Stress-induced phase transformation in nanocrystalline UO2

Abstract

We report a stress-induced phase transfonnation in stoichiometric U02 from fluorite to a-Pb02 structure using molecular dynamics (MD) simulations and density functional theory (DFT) calculations. MD simulations, performed on nanocrystalline microstructure under constant-stress tensile loading conditions, reveal a heterogeneous nucleation of a Pb02 phase at the grain boundaries followed by the growth of this phase towards the interior of the grain. The DFT calculations confinn the existence of the a-Pb02 structure, showing that it is energetically favored under tensile loading conditions. Uranium dioxide (U02) is one of the most widely used materials for nuclear fuel because it is easy to fabricate and is highly stable under intense heat and radiation conditions in the reactor core. It has a high melting temperature (~3100 K) [1,2] but poor thermal conductivity. In the reactor, as heat is removed from the exterior regions of the U02 pellet via a coolant fluid, the temperature in the interior remains comparatively high, leading to large thermal gradients and a complex stress state across the fuel pellet.In addition, at high burnups and in presence of fission products, voids, and gas bubbles, the U02 matrix is likely to be under high stress. However, little is known about any stress-induced structural transformations U~. Experiments and simulations have confirmed that U0 2 , which is primarily found in the fluorite structure, transforms to the cotunnite structure under compression at 69 GPa [3]. However, to the best of our knowledge, there has been no examination of structural transformations under tension. In this work, we use two different computational techniques -molecular dynamics (MD) and density functional theory (DFT) -to study the structural changes of U0 2 under high stress tensile loading conditions. We find that U02 shows a stress-induced phase transformation from fluorite (space group 225) to the a-Pb02 structure (space group 60).The MD simulations were performed on idealized, nanocrystalline model structures of stoichiometric U0 2 • Under constant-stress tensile loading conditions of 4.5 GPa and at a temperature T 800 K, we observe the heterogeneous nucleation of U02 in the a-Pb02 structure at the grain boundaries (GBs) with the subsequent growth of this phase towards the grain interior. Newton's equation using a 5 th order predictor-corrector algorithm with a time step of M = 0.5xlO-15 s. In order to obtain thermally equilibrated GBs at the temperature of interest, the unrelaxed zero-temperature structure is gradually heated up to 3000 K, slowly cooled down in a step-wise fashion, and finally annealed at T 800 K for 7 ns. The temperature is maintained using a velocity-rescaling thermostat.In order to induce a tensile loading condition, a constant stress of 4.5 GPa is applied in the z-direction (see Fig. 2), which is normal to the columnar axis, by using the Parrinello-Rahman constant-stress technique [8]. Figure 1 illustrates the observed tensile the loading direction as a LUHvUUll During...

Crack tip plasticity in single crystal UO2: Atomistic simulations

“…Under compressive pressure, recent DFT calculations have shown that UO 2 can take several metastable phases [35,36]. While under tensile state, recent MD simulations using the same potential found that UO 2 transforms from Fluorite phase to Scrutinyite phase, and the results are validated by DFT calculations [17]. The Rutile phase of UO 2 has never been reported before, even though the coexistence of Rutile and Scrutinyite phases has been observed both in experiments and simulations on TiO 2 [37,38].…”

“…This potential was selected from the many available UO 2 empirical potentials because a recent comparison shows that the rigid ion type potentials with non-formal charges give better descriptions than others [15,16]. Furthermore, the Basak potential has successfully predicted the Fluorite to Scrutinyite phase transformation under tensile deformation, in reference to density-functional-theory (DFT) calculations [17,18]. The surface energies calculated using the Basak potential are 1.02 and 1.85 J/m 2 for the {1 1 1} and {1 1 0} surfaces respectively, in good agreement with previous MD simulations [19] using the Catlow potential [20].…”

“…To calculate the Coulombic energy, we adopted the Wolf summation [22,23] with a cutoff radius of 2a 0 and a damping constant of 1.5; these two parameters have been tested for UO 2 interfaces in previous MD simulations [17]. Here a 0 is the lattice constant of Fluorite phase UO 2 , and its value predicted by the Basak potential is 5.45 Å [14].…”

Anisotropic shock response of columnar nanocrystalline Cu

“…30 We first construct an idealized hexagonal columnar nanocrystalline microstructure unit with the Voronoi tessellation method. 7,8,31 It contains six grains of identical hexagonal shape and diameter within the three-dimensional ͑3D͒ periodic cell. We choose a grain diameter of about 13 nm for exploring the GB void nucleation under given loading conditions.…”

Microstructure design for fast oxygen conduction

“…A similar strain-induced phase transformation has been previously observed in fluoritebased nanocrystalline UO 2 providing some support to these recent observations. 121 On a broader view, the strain-induced phase transformations could be also expected in other materials, and future investigations are required to understand such structure-property relationships, and their effect on oxygen diffusion near interfaces.…”