point. There are many causes suggested for the dip; here the distribution of the cation Frenkel energy for the mixed oxides caused by the local environment is proposed as a contributor. Furthermore, a variant of the moving interface method which yields information on the position of the solidus and liquidus boundaries, is used to predict the phase diagrams of these systems

A molecular dynamics method to identify the liquidus and solidus in a binary phase diagram

A method is presented adopting the phase coexistence technique within molecular dynamics simulations to identify the liquidus and solidus of binary systems. The Compositional Moving Interface method is applied to the case study of the Cu–Ni system and compared against a thermodynamic end-point model where the input parameters are determined using the same MD potential. This is a simple and powerful method to predict the solidus and liquidus boundary of a binary phase diagram for mixed systems calculated from the dynamics of a simulation

Thermophysical properties of urania-zirconia (U,Zr)O<inf>2</inf> mixed oxides by molecular dynamics

Molecular dynamics simulations were used to investigate the thermophysical properties of (U,Zr)O2 between 300 K and 3500 K. For compositions with 25% UO2, which are in the cubic fluorite phase, is similar to that of UO2. A superionic transition is observed in cubic (U,Zr)O2 at temperatures between 1500 K and 3000 K, occurring at progressively lower temperatures with increasing ZrO2 content. The heat capacity of these mixed oxides increases from 80 J/mol.K up to 130 J/mol.K at temperatures relevant to accident conditions, possibly retarding temperature increase in fuels with a significant pellet-clad bonding layer

The predicted shapes of voids and Xe bubbles in UO2

Morphology is a fundamental attribute when investigating voids and bubbles in UO 2 . This study uses molecular dynamics and Monte Carlo simulations to predict the lowest energy shapes for voids and bub- bles in UO 2 . The energies of the { 100 } , { 110 } and { 111 } surfaces have been calculated and used to predict the equilibrium void shape from Wulff construction. This equilibrium shape is compared to low energy faceted voids exhibiting different relative proportions of each family of terminating surfaces. It is found that the equilibrium Wulff shape does not represent the lowest energy morphology for nm void sizes at temperatures between 30 0 K and 120 0 K. Furthermore, the lowest energy faceted voids are slightly more energetically favourable than spherical voids, and as Xe is added, and bubble pressure increases, the faceted morphology becomes even more favourable than the spherical shape

Fission gas in thoria

The fission gases Xe and Kr, formed during normal reactor operation, are known to degrade fuel performance, particularly at high burn-up. Using first-principles density functional theory together with a dispersion correction (DFT + D), in ThO₂ we calculate the energetics of neutral and charged point defects, the di-vacancy (DV), different neutral tri-vacancies (NTV), the charged tetravacancy (CTV) defect cluster geometries and their interaction with Xe and Kr. The most favourable incorporation point defect site for Xe or Kr in defective ThO₂ is the fully charged thorium vacancy. The lowest energy NTV in larger supercells of ThO₂ is NTV3, however, a single Xe atom is most stable when accommodated within a NTV1. The di-vacancy (DV) is a significantly less favoured incorporation site than the NTV1 but the CTV offers about the same incorporation energy. Incorporation of a second gas atom in a NTV is a high energy process and more unfavourable than accommodation within an existing Th vacancy. The bi-NTV (BNTV) cluster geometry studied will accommodate one or two gas atoms with low incorporation energies but the addition of a third gas atom incurs a high energy penalty. The tri-NTV cluster (TNTV) forms a larger space which accommodates three gas atoms but again there is a penalty to accommodate a fourth gas atom. By considering the energy to form the defect sites, solution energies were generated showing that in ThO₂−x the most favourable solution equilibrium site is the NTV1 while in ThO₂ it is the DV