Thermal conductivity and energetic recoils in UO₂using a many-body potential model

Abstract

Classical molecular dynamics simulations have been performed on uranium dioxide (UO2) employing a recently developed many-body potential model. Thermal conductivities are computed for a defect free UO2 lattice and a radiation-damaged, defect containing lattice at 300 K, 1000 K and 1500 K. Defects significantly degrade the thermal conductivity of UO2 as does the presence of amorphous UO2, which has a largely temperature independent thermal conductivity of ∼1.4 Wm(-1) K(-1). The model yields a pre-melting superionic transition temperature at 2600 K, very close to the experimental value and the mechanical melting temperature of 3600 K, slightly lower than those generated with other empirical potentials. The average threshold displacement energy was calculated to be 37 eV. Although the spatial extent of a 1 keV U cascade is very similar to those generated with other empirical potentials and the number of Frenkel pairs generated is close to that from the Basak potential, the vacancy and interstitial cluster distribution is different.

Modelling the thermal conductivity of (U Th1−)O2 and (U Pu1−)O2

“…For example, Qin et al [14] using the same many-body potential employed here predicted a strong reduction in thermal conductivity due to oxygen Frenkel disorder. Alternatively, Arima et al [10,11] and Ma et al [12] investigated the thermal conductivity of (U x Pu 1Àx )O 2Ày as a function of both 0 x 0.3 and 0 y 0.25, revealing a small degradation of conductivity due to the mixed cation lattice but a much stronger dependence on the oxygen to metal ratio.…”

“…For example when fitting Equation (17) to the degradation of UO 2 thermal conductivity due to oxygen Frenkel pair concentration (as derived previously by Qin et al [14]) the following parameter set was obtained: a ¼ 1.05 Â 10 À2 m K W À1 , b ¼ 1.95 Â 10 À4 m W À1 and c ¼ 13.00 Â 10 À2 m K W À1 (with r 2 ¼ 0.992). Although Equation (17) is suitable for describing a single defective lattice over a range of temperatures and defect concentrations, it is not suitable for describing the behaviour of a mixed oxide system over the full compositional range.…”

Superionic UO2: A model anharmonic crystalline material

“…The use of periodic boundary conditions easy achieves this end as a non-equilibrium state and our crystalline UO2 persists in a long-lived metastable state for temperatures up to about around 3600 K for our potential model. 37 This degree of superheating is typical for crystals with periodic boundary conditions. 82 For example, the ratio of the "mechanical stability temperature" to the equilibrium melting temperature Tm has been found to be near 1.22  0.09 for a wide range of metals (Ag, Al, Au, Cu, Mg, Mo, Ni, Ti, V, Zn, Zr) 82 , compared to the ratio near 1.18 obtained from our potential model.…”

Structural stability and fission product behaviour in U3Si

“…In comparison, uranium dioxide has a density of 10.96 g/cm 3 [6] with a maximum uranium loading of 9.66 g/cm 3 . Additionally, as the compounds exhibit metallic bonding, the thermal conductivities are significantly higher than UO 2 [7], especially after irradiation [8,9]. The UO 2 system degrades significantly because the defect population shortens the mean free path of phonons, which are the main transmitter of heat in ionic solids.…”