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.     

Thermoelectric power studies of ferromagnet U2ScB6C3

The thermoelectric power (TEP) of a ferromagnet U₂ScB₆C₃ (T_C = 61 K) has been measured in the temperature range 5-300 K. The TEP is positive over the whole measured temperature range and reaches a relatively large value at room temperature of 29 μV/K. Below 30 K and above 200 K the TEP follows a straight line S(T) ∼AT, with slope of 0.23 and 0.085 μV/K², respectively. The change in the slope can be explained by the electron-phonon interaction renormalization effects or spin-reorientation associated with a change in the electronic structure. Analysing the temperature dependence of the ratio [S(T)/T]/[S_300 K/300] and taking into account the specific heat data, we suggest that spin fluctuations are another important factor in determining the thermoelectric power behaviour of U₂ScB₆C₃.     

The heat capacity and enthalpy of condensed UO2: Critical review and assessment

Having established the role of the heat capacity, C_p(T), of condensed UO₂ in various FBR accident scenarios, e.g. HCDA and PAHR, and having noted the unsatisfactory state of present knowledge concerning this basic thermophysical property of the fuel, all existing enthalpy and heat capacity data are collated and assessed, and certain recommendations made. The conventional method of obtaining C_p(T) by analytical differentiation of some adopted fit to this enthalpy data is then critically examined. The attendant problems are illustrated both for solid UO₂, where the contribution to C_p(T) from the weak, sigmoidal, enthalpy structure (which is just discernible in the data of Hein and Flagella) is missed and for molten UO₂, where not even the direction of the trend of C_p(T) with T can be definitively established, resulting, upon extrapolation to 5000 K, in C_p values which can differ by as much as 60 J mol⁻¹K⁻¹.Some recent progress towards a more acceptable, “model-independent” approach, known as quasi-local linear regression (QLLR), is then reviewed and applied to enthalpy data of UO₂ on both sides of its melting point, T_m. In the case of solid UO₂, a pronounced heat capacity peak, extending over about 100 K and centred on 2610 K., is revealed, whose magnitude and location is very similar to that found in other fluorite structured materials near 0.8T_m wherein it indicates a (Bredig) transition to a state characterised by giant ionic conductivities.Whilst it is impossible to establish any definite T-dependence for the C_p(QLLR) values in molten UO₂, the tendency to slightly decrease appears to marginally outweigh the converse, in qualitative accord with the dependence advocated by Hoch and Vernardakis. In the post-transitional region T_t<T<T_m the opposite holds, as is necessary for consistency between the independently established T-dependences of the thermal conductivity and diffusivity, which requires that C_p(T) increases with T faster than the density decreases.Attention is then drawn to some interesting comparisons which exist between the behaviour of UO₂ and other (non-actinide) fluorites near their melting points, which suggest the existence, in UO₂ of a significant degree of (i) cation disorder in the post-transitional region, 0.8T_m<T <T_m and (ii) electronic disorder in the melt. The review concludes with an extended, retrospective overview of the present situation regarding the heat capacity of condensed UO₂, and identifies some specific experimental goals in connection with the current experimental programme of the Joint Research Centre, Karlsruhe, Fed. Rep. Germany.     

Chaleur specifique de 2 a 300 K du monocarbure de thorium

The specific heat of ThC has been measured in the range 1.5–300 K. From the results, we have calculated the thermodynamic functions at 298.15 K, the Debye and Einstein temperatures linked with the acoustic and optical branches, respec tively, of the lattice vibrations, and the coefficient (γ = 2.12 mJ/mol · K²) of electronic specific heat.     

Magnetic and related properties of PuPdSn

A new phase PuPdSn was prepared and studied by X-ray diffraction, magnetization and heat capacity measurements, performed in the temperature range 2-300 K and in magnetic fields up to 14 T. The crystal structure determined from single-crystal X-ray data is the hexagonal ZrNiAl-type [space group ] with lattice parameters: a = 7.5057 Å and c = 4.0853 Å. PuPdSn orders antiferromagnetically at T_N = 21 K. Moreover, another antiferromagnetic-like transition takes place at 9.6 K. Above T_N the susceptibility follows a modified Curie-Weiss law with μ_eff = 1.0 μ_B, Θ_p = −14 K and χ₀ = 2.1 × 10⁻⁴ emu/mol. The low-temperature linear specific heat coefficient is small (γ ∼ 8 mJ/mol K²) pointing to well localized 5f electrons.     

Investigation of high temperature phase stability, thermal properties and evaluation of metallurgical compatibility with 304L stainless steel, of indigenously developed ferroboron alternate shielding material for fast reactor applications

Towards the cause of serving economic power production through fast reactors, it is necessary to bring in functionally more efficient and innovative design options, which also includes exploration of cheaper material alternatives, wherever possible. In this regard, the feasibility of using a commercial grade ferroboron alloy as potential alternate shielding material in the outer subassemblies of future Indian fast reactors has been recently investigated from shielding physics point of view. The present study explores in detail the high temperature thermal stability and the metallurgical compatibility of Fe-15.4B-0.3C-0.89Si-0.17Al-0.006S-0.004P-0.003O (wt.%) alloy with SS 304L material. In addition, the high temperature specific heat and lattice thermal expansion characteristics of this alloy have also been investigated as a part of the present comprehensive characterisation program. The Fe-15 wt.%B alloy is constituted of principally of two boride phases, namely tetragonal Fe₂B and orthorhombic FeB phases, which in addition to boron also contains some amount of C and Si dissolved in solid solution form. This Fe-B alloy undergoes a series of phase transformation as a function of increasing temperature; the major ones among them are the dissolution of Fe₂B-lower boride in the matrix through a eutectic type reaction, which results in the formation of the first traces of liquid at 1500 K/1227 °C. This is then followed by the dissolution of the major FeB boride phase in liquid and the melting process is completed at 1723 K/1450 °C. In a similar manner, the thermal stability studies performed on combined Fe-B + 304L steel reaction couples revealed that a pronounced pre-melting or liquid phase formation occurs at a temperature of 1471 K/1198 °C, which is lower than the melting onset of both Fe-B and SS 304L. It is found that within the limits of experimental uncertainty, this pre-melting phenomenon occurred at the same fixed temperature of 1471 K/1198 °C, irrespective of the mass ratios of Fe-B and 304L steel. Further, it is also found that SS 304L is completely soluble in Fe-B alloy and the fused product upon solidification formed a mixture of complex intermetallic borides, such as (Fe,Cr)(B,C), (Fe,Cr)₂(B,C) and (Fe,Ni)₃B. In the temperature range 823-1073 K (550-800 °C), the SS 304L clad is found to interact strongly with the Fe-B alloy. The diffusion layer thickness or the attack layer depth (x) is found to vary with time (t) up to about 5000 h, according to the empirical rate law, x² = k(T)t. The temperature sensitivity of the rate constant, k(T) is found to obey the Arrhenius law, k(T) = k_o exp(−Q/RT), with Q = 57 kJ mol⁻¹, being the effective activation energy for the overall diffusional interaction of Fe-B and SS 304L. The room temperature specific heat capacity of Fe-B alloy is found to be 538 kJ kg⁻¹ K⁻¹. The C_P values measured over 300-1350 K, is found vary smoothly with temperature according to the expression, C_P/kJ kg⁻¹ K⁻¹ = 0.62094 + 0.00012T + 10685.81T⁻². The lattice thermal expansion of both FeB and Fe₂B phases are found to be anisotropic in that the c-axis expansion is found to be more than that along a and b axes. The room temperature volume thermal expansivity of FeB and Fe₂B phases are found to be of the order of 48 × 10⁻⁶ K⁻¹ and 28 × 10⁻⁶ K⁻¹, respectively. The thermal expansion of FeB is found to be more temperature sensitive than that of Fe₂B.     

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.     

An electrical conductivity and X-ray study of a high-temperature transition in U4O9

The high temperature transition in U₄O₉ has been studied by electrical conductivity measurements and X-ray diffraction. From the electrical conductivity measurements, a similar variation of log σ$\text{T}$ with reciprocal temperature to that in the transition range near room temperature is observed in the temperature range from about 300 to 800°C. Like the low temperature transition, a small lattice contraction is also observed in that temperature range by means of X-ray diffractometry, and the transition temperature increases from 530 to 620°C with increasing O/U ratio. After the transition the intensity of $\text{4a}{}_0$ superlattice reflections increases, but that of $\text{8a}{}_0$ superlattice reflections disappears. The mechanism of this high temperature transition is considered to be a second-order transition of the order—disorder type based on the configurational change of U⁴⁺ and U⁵⁺ with the shift of some portions of the lattice oxygen atoms from the lattice sites to the interstitial positions. The phase diagram of U₄O₉ is presented on the basis of the electrical conductivity and X-ray data.     

The optical properties of liquid plutonium at 632.8 nm

The optical properties and normal, spectral emissivity of liquid plutonium at 632.8 nm were measured over a temperature range of 2016–2189 K using rotating analyzer ellipsometry. The purity of the liquid was maintained in a containerless environment using electromagnetic levitation and heating. The material investigated contained 1 wt% Ga that was added during the casting process. The measured values of the optical property results are given as a function of temperature by ϵ_λ=5.38×10⁻⁵T+0.250, n_λ=−1.29×10⁻⁴T+3.82, and k_λ=−7.04×10⁻⁴T+5.77 over the investigated temperature range.     

Transient gasification in an NBG-18 coolant channel of a VHTR prismatic fuel element

This paper investigates the transient gasification of NBG-18 nuclear graphite with atmospheric air ingress in a 0.8-m long coolant channel of a prismatic Very High Temperature Reactor fuel element. Analysis varied the initial graphite and air inlet temperature, T_o, from 800 to 1100 K at air inlet Reynolds number, Re_in = 5, 10 and 20. The analysis employs a Generic Interface that couples a multi-species diffusion and flow model to readout tables of the CO and CO₂ production fluxes. These fluxes are functions of the graphite local surface temperature, oxygen partial pressure and graphite weight loss fraction and calculated using a chemical-reactions kinetics model for the gasification of nuclear graphite. The analysis accounts for the heats of formation of CO and CO₂ gases, the heat conduction in the graphite sleeve, and the change in the oxygen partial pressure in the bulk gas flow mixture along the channel. Transient calculations performed up to a weight loss fraction of 0.10 at the entrance of the channel, t₁₀. They include the local graphite surface temperature and composition of bulk gas flow, the local and total graphite weight losses and the local and total production rates of CO and CO₂ gases. The heat released in the exothermic production reactions of these gases increases the local graphite surface temperature, accelerating its gasification. At the end of the calculated gasification transient, t = t₁₀, the graphite weight loss is highest at the channel entrance and decreases rapidly with axial distance into the channel, to its lowest value where oxygen in the bulk gas flow is depleted. Increasing T_o decreases t₁₀ and the total graphite loss, while increasing Re_in decreases t₁₀ but increases graphite loss.     

Vaporization of solid lithium nitride

The vaporization of solid lithium nitride has been studied by a mass spectrometric Knudsen effusion method. The solid was found to vaporize congruently to Li(g), Li₂(g) and N₂(g), and partial pressures of them may be represented by the equations: logp_Li (Pa) = (11.487 ± 0.102) ? (9.670 ± 0.081) 10³/T, logp_Li2 (Pa) = (14.306 ± 0.209) ? (14.090 ± 0.166) 10³/T and logp_N2 (Pa) = (10.959 ± 0.157) ? (9.638 ± 0.124) 10³/T in the temperature range 739–859 K. No identification of LiN(g) was made. From the combination of the determined enthalpy of the reaction Li₃N(c) = 3Li(g) + 0.5N₂(g) with appropriate literature data, the enthalpy ΔH°_f298, the free energy ΔG°_f298 and the entropy ΔS°_f298 of formation for solid lithium nitride have been obtained to be (?171.3 ± 7.7) kJ/mol, (?135.4 ± 7.7) kJ/mol and (?120.4 ± 36.5) J/mol · K, respectively.     

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.     

Stability studies of Hg implanted YBa2Cu3O6+x

High quality YBa₂Cu₃O_6+x (YBCO) superconducting thin films were implanted with the radioactive ^197mHg (T_1/2=24 h) isotope to low fluences of 10¹³ atoms/cm² and 60 keV energy. The lattice location and stability of the implanted Hg were studied combining the Perturbed Angular Correlation (PAC) and Emission Channeling (EC) techniques. We show that Hg can be introduced into the YBCO lattice by ion implantation into a unique regular site. The EC data show that Hg is located on a highly symmetric site on the YBCO lattice, while the PAC data suggests that Hg occupies the Cu(1) site. Annealing studies were performed under vacuum and O₂ atmosphere and show that Hg starts to diffuse only above 653 K.     

Double differential neutron scattering cross sections of materials for ultra high temperature reactors

The five materials with the highest melting point are hafnium, tantalum, niobium and zirconium (ZrC) carbides and graphite (that sublimes). Graphite is the material of choice for very high temperature reactors (VHTR); ultra high temperature reactors (UHTR), like the thermal nuclear propulsion reactor NERVA use a dispersion of ZrC and UC in graphite as the material in the reactor core. Presently there are neither inelastic nor elastic double differential scattering data available that describe the thermalization process in ZrC. We therefore, calculated coherent elastic and incoherent inelastic cross sections for the ZrC crystal which has a face centered cubic (fcc) lattice. The phonon spectrum for the ZrC lattice was calculated with the computer code PHONON using the Hellman–Feynman forces computed with ab-initio methods [Jochyn, P.T., Parlinski, K., 2000. Ab initio lattice dynamics and elastic constants of ZrC. Eur. Phys. J. B 15, 265–268]. This phonon spectrum was then used to compute the S(α, β, T) matrices for the inelastic scattering cross sections for C and Zr in the ZrC lattice using modified versions of the computer codes GASKET, HEXSCAT and NJOY. The results were applied to calculate, with the proper S(α, β, T), criticality and reactivity coefficients of temperate of reactor systems containing ZrC and UC. For comparisons, these parameters were also calculated with approximations of S(α, β, T), i.e. the gas or the graphite scattering kernels. Depending on the degree of thermalization, k_eff is underestimated between 0.6% and 1%, and the values and the shape of the reactivity coefficients as a function of temperature change by substantial amounts.     

Uranium mononitride: Heat capacity and thermal conductivity from 298 to 1000 °K

The heat capacity and the thermal diffusivity of uranium mononitride were measured by a laser flash method at temperatures ranging from 298 to 1000 °K. The samples were arc-melted UN having nearly zero porosity and sintered UN having porosity of 10.1%. The heat capacity of UN was represented by C_p = 12.08 + 2.548 × 10 ^{− 3}T − {−1.252 × 10⁵T⁻² cal/mol · deg K (298–1000 °K)}. From the heat capacity data, entropy, enthalpy and the Gibbs energy function of UN were calculated. The thermal conductivities of arc-melted UN, calculated from the heat capacity and the thermal diffusivity data, at 350 and 1000 °K were 0.031 and 0.045 cal/ cm · sec · deg K, respectively. The results agreed reasonably well with those of Moore et al. obtained at lower temperatures.     

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.     

Vaporisation thermodynamics of caesium iodide and caesium chromate

Vaporisation studies over solid and liquid CsI have been carried out by Knudsen effusion (753 – 897 K) transpiration (862 – 1125 K) and boiling temperature (977 – 1430 K) methods. The Knudsen mass loss and transpiration data over CsI(s) are consistent with the presence of monomeric species upto 873 K and the vapour pressure values can be represented by the equation: log₁₀P(kPa) = (7.59 ± 0.12) ? 0.00113 T(K) ? (10301 ± 95)/T(K) ? 21088/T²(K) + 1.13 log₁₀T(K) Transpiration and boiling temperature data were used to calculate partial pressures of monomeric (p₁°) and dimeric (p₂°) species over CsI(1) and the values obtained can be represented by the following equations: log₁₀P₁°(kPa) = (19.67 ± 0.38) ? (9628 ± 344)T(K) ? 3.65 log₁₀T(K)log₁₀P₂°(kPa) = (28.42 ± 0.41) ? (9986 ± 415)T(K) ? 6.56 log₁₀T(K) Vapour pressure of Cs₂CrO₄(1) was measured in the temperature range 1225 to 1405 K by transpiration method and vapour pressure calculated assuming monomeric species can be represented by the equation: log₁₀p(kPa) = (8.32 ± 0.26) ? (13434 ± 344)T(K)     

Crystal structure and physical properties of PuPd5Al2

We report on the crystallographic aspects and the basic properties of the plutonium based compound PuPd₅Al₂. This material is antiferromagnetic at T_N = 5.6 K and does not present any hint of superconductivity down to 2 K. This material crystallizes in the ZrNi₂Al₅-type of structure with lattice parameters: a = 4.1302 Å and c = 14.8428 Å. The magnetization, heat capacity and electrical resistivity measurements indicate clearly antiferromagnetic order at T_N = 5.6 K. This material is compared to the structurally related cerium based material CePd₅Al₂ presenting superconductivity induced by pressure.     

Measurement of transformation temperatures and specific heat capacity of tungsten added reduced activation ferritic-martensitic steel

The on-heating phase transformation temperatures up to the melting regime and the specific heat capacity of a reduced activation ferritic-martensitic steel (RAFM) with a nominal composition (wt%): 9Cr-0.09C-0.56Mn-0.23V-1W-0.063Ta-0.02N, have been measured using high temperature differential scanning calorimetry. The α-ferrite + carbides → γ-austenite transformation start and finish temperatures, namely Ac₁, and Ac₃, are found to be 1104 and 1144 K, respectively for a typical normalized and tempered microstructure. It is also observed that the martensite start (M_S) and finish (M_f) temperatures are sensitive to the austenitising conditions. Typical M_S and M_f values for the 1273 K normalized and 1033 K tempered samples are of the order 714 and 614 K, respectively. The heat capacity C_P of the RAFM steel has been measured in the temperature range 473-1273 K, for different normalized and tempered samples. In essence, it is found that the C_P of the fully martensitic microstructure is found to be lower than that of its tempered counterpart, and this difference begins to increase in an appreciable manner from about 800 K. The heat capacity of the normalized microstructure is found to vary from 480 to 500 J kg⁻¹ K⁻¹ at 500 K, where as that of the tempered steel is found to be higher by about, 150 J kg⁻¹ K⁻¹.     

Ga NMR and magnetic susceptibility in δ-phase of Pu1−xGax (x = 0.05, x = 0.08) alloys

⁶⁹Ga nuclear magnetic resonance spectra, line shifts (⁶⁹K) and nuclear spin-lattice relaxation rate have been measured in the 20 years aged Pu_0.95Ga_0.05 and in fresh prepared Pu_0.92Ga_0.08 alloys, stabilized δ-phase, at magnetic field of 9.4 T in the temperature range (10-500) K. The line shift and are determined correspondingly by the static and fluctuating-in-time parts of the local magnetic field that originates in transferred hyperfine coupling the Ga nuclear spin with the nearest f-electron environment of more magnetic Pu.Temperature behavior of the resonance properties is found the same in fresh Pu_0.92Ga_0.08 and aged Pu_0.95Ga_0.05 alloy. The NMR results are in favor that δ-phase of Pu_1−xGa_x alloys represents at T > 200 K the Kondo lattice, in which the localized electronic spins fluctuate independently from each other without any macroscopic coherence. The coherent state like in heavy-fermion liquids emerges in Pu_0.95Ga_0.05 below T^∗ = 200 K. A little bit higher estimate of crossover temperature T^∗ = 250 K was founded for Pu_0.92Ga_0.08.