Thermal conductivities of hypostoichiometric (U, Pu, Am)O2−x oxide

The thermal conductivities of (U_0.68Pu_0.30Am_0.02)O_2.00−x solid solutions (x = 0.00-0.08) were studied at temperatures from 900 to 1773 K. The thermal conductivities were obtained from the thermal diffusivities measured by the laser flash method. The thermal conductivities obtained experimentally up to about 1400 K could be expressed by a classical phonon transport model, λ = (A + BT)⁻¹, A(x) = 3.31 × x + 9.92 × 10⁻³ (mK/W) and B(x) = (−6.68 × x + 2.46) × 10⁻⁴ (m/W). The experimental A values showed a good agreement with theoretical predictions, but the experimental B values showed not so good agreement with the theoretical ones in the low O/M ratio region. From the comparison of A and B values obtained in this study with the ones of (U,Pu)O_2−x obtained by Duriez et al. [C. Duriez, J.P. Alessandri, T. Gervais, Y. Philipponneau, J. Nucl. Mater. 277 (2000) 143], the addition of Am into (U, Pu)O_2−x gave no significant effect on the O/M dependency of A and B values.     

Thermal transport properties of hafnium hydrides and deuterides

The thermal conductivities of δ′-, δ-, δ+ε-, and ε-phase hafnium hydrides and deuterides with various hydrogen isotope concentrations (HfH_x, 1.48 ? x ? 2.03; HfD_x, 1.55 ? x ? 1.94) were evaluated within the temperature range of 290-570 K from the measured thermal diffusivity, calculated specific heat, and density. The thermal conductivities of δ′-, δ-, δ+ε-, and ε-phase HfH_x and HfD_x are independent of the temperature within the range 300-550 K and are in the range 0.15-0.22 W/cm K and 0.17-0.23 W/cm K, respectively; these values are similar to and lower than the observed thermal conductivities of α-phase Hf. The experimental results for the electrical resistivities of δ′-, δ-, δ+ε-, and ε-phase HfH_x and HfD_x and the Lorenz number corresponding to the electronic conduction, obtained from the Wiedemann-Franz rule, indicated that heat conduction due to electron migration significantly influences the thermal conductivity values at high temperatures. On the other hand, heat conduction due to phonon migration significantly affects the isotope effects on the thermal transport properties.     

First-principles theory for helium and xenon diffusion in uranium dioxide

The diffusion properties of He and Xe in UO₂ have been investigated, using density-functional calculations employing the projector-augmented-wave (PAW) method and the generalized gradient approximation (GGA). The migration energies corresponding to both interstitial and vacancy-assisted mechanisms have been calculated and the results for the two noble gas atoms are compared with each other. We suggest that He likely diffuses by hopping through a single vacancy with computed low migration energies smaller than 0.79 eV and its diffusivity is much higher than that of Xe. Xe has a quite large migration energy compared to He; the strain energy plays a key role in Xe diffusion in UO₂.     

Caesium isothermal migration behaviour in sintered titanium nitride: New data and comparison with previous results on iodine and xenon

Titanium nitride has been proposed as a fission product barrier in fuel structures for gas cooled fast reactor (GFR) systems. The thermal migration of Cs was studied by implanting 800 keV ¹³³Cs⁺⁺ ions into sintered samples of TiN at an ion fluence of 5 × 10¹⁵ cm⁻². Thermal treatments at temperatures ranging from 1500 to 1650 °C were performed under a secondary vacuum. Concentration profiles were determined by 2.5 MeV ⁴He⁺ elastic backscattering. The results reveal that the global mobility of caesium in the host matrix is low compared to xenon and iodine implanted in the same conditions. Nevertheless, the evolution of caesium depth profile during thermal treatment presents similarities with that of xenon. Both species are homogeneously transported towards the surface and the transport rate increases with the temperature. In comparison, iodine exhibits singular migration behaviour. Several assumptions are proposed to explain the better retention of caesium in comparison with both other species. The potential role played by the oxidation is underlined since even a slight modification of the surface stoichiometry may modify species mobility. More generally, the apparition of square-like shapes on the surface of the samples after implantations and thermal treatments is discussed.     

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.     

Atomistic modeling of the vibrational and thermodynamic properties of uranium dioxide, UO2

Modeling the thermal properties of uranium oxide is of immense interest to the nuclear industry. UO₂ belongs to the family of superionic conductors whose solid-state diffusion coefficients at high temperatures are comparable to that of liquids. We report lattice dynamics and molecular dynamics studies carried out on oxide UO₂ in its normal as well as superionic phase. Lattice dynamics calculations have been carried out using shell model in the quasiharmonic approximation. The calculated equilibrium structure, elastic constants, bulk modulus, phonon frequencies and specific heat are in excellent agreement with the reported experimental data. Pressure variation of the phonon dispersion and equation of state have also been predicted. Molecular dynamics simulations have been carried out to study the diffusion behavior and the thermodynamic properties in UO₂. The diffusion constant of O in UO₂ has been determined. The pair correlation functions, O-U-O bond angle and thermal amplitude of vibration for the oxygen atom provide a microscopic picture of the local structure thereby throwing light on the gradual increase in the disorder of the oxygen sub-lattice which is a signature of superionic transition. The calculated transition temperature of UO₂ is 2300 K, which compares well with experimental value of about 2600 K.     

Helium solubility and behaviour in uranium dioxide

A set of devices was developed in order to infuse UO₂ disks with helium, at high temperature and pressure, to measure the helium infused quantity and from these data to calculate the helium solubility in the UO₂ matrix. Samples of UO₂ single crystal and UO₂ polycrystal were infused at a temperature of 1473 and 1743 K in a helium atmosphere ranging between 50 and 100 MPa. These samples were then annealed and the helium released was measured with a mass spectrometer. From the obtained spectra it was possible to give an interpretation of the helium release mechanism and to calculate its solubility in the UO₂ lattice in these specific thermodynamic conditions. Additionally to the helium solubility measurement from infused samples, a 37 years old sample of ²³⁸PuO₂, retrieved from an old ²⁴²Cm radioisotope thermoelectric generator (RTG), containing radiogenic helium, was also measured to widen perspectives of this kind of measurements to damaged sample more representative of spent fuel.     

Prediction of point-defect migration energy barriers in alloys using artificial intelligence for atomistic kinetic Monte Carlo applications

We significantly improved a previously proposed method to take into account chemical and also relaxation effects on point-defect migration energy barriers, as predicted by an interatomic potential, in a rigid lattice atomistic kinetic Monte Carlo simulation. Examples of energy barriers are rigorously calculated, including chemical and relaxation effects, as functions of the local atomic configuration, using a nudged elastic bands technique. These examples are then used to train an artificial neural network that provides the barriers on-demand during the simulation for each configuration encountered by the migrating defect. Thanks to a newly developed training method, the configuration can include a large number of neighbour shells, thereby properly including also strain effects. Satisfactory results have been obtained when the configuration includes different chemical species only. The problems encountered in the extension of the method to configurations including any number of point-defects are stated and solutions to tackle them are sketched.     

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.     

Potential mechanisms involved in resistant phenotype of MCF-7 breast carcinoma cells to ionizing radiation induced apoptosis

In the present study, we investigated the mechanisms of apoptosis resistance and the roles of the phosphorylation of BRCA1, p21, the Bax/Bcl-2 protein ratio and cell cycle arrest in IR-induced apoptosis in MCF-7 cells. X-irradiation, in particular at low dose (1 Gy), but not carbon ion irradiation, had a significant antiproliferative effect on the growth of MCF-7 cells. 1 Gy X-irradiation resulted in G1 and G2 phase arrest, but 4 Gy induced a significant G1 block. In contrast, carbon ion irradiation resulted in a significant accumulation in the G2 phase. Concomitant with the phosphorylation of H2AX induced by DNA damage, carbon ion irradiation resulted in an approximately 1.9-2.8-fold increase in the phosphorylation of BRCA1 on serine residue 1524, significantly greater than that detected for X-irradiation. Carbon ion irradiation caused a dramatic increase in p21 expression and drastic decrease in Bax expression compared with X-irradiation. The data implicated that phosphorylation of BRCA1 on serine residue 1524 might, at least partially, induce p21 expression but repress Bax expression. Together, our results suggested that the phosphorylation of BRCA1 at Ser-1524 might contribute to the G2 phase arrest and might be an upstream signal involved in preventing apoptosis signal via upregulation of p21 and downregulation of the Bax/Bcl-2 ratio.     

On surface elementary processes and polymer surface modifications induced by double pulsed dielectric barrier discharge

In this paper, polyethylene terephthalate and polyethylene films were plasma treated using a dielectric barrier discharge (DBD), which presents a second current pulse with inverse polarity induced at the decreasing applied voltage flank in addition to the main current pulse. The surface modifications of polymer samples were pointed out by two complementary methods: the contact angle method and atomic force microscopy (AFM) technique. The influence of the voltage pulse parameters as width, falling rate and frequency on the secondary discharge formation was studied and correlated with polymer surface modifications. The plasma treatment is improved when the discharge is driven by a voltage pulses with very fast falling flank. After plasma treatment, roughness of the PE films is not considerably changed; only topographical modifications of PE treated samples being observed, while treated PET surfaces present an increased roughness and a new granular structure, with much smaller features compared to the pristine ones.     

First-principles modeling of He-clusters in UO2

We have investigated the behavior of He in UO₂, using the projector-augmented-wave (PAW) method and the generalized gradient approximation (GGA) based on the density functional theory. Total energy calculations with atomic relaxation included have been performed in a 96-atom large supercell. We have found that He has a strong tendency to form a cluster in vicinity of an octahedral interstitial site (OIS) in the UO₂ matrix. In addition, the strain energy produced by a He-cluster was found to be sufficient to create point defects of the host atoms in UO₂. Our study suggests that He-clusters and He-induced point defects play an important role for the local mechanical properties of UO₂.     

Atomic diffusion mechanism of Xe in UO2

We have investigated vacancy-assisted diffusion of Xe in uranium dioxide (UO₂) calculating incorporation, binding, and migration energies. All the energy values have been obtained using the density functional theory (DFT) within the generalized gradient approximation (GGA) and the projector-augmented-wave (PAW) method. Considering spin-polarization effect, we find that the computed migration energy is reduced by and agrees well with experimental data compared to those obtained from non-magnetic calculations. We also find that an oxygen vacancy lowers the migration energy of a uranium vacancy by about 1 eV, enhancing an effective movement of vacancy clusters consisting of both uranium and oxygen vacancies. Furthermore, the strain energy of Xe is large enough to contribute to the clustering of vacancies making it the driving force for the vacancy-assisted diffusion of Xe in UO₂. In summary all the calculated results suggest that the trivacancy is a major diffusion pathway of Xe in UO₂.     

Microstructure characterization of oxidation of aluminized coating prepared by a combined process

Alumina layer is a good candidate for the tritium penetration barrier that is important in the control of tritium losses due to permeation through structural materials used in high-temperature gas-cooled reactors and in fusion reactors. This paper describes the microstructure of the oxide film of the tritium penetration barrier formed on 316L stainless steel, which was prepared by a combined process, namely, aluminizing and oxidizing treatments using a double glow plasma technology. Microstructure and phase structure of the coatings investigated were examined by scanning electronic microscope (SEM), X-ray diffraction analysis (XRD) and transmission electron microscopy (TEM), respectively. The chemical composition and the chemical states of Al, O elements in the oxidation film were identified by X-ray photoelectron spectroscopy (XPS). After aluminization, the typical microstructure of the coating mainly consisted of an outer high aluminum-containing intermetallic compound layer (Fe₂Al₅ and FeAl) and intermediate ferritic stainless steel (α Fe(Al))layer followed by the austenitic substrate. After the combined process, an oxide layer that consisted of Al₂O₃ and spinel FeAl₂O₄ had been successfully formed on the aluminizing coating surface, with an amorphous outmost surface and an underlying subsurface nanocrystalline structure.     

Behaviour of helium after implantation in molybdenum

This study deals with the behaviour of helium in a molybdenum liner dedicated to the retention of fission products. More precisely this work contributes to evaluate the release of implanted helium when the gas has precipitated into nanometric bubbles close to the free surface. A simple model dedicated to calculate the helium release in such a condition is presented. The specificity of this model lays on the assumption that the gas is in equilibrium with a simple distribution of growing bubbles. This effort is encouraging since the calculated helium release fits an experimental dataset with a set of parameters in good agreement with the literature.     

Ab initio approach to the effect of Fe on the diffusion in hcp Zr II: The energy barriers

Using density functional theory calculations together with the Monomer method for the search of saddle points (combined for the first time with an ab initio algorithm), we obtain the vacancy formation energies and the migration barriers for α-Zr self-diffusion and for the diffusion of interstitial impurities, including the ultra-fast diffuser Fe. Good agreement with measured diffusion coefficients is obtained, as a much lower energy barrier for the ultra-fast diffuser is found. We also suggest a possible mechanism for the increase in self-diffusion due to the Fe impurity, always present in the experimental samples.