Thermodynamic properties of neptunium nitride: a first principles study

The thermal and mechanical properties of neptunium nitride (NpN) were investigated by first principles calculations. From the Helmholtz free energy equilibrium lattice constants, thermal expansion coefficients, bulk moduli and specific heat capacities were calculated for temperatures up to 1500 K. The electronic specific heat capacity was also calculated from the electronic density of states at the Fermi energy. The obtained specific heat capacity reproduced the experimental data well. It was thus clarified that the specific heat capacity of NpN consists of the lattice and electronic specific heat capacities and the contribution of the lattice dilatation to the specific heat capacity.     

U—10Zr合金的电阻率,热导率和相变

用四端引线法测量了Ｕ－１０Ｚｒ（即Ｕ０．７７Ｚｒ０．２３）合金在３００－１３０Ｋ温度范围的电阻，计算出了此温度范围的电阻率。由电阻率－温度由线得到４个相变温度，分别为８８１、９３７、９７１和９９０Ｋ。根据Ｗｉｅｄｅｍａｎｎ－Ｆｒａｎｚ定律计算了各温度的热导率值，它在８８０Ｋ以下的激光脉冲法得到的热导率值符合得很好。在８８０Ｋ以上的值和Ｌｅｉｂｏｗｉｔｚ等得到的Ｕ０．７５Ｚｒ０．２５的热导率值相一致     

Electrical resistivity of pure transuranium metals under pressure

The influence of electronic structure evolution upon pressure on the temperature dependence of the electrical resistivity of pure Np, Pu, Am, and Cm metals has been investigated within the coherent potential approximation (CPA) for the many-bands conductivity model. The densities of states calculated within the local density approximation accounting for the Hubbard U and spin-orbit coupling (LDA+U+SO method) for the cubic (bcc- or fcc-) structures with the volumes fitted to the relative volumes of real metal phases were used as a starting point for model investigations of the electrical resistivity. The obtained results were found in good agreement with a number of available experimental data. The origin of large magnitude of the resistivity of the actinides was discussed.     

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.     

Ion implantation in TiO2: damage production and recovery,lattice site location and electrical conductivity

Ions with different oxidation states were implanted in TiO₂ (rutile). The lattice disorder as well as the lattice site location of the implanted ions were determined using Rutherford Backscattering and Channeling (RBS-C) spectrometry. The production of disorder as a function of dose and temperature, and its recovery was studied in detail. Important results are the observation of dynamic recovery at 293 K and above, and one isothermal at 77 K and two thermal recovery stages between 170 and 210 K and between 260 and 293 K. The recovery at 77 K is proportional to ln t, indicating that the activation energy increases with decreasing disorder density. The results concerning the lattice site location of 14 ion species reveal that 13 ions occupy Ti lattice site. With increasing netcharge, the maximum soluble concentration decreases by the formation of impurity–defect complexes probably enforced by charge compensation. Directed displacements from the substitutional lattice site provide some hints on the structures of these complexes. The electrical conductivity of the implanted samples increased by about 12 orders of magnitude irrespective of the oxidation state of the implanted species. From the temperature and dose dependence of the electrical conductivity as well as from its similar behaviour for noble-gas ions and other species it is concluded, that the carrier transport occurs by single energy states excitation at low doses and by variable range hopping between localized states at high doses.     

Thermal Conductivity Measurement with Silica-Coated Hot Wire for Li4SiO4 Pebble Bed

The effective thermal conductivity of a Li₄SiO₄ pebble bed was measured by the hot wire method. The bare and silica-coated Nichrome heaters were used as the hot wires. At 975 K, effective thermal conductivity was not measured correctly by the bare hot wire. This is due to the fact that the electrical signal of a bare thermocouple is distorted due to the electrical conductivity of Li₄SiO₄. Using a silica-coated hot wire, effective thermal conductivity can be measured at temperatures ranging from room temperature to 975 K. The effect of the coating layer on the measured effective thermal conductivity was estimated to be small and corresponded to the experimental data. The hot wire method with silica coating can be applied to other ceramic breeder materials.     

Thermal conductivity and electrical resistivity of liquid Ag–In alloy

To understand and predict the progression of core meltdown accidents in nuclear power plants, it is important to understand the behavior of molten core materials. We focused on the melting behavior of Ag–In–Cd alloys used in the control rods of pressurized water reactors that are known to melt first when a severe accident occurs. To obtain fundamental knowledge about these alloys, we studied the thermal conductivity of Ag–In binary alloys in this study. We evaluated thermal conductivity using two approaches: evaluating from thermal diffusivity up to 873 K measured by the laser-flash method, and calculating based on the Wiedemann–Franz law using the electrical resistivity up to 1273 K measured by the four-probe method. The values of thermal conductivity of liquid Ag–In alloys obtained by these two methods agreed well except for pure indium. Although Ag is known as a material that has one of the highest thermal conductivities, the thermal conductivity of liquid Ag–In alloys is much lower than that of pure liquid Ag (177 W/mK at 1273 K), but almost the same or less than that of liquid In (59.2 W/mK at 1273 K) in all Ag_1?xIn_x (x = 0.2–0.8) alloys at all temperatures in this measurement.     

Heat capacity and electrical conductivity of non-stoichiometric U4O9-y from 300 to 1200 K

The heat capacity and the electrical conductivity of non-stoichiometric U₄O_9-y with various compositions were measured simultaneously by direct heating pulse calorimetry from 300 to 1200 K. As well as the heat capacity anomaly due to the α-β transition around 350 K, two small heat capacity anomalies due to the β-γ transition were observed around 1000 and 1100 K, which are superimposed on a monotonie increase in the heat capacity above 800 K, presumably due to the onset of the γ-U₄O_9-y-UO_2+x transition. The change in the slope of the electrical conductivity curve as also observed at the phase transitions. The excess entropy due to the overall transition from α-U₄O₉to UO_2+x was evaluated to be 5.95 J K^-1mol^-1, which is in agreement with the value calculated on the assumption that the excess entropy consists of the contribution of the electronic disordering of U⁴⁺ and U⁵⁺ ions and that of the atomic disordering of oxygen atoms.     

Thermal conductivity of BaPuO3 at temperatures from 300 to 1500 K

Polycrystalline specimens of barium plutonate, BaPuO₃, have been prepared by mixing the appropriate amounts of PuO₂ and BaCO₃ followed by reacting and sintering at 1600 K under the flowing gas atmosphere of dry-air. The sintered specimens had a single phase of orthorhombic perovskite structure and were crack-free. The Debye temperature of BaPuO₃ was determined from the sound velocity and lattice parameter measurements. The elastic moduli were also determined from the longitudinal and shear sound velocity. The thermal conductivity of BaPuO₃ was calculated from the measured density at room temperature, literature values of heat capacity, and thermal diffusivity measured by a laser flash method in vacuum. The thermal conductivity of BaPuO₃ was roughly independent of the temperature and was almost the same magnitude as that of BaUO₃. This was markedly lower than the conductivities of other perovskite type oxides and was about one-tenth that of UO₂ around room temperature. The temperature dependence of the thermal conductivity of BaPuO₃ was found to be quite similar to that of BaUO₃.     

On the thermal conductivity of UO2 nuclear fuel at a high burn-up of around 100 MWd/kgHM

A study of the thermal conductivity of a commercial PWR fuel with an average pellet burn-up of 102 MWd/kgHM is described. The thermal conductivity data reported were derived from the thermal diffusivity measured by the laser flash method. The factors determining the fuel thermal conductivity at high burn-up were elucidated by investigating the recovery that occurred during thermal annealing. It was found that the thermal conductivity in the outer region of the fuel was much higher than it would have been if the high burn-up structure were not present. The increase in thermal conductivity is a consequence of the removal of fission products and radiation defects from the fuel lattice during recrystallisation of the fuel grains (an integral part of the formation process of the high burn-up structure). The gas porosity in the high burn-up structure lowers the increase in thermal conductivity caused by recrystallisation.     

Thermal conductivity of near-stoichiometric (U, Ce)C and (U, Pu, Ce)C solid solutions

The thermal conductivities of near-stoichiometric (U, Ce)C and (U, Pu, Ce)C solid solutions containing CeC up to 10 mol% were determined in the temperature range from 740 to 1600 K by the laser flash method. The thermal conductivity decreased with the cerium content in the solid solutions. The electrical resistivities were also measured for the purpose of analyzing the heat conduction mechanism. It was found that the decrease of electronic heat conduction caused by the addition of cerium resulted in decreasing the thermal conductivities of (U, Ce)C and (U, Pu, Ce)C compared with UC and (U, Pu)C.     

Thermal spike model of defect production by electron excitation in Cu

The thermal spike model is investigated theoretically in order to clarify the mechanism of defect production by electron excitation in Cu [Iwase et al., J. Phys. Soc. Jpn 61 (1992) 3878]. The electron temperature and the lattice temperature are calculated numerically solving the heat-flow equation. It is clarified that the maximum lattice temperature is about 100 K when the electronic stopping power of the incident ion is 100 keV/nm and the initial lattice temperature is 10 K. This means that no defect is created by purely thermal processes induced by electron excitation in the case of Cu.     

Molecular Dynamics Studies of Americium-Containing Mixed Oxide Fuels

The molecular dynamics (MD) calculation was performed for americium-containing mixed oxide fuels, (U_0-7--x Pu_0.3Am _x )O₂ (x=0,0.016; 0.03; 0.05; 0.1; 0.15), in the temperature range from 300 to around 2,500 K to evaluate the lattice parameter, heat capacity and thermal conductivity. The MD results reveal that the calculated heat capacity and thermal conductivity are at a similar level in the entire composition range, in other words they are scarcely influenced by adding americium up to 15%. This behavior was examined from a view point of a phonon-impurity scattering mechanism.     

Physical properties of monolithic U8 wt.%-Mo

As a possible high density fuel for research reactors, monolithic U8 wt.%-Mo (“U8Mo”) was examined with regard to its structural, thermal and electric properties. X-ray diffraction by the Bragg-Brentano method was used to reveal the tetragonal lattice structure of rolled U8Mo. The specific heat capacity of cast U8Mo was determined by differential scanning calorimetry, its thermal diffusivity was measured by the laser flash method and its mass density by Archimedes’ principle. From these results, the thermal conductivity of U8Mo in the temperature range from 40 °C to 250 °C was calculated; in the measured temperature range, it is in good accordance with literature data for UMo with 8 and 9 wt.% Mo, is higher than for 10 wt.% Mo and lower than for 5 wt.% Mo. The electric conductivity of rolled and cast U8Mo was measured by a four-wire method and the electron based part of the thermal conductivity calculated by the Wiedemann-Frantz law. Rolled and cast U8Mo was irradiated at about 150 °C with 80 MeV ¹²⁷I ions to receive the same iodine ion density in the damage peak region as the fission product density in the fuel of a typical high flux reactor after the targeted nuclear burn-up. XRD analysis of irradiated U8Mo showed a change of the lattice parameters as well as the creation of UO₂ in the superficial sample regions; however, a phase change by irradiation was not observed. The determination of the electron based part of the thermal conductivity of the irradiated samples failed due to high measurement errors which are caused by the low thickness of the damage region in the ion irradiated samples.     

Neutron irradiation effects in uranium monosulfide

Uranium monosulfide (US) was irradiated to investigate the effects of fission damage. Post-irradiation examinations were done by measuring the electrical resistivity, and partly the magnetic properties, at low temperature. The lattice parameter and the electrical resistivity measured at room temperature just after the irradiations showed an increase starting at a fission dose of 1 × 10¹⁶ fissions/cm³ and attaining a maximum at 3 × 10¹⁶ fissions/cm³. After that, a saturation of both increases persiste until 3 × 10¹⁷ fissions/cm³. The low-temperature electrical resistivity in the magnetic ordered state (ferromagnetic transition, T_c, at about 180 K) increased remarkably, while decreasing drastically in the magnetization, with increasing fission dose, apparently corresponding to the lattice expansion. In addition, the Curie point (T_c) shifted to lower temperatures with accumulating fission damage.     

The thermal conductivity of hexagonal boron nitride parallel to the basal planes

Detailed measurements of the basal thermal conductivity and specific heat of boron nitride have recently been reported over the range 1.5–350 K. Boron nitride is a lattice conductor with a conductivity varying as T^2.4 below 10 K, while the lattice specific heat varies as T³ over the same range. This anomalous difference in the temperature dependence of lattice conductivity and specific heat is similar to that observed in graphite. The theory of the lattice conductivity of graphite parallel to the basal planes is applied to the explanation of this anomaly. It appears that the value of C₄₄ in this lattice must be smaller than that of graphite.     

Thermal conductivity of U3O8 from 300 to 1100 K

The thermal conductivity of orthorhombic α-U₃O₈ has been measured in air from 300 to 1100 K using an axial heat flow comparative set-up. The results show that the conductivity decreases monotonically with increasing temperature. The observed conductivity can be explained in terms of the phonon-defects and phonon–phonon interaction processes. It is also shown that the intrinsic thermal resistivity can be quantitatively explained by the modified Leibfried–Schlomann (LS) equation proposed by G.A. Slack (in: H. Ehrenreich, F. Seitz, D. Turnbull (Eds.), Solid State Physics, vol. 34, Academic Press, New York, 1979, p. 1).     

Thermophysical properties of americium-containing barium plutonate

Polycrystalline specimens of americium-containing barium plutonate have been prepared by mixing the appropriate amounts of (Pu_0.91Am_0.09)O₂ and BaCO₃ powders followed by reacting and sintering at 1600 K under the flowing gas atmosphere of dry-air. The sintered specimens had a single phase of orthorhombic perovskite structure and were crack-free. Elastic moduli were determined from longitudinal and shear sound velocities. Debye temperature was also determined from sound velocities and lattice parameter measurements. Thermal conductivity was calculated from measured density at room temperature, literature values of heat capacity and thermal diffusivity measured by laser flash method in vacuum. Thermal conductivity of americium-containing barium plutonate was roughly independent of temperature and registered almost the same magnitude as that of BaPuO₃ and BaUO₃.     

Isotopie Effect on Thermal Physical Properties of Isotopically Modified Boron Single Crystals

The measurement of specific heat and thermal conductivity at low temperature for isotopically modified boron single crystals was performed between 0.5 and 100 K using relaxation method and steady heat flow method, respectively. The results indicate that the specific heat has obvious divergences at T <5 K. At 40 K, the thermal conductivity of ¹⁰B- enriched crystal is about 570W/m-K, which is 40% larger than that of natural boron crystal. The influence of lattice vibration modes and the isotopic effect on specific heat and thermal conductivity for isotopically modified boron are discussed.     

Thermal conductivity of uo2 and u4o9 from 100 to 300 k

The thermal diffusivities of UO₂ and U₄O₉ were measured by the laser flash method at temperatures ranging from 100 to 300 K. The phonon mean free path and the thermal conductivity were calculated from the obtained thermal diffusivity data and the heat capacity. The structure of the u₄o₉ is closely related to the UO₂ structure with an excess oxygen atom per unit cell in U₄O₉. As the excess oxygen atoms increase the anharmonicity of the lattice vibration, the phonon mean free path in U₄O₉ decreases. Therefore, the thermal conductivity of U₄O₉ is much lower than that of UO₂ and increases slightly with increasing temperature due to the rise in heat capacity.