Molecular dynamics calculations of heat conduction in actinide oxides under thermal gradient

Thermal conductivities of UO₂, PuO₂ and (U_0.8,Pu_0.2)O₂ have been investigated by non-equilibrium molecular dynamics (NEMD) simulation between 300 K and 2000 K. The thermal conductivity was directly calculated by the temperature gradient on the system according to Fourier's law in NEMD simulation. The thermal conductivity obtained from the NEMD simulation decreases with a decrease of the supercell size, which means the phonon scattering occurs at the system boundaries in the microsystem. In addition, the present NEMD simulation, as well as previous EMD simulation studies, clearly shows that the Umklapp process causes the decrease of thermal conductivity at high temperatures. When comparison is made with literature data, the calculated results obtained from the relatively small supercell are in good agreement with the measured ones for the above actinide dioxides.     

A classical molecular dynamics study of the correlation between the Bredig transition and thermal conductivity of stoichiometric uranium dioxide

Thermophysical properties of uranium dioxide are investigated by classical molecular dynamics for temperatures from 300 K to 3000 K. An increase of specific heat in the temperature range from 1300 K to 2500 K is noted. Comparison with a theoretical model shows that the origin of this behavior is only due to anharmonicity. Such characteristic features of the Bredig transition as the peak in specific heat and high ionic conductivity are investigated. We show that one more important feature was left unnoticed: the rise in the lattice contribution to thermal conductivity at high temperatures. An explanation is provided for this effect which is specific to superionic conductors. Reasonable agreement with experimental data up to 3000 K is obtained for thermal conductivity, even in the absence of electronic excitations.     

Effect of the cascade energy on defect production in uranium dioxide

The primary damage induced by a displacement cascade in a pure uranium dioxide matrix was investigated using classical molecular dynamics simulations. Cascades were initiated by accelerating a uranium primary knock-on atom (PKA) to a kinetic energy ranging from 1 keV to 80 keV inside a perfect UO₂ lattice at low temperature (300 K and 700 K). There is little effect of temperature in the temperature range studied. Following the cascade event, the damage level, defined as the total number of defects irrespective of whether they form clusters or not, is proportional to the initial kinetic energy of the PKA, in agreement with the literature relating to other materials. The linear dependence of damage upon initial PKA energy results from the formation of subcascades at high energy and constitutes a simple law which can be applied to any material and used in order to extrapolate molecular dynamics results to high energy PKAs. The nature of irradiation induced defects has also been studied as a function of the cascade energy.     

A molecular dynamics study of radiation induced diffusion in uranium dioxide

The nuclear oxide fuels are submitted ‘in-pile’ to strong structural and chemical modifications due to the fissions and temperature. The diffusion of species is notably the result of a thermal activation and of radiation induced diffusion. This study proposes to estimate to what extent the radiation induced diffusion contributes to the diffusion of lattice atoms in UO₂. Irradiations are simulated using molecular dynamics simulation by displacement cascades induced by uranium primary knock-on atoms between 1 and 80 keV. As atoms are easier to displace when their vibration amplitude increases, the temperature range which have been investigated is 300-1400 K. Cascade overlaps were also simulated. The material is shown to melt at the end of cascades, yielding a reduced threshold energy displacement. The nuclear contribution to the radiation induced diffusion is compared to thermally activated diffusion under in-reactor and long-term storage conditions.     

Establishing the isotropy of displacement cascades in UO2 through molecular dynamics simulation

Simulation of displacement cascades is a valuable approach in furthering our understanding of how the physical properties of nuclear fuel evolve. Molecular dynamics simulations of displacement cascades in uranium dioxide have been performed at three different primary knock-on atom energies. Various properties of the cascade (such as the spatial extent and total number of defects) are monitored as the cascade progresses. Both the statistical variation of these properties and the dependence on the crystallographic direction of the primary knock-on atom are investigated in order to determine the isotropy of these events.     

二氧化铀热导率的气孔效应

本工作测量了气孔率为2.10,3.47,4.32,5.84,和8.67%的UO_2在573—2273K温度范围的热扩散率。算出了相应的热导率。基于UO_2的热传导模型,求得了热导率与温度、气孔率的关系式。与改进的Maxwell-Eucken气孔效应修正式对照,推算了气孔系数的表达式,还给出了UO_2的德拜温度。     

Measurements of the thermal conductivity of uranium dioxide at 670–1270 k

Thermal conductivity measurements have been made on porous stoichiometric and hyperstoichiometric uranium dioxide in the temperature range 670–1270 K using the laser-flash method. The pore volume fraction varied between 0.014 and 0.096 and the O/U ratios between 2.00 and 2.11. The variation of thermal conductivity ($\text{k}$) with pore volume fraction ($\text{P}$) at O/U ratios of 2.00 and 2.015 followed an equation of the form $\text{k = k}{}_0\text{(1 ? βP)}$, where $\text{k}{}_0$ is the thermal conductivity of fully-dense material, at all temperatures in the range covered. The value of β was independent of the temperature in both the stoichiometric and hyperstoichiometric material but had different values, 2.8 and 1.5 respectively, in the two materials. No explanation can at present be offered for this difference. The individual effects of three kinds of phonon scattering centres, viz. other phonons, impurities in the lattice and excess oxygen ions, have been measured. The average phonon scattering cross-section for excess oxygen ions has been evaluated as $\text{6.3 × 10}{}^{\mathrm{?19}}\text{m}{}^2$ The thermal resistivity ($\text{R}$) of material having 96% of the maximum theoretical density, has been shown to conform to an equation of the type $\text{R = A + BT}$ where both $\text{A}$ and $\text{B}$ are parameters which depend on the O/U ratio, and $\text{T}$ is the absolute temperature. On the basis of our observations we suggest that the thermal conductivity of uranium dioxide might well become independent of both temperature and O/U ratio at temperatures above about 850 K and at O/U ratios in excess of about 2.13.     

Thermal transport properties of uranium dioxide by molecular dynamics simulations

The thermal conductivities of single crystal and polycrystalline UO₂ are calculated using molecular dynamics simulations, with interatomic interactions described by two different potential models. For single crystals, the calculated thermal conductivities are found to be strongly dependent on the size of the simulation cell. However, a scaling analysis shows that the two models predict essentially identical values for the thermal conductivity for infinite system sizes. By contrast, simulations with the two potentials for identical fine polycrystalline structures yield estimated thermal conductivities that differ by a factor of two. We analyze the origin of this difference.     

On the relative importance of the electronic and radiative contributions to the thermal conductivity of uranium dioxide

The relative contributions of radiative and electronic heat transfer mechanisms to the thermal conductivity of uranium dioxide are considered. Uncertainties in the extrapolation of data to high temperatures are discussed, with particular reference to the thermal conductivity of the liquid phase.     

Electrical conductivity measurement and thermogravimetric study of pure and niobium-doped uranium dioxide

The electrical conductivity and nonstoichiometric composition of UO_2+x and (U_1?yNb_y)O_2+x (y = 0.01, 0.05 and 0.10) were measured in the range $\text{1282 ≦ T ≦ 1373}$ K and Pa by tie four inserted wires method and thermogravimetry, respectively. The electrical conductivity of (U_1?yNb_y)O_2+x plotted against the oxygen partial pressure indicated a minimum corresponding to the transition between $\text{n}$- and $\text{p-}\text{type}$ cone uction. The band-gap energy of (U_1?yNb_y)O_2+x was calculated to be $\text{(248 ± 12)}$ kJmol.^?1, independent of niobium content, which is nearly the same as that of UO_2+x. From the oxygen partial pressure dependences of both the electrical conductivity and the deviation $\text{x}$ of UO_2+x and (U_1?yNb_y)O_2+x, the defect structures in these oxides were discussed with the complex defect model consisting of oxygen vacancies and two kinds of interstitial oxygens.     

Thermal conductivity of non-stoichiometric americium oxide: A molecular dynamics study

The aim of this paper is to investigate the influence of multi-valency of americium in its oxide for the lowering of the thermal conductivity and the uncertainty in measurement. In the present study, thermal conductivity of non-stoichiometric americium oxide was evaluated up to 2000 K by the non-equilibrium molecular dynamics calculations using the Born-Mayer-Huggins interatomic potential with the partially ionic model. The oxygen-to-americium ratio (O/Am) was varied from 1.6 to 1.9, which corresponded to the variation of the ratio of Am³⁺/Am⁴⁺. So, we prepared potential parameters for both Am³⁺ and Am⁴⁺. The calculated thermal conductivity of non-stoichiometric americium oxide decreased with an increase of temperature, and the degree of the temperature dependence became smaller with a decrease of the O/Am ratio. This was mainly caused by the phonon-scattering due to oxygen vacancies induced with Am³⁺ ions. Comparing two supercells in which (1) short-range ordered Am³⁺ clusters were contained and (2) Am³⁺ ions were randomly distributed, the thermal conductivity of the former seemed to be somewhat larger than that of the latter.     

Effect of burn-up on the thermal conductivity of uranium dioxide up to 100.000 MWd t

The thermal diffusivity and specific heat of reactor-irradiated UO₂ fuel have been measured. Starting from end-of-life conditions at various burn-ups, measurements under thermal annealing cycles were performed in order to investigate the recovery of the thermal conductivity as a function of temperature. The separate effects of soluble fission products, of fission gas frozen in dynamical solution and of radiation damage were determined. In this context, particular emphasis was given to the behaviour of samples displaying the high burn-up rim structure. Recovery stages could be thoroughly investigated in samples that were irradiated at low burn-ups and/or at high irradiation temperatures. Other samples, in particular those exhibiting the characteristic rim structure, disintegrated at temperatures slightly higher than the irradiation temperature. Finally, from a database of several thousand measurements, an accurate formula for the in-pile thermal conductivity of UO₂ up to 100 GWd t⁻¹ was developed, taking into account all the relevant effects and structural changes induced by reactor burn-up.     

熔融物材料扩散系数的从头算分子动力学计算

为获得核反应堆严重事故后期反应堆压力容器（RPV）下腔室内熔融物微观组织的演化规律,需要对熔融物的材料物理性质进行研究。以熔融池中发生熔化过程的实际材料,包括燃料芯块UO₂、包壳管熔融后的U-Zr-O材料以及不锈钢构件熔融后的U-Fe-O材料为研究对象,采用基于第一性原理的从头算分子动力学模拟了熔融物材料高温液态下的原子扩散行为。研究结果表明,在高温液相中的U、Zr、Fe、O的原子扩散系数与原子质量呈负相关,且在相同温度下受组分的影响较小,仍保持相对稳定的比例关系。不同原子扩散系数的差异理论上会导致熔融池形成分层结构,因此,可对比上述3种材料在高温液态下各种原子的扩散系数,确定直接的量化关系,为在大尺度下进一步研究熔融物微观组织的演化奠定基础。     

The interaction of defects in titanium: A molecular dynamics study

Behaviors and properties of helium in titanium were explored by molecular dynamics (MD) simulation in this study. The influence of He number, vacancy number and He density (ratio of helium to vacancy) on the thermal stability of HenVm clusters (where n and m denote the number of He atoms and vacancies) were investigated. Meanwhile, interactions among He atoms, SIA atoms and vacancies were discussed. The results demonstrate that the binding energies of an interstitial helium atom primarily depend on He and vacancy numbers rather than the helium-to-vacancy ratio (n/m). It is different from the previous report of other researchers. The binding energies of an isolated vacancy and a self-interstitial titanium atom depend on both the number of helium atoms and the helium-to-vacancy ratio (n/m) of clusters. The thermal stability of clusters is decided by the competitive processes among thermal emissions of vacancy, SIA and helium atom.     

Recycling process of defective aged uranium dioxide pellets

The work was devoted to study the recycling process of the unirradiated defective uranium dioxide pellets stored during 10 years. The effect of the temperature on the kinetics of the oxidation and reduction of dioxide uranium was investigated, respectively, in the range 250 °C–600°C and 300 °C–800 °C. For the oxidation process, the kinetics is low below 350 °C, and is fast at higher temperatures. The same phenomenon is observed for the reduction process, where the rate accelerates at above 500 °C and the reduction completed in shorter time. Using a laser particle size analyzer and a Brunauer, Emmett, Teller (BET) analyzer, it was determined that the oxidation at 400 °C gives a triuranium octoxide powder with an adequate particle size (33 μm) and a specific surface area of 0.9 m²/g in reasonable time (210 min). Reducing the triuranium octoxide to uranium dioxide powder at 600 °C in pure hydrogen was completely achieved after only 16 minutes, without affecting its characteristics. To ameliorate the specific surface area, several oxidation–reduction cycles were performed on the obtained uranium dioxide powder. It is found that after five cycles, the specific surface area of uranium dioxide was improved to more than 2.5 m²/g, minimum value required to the powder sinterability.     

A molecular dynamics study of the clustering of implanted potassium in multiwalled carbon nanotubes

We have studied the low energy irradiation of carbon nanotubes (CNT) with K ions using classical molecular dynamics simulations with analytical potentials. The studied CNTs had diameters of about 0.5–1.2 nm and single or multiple walls. The average penetration depth and probabilities to introduce an impurity atom into CNT were studied with simulations on irradiating the CNT with single K ion. The number of potassium clusters, their average sizes and the damage produced into the CNT due to the irradiation were studied using multiple K ion irradiations. We found that the K ions are mobile in CNTs right after the implantation event and that they cluster together. For CNTs with 1–3 coaxial tubes, the highest ratio of K atoms in clusters per total number of K ions was obtained by using an irradiation energy of about 100 eV. Also the least damage per K ion was found to be produced into the CNT with this energy when those energies high enough for the ion to penetrate the outermost wall of the CNT were considered.