C.K. Mathews

Thermodynamic properties of ternary oxides of fission products from calorimetric measurements

Abstract The alkaline earth elements barium and strontium, which are produced in a mixed oxide fuel during irradiation, can combine with other major fission products such as zirconium, molybdenum and cerium under favourable conditions of temperature and oxygen potential. As a part of our programme to investigate the thermodynamic properties of these fission product compounds, we have already reported thermodynamic data on BaZrO 3 , SrZrO 3 , BaMoO 4 and BaCeO 3 . To complete the series, enthalpy increment measurements were carried out on SrCeO 3 , SrMoO 4 , CaZrO 3 and CaMoO 4 using a high temperature microcalorimeter. These data along with the derived thermodynamic functions are reported in this paper. The stabilities of the whole set of fission product compounds, under conditions of oxygen potentials and temperature prevalent in the mixed oxide fuel, are examined in the light of all the data mentioned above.

Thermochemistry of metal-rich chromium telluride and its role in fuel-clad chemical interactions

Abstract Vaporisation of Cr-Te alloys was studied by Knudsen-effusion mass spectrometry. The partial pressures of Te 2 (g) and Te(g) over the two-phase field, Cr + CrTe x , were determined in the temperature ranges 1015 to 1138 K and 1180 to 1285 K, respectively. The temperature dependencies of the partial pressures have indicated that nearly equimolar proportions of Te and Te 2 are present in the vapor phase and that there is a phase transformation in CrTe x at 1160 ± 20 K . The Cr-rich phase boundaries of the nonstoichiometric CrTe x were delineated at 1075 K (50.72 ± 0.7 at % Te ) as well as at 1235 K (48.25 ± 0.9 at% Te) by a continuous monitoring of the intensities of Te + and Te + 2 as a function of time, starting with samples having 55.63 and 57.22 at% Te. Enthalpies and Gibbs energy changes were derived for the equilibria. CrTe x (s) ai Cr(s) + ( x i )Te i (g) [ x = 1.029 and 0.932; i = 1 and 2] and Te 2 ( g ) ai 2 Te ( g ). Enthalpies and Gibbs energies of formation of CrTe 1.0.29 and CrTe 0.932 were arrived at. The tellurium potentials which would be required for the formation of MTe x ( M = Fe , Cr , and Ni ) in Type 316 stainless steel and those likely to exist in the fuel-cladding gap of a mixed-oxide fuel pin were computed.

Carbon potential and carbide equilibrium in 18/8 austenitic steels

Abstract Austenitic stainless steels are in contact with high temperature liquid sodium in a fast reactor. In order to understand the carbon transport occurring between various regions of these materials through liquid sodium, an accurate knowledge of the carbon activity-concentration relationship in the austenitic steels is essential. A new method involving equilibration of the steel sample with liquid sodium coupled with the measurement of carbon activity in sodium, employing an electrochemical carbon meter, is reported for the measurement of the carbon potential of the steel at different carbon concentrations in the temperature range of 860–960 K. The results obtained are discussed and a new expression is proposed relating carbon activity with the composition of steel. The thermodynamics of carbide equilibrium in the steel are studied by isolating and characterising the carbide phase. These data have been analysed using a regular solution model for the carbide phase, M 23 C 6 and based on this the observed composition of M 23 C 6 as well as the variation of the carbon potential of steel with temperature have been explained.

Phase diagram studies on the Na-Mo-O system

The phase diagram of the Na-Mo-O ternary system is of interest in interpreting the behaviour of structural materials in the sodium circuits of fast breeder reactors and sodium-filled heat pipes. Experiments involving heating of sodium oxide with molybdenum metal under vacuum, selective removal of oxygen from polymolybdates by reducing them under hydrogen and confirmation of the coexistence of various phase mixtures were conducted in the temperature range of 673 to 923 K. Phase fields involving molybdenum metal, dioxide of molybdenum and ternary compounds were derived from these results. The ternary phase diagram of the Na-Mo-O system was constructed and isothermal cross sections of the phase diagram are presented.

VAPORISATION THERMODYNAMICS OF THE NICKEL-RICH PHASES IN THE Ni-Te BINARY SYSTEM - A HIGH TEMPERATURE MASS SPECTROMETRIC STUDY

The vaporisation of the metal-rich nickel tellurides (βh2, gb1, and β1) was studied by Knudsen effusion mass spectrometry. The vapour phase was found to consist of Te2(g) and Te(g). Partial pressure-temperature relationships over the Ni + β2 phase field were determined to be log(PTe2Pa) = (−12563 ± 223)/T(K) + 10.945 ± 0.236 and log(PTePa) = (−13350 ± 425)/T(K) + 11.109 ± 0.450, in the temperature range 893–993 K. Enthalpy and Gibbs energy of formation of the β2 phase, at the nickel-rich boundary (NiTe0.634), at 298 K were derived as −33.3 ± 5.1 and −35.0 kJ mol−1, respectively. The temperature of phase transition from Ni+β2 to Ni+ǵb1 was confirmed to be 1009 ± 10 K. P−T equations over the Ni+gb1 phase field (1020−1055 K) were deduced as log(PTe2Pa) = (−10883 ± 461)/T(K) + 9.305 ± 0.444 and log(PTePa) = (−11189 ± 510)/T(K) + 8.949 ± 0.492. Those over the Ni + β1 phase field (1090–1190 K) were log(PTe2Pa) = (−11187 ± 344)/T(K) + 9.649 ± 0.301 and log(PTePa) = (−11930±400)/T(K) + 9.671±0.349. At the nickel-rich boundary (NiTe0.587), ΔfH° and ΔfG° for the gb1 phase were determined to be −27.7 ± 4.1 and −30.1 kJ mol−1 at 1038 K and those for the β1 phase as −30.5±3.2 and −30.7 kJ mol−1 at 1140 K.

Enthalpy of formation of UZr2 by calorimetry

Abstract The enthalpy of formation of the intermetallic compound UZr2 at 298 K was determined by high temperature solution calorimetry in which liquid aluminium was used as the solvent. The thermal effects of dissolution of U, Zr and UZr2 in liquid aluminium were measured in separate experiments by dropping the samples held at ambient temperature into liquid aluminium maintained at 991 K in the calorimeter. The thermal effects of dissolution of the samples in liquid aluminium at infinite dilution were derived from these measurements, and based on these data the enthalpy of formation of UZr2 at 298 K was computed as −4.0 kJ mol−1. The partial enthalpy of solution of zirconium in liquid aluminium at infinite dilution was also computed and the value thus obtained was −(257.7 ± 9.5) kJ mol−1.

Vaporization thermodynamics of (iron + tellurium): a high-temperature mass-spectrometric study

Abstract Vaporization of (iron + tellurium) alloys was studied using Knudsen-effusion mass spectrometry. The partial pressures of Te 2 (g) and Te 3 (g) over the two-phase fields {FeTe 0.939 (s) + FeTe 1.994 (s)} and {FeTe 0.939 (s) + FeTe 1.451 (s)} were determined in the temperature ranges 659 to 759 K and 803 to 818 K, respectively. The partial pressures over the two-phase field {FeTe 0.939 (s) + FeTe 1.427 (s)} were determined at 868 K. Enthalpy or Gibbs energy changes were derived from the partial pressures for the equilibria: FeTe x (s) = FeTe y (s) + { (x−y) i } Te i (g) , where x = 1.994 (e-phase), 1.451 (δ′-phase), and 1.427 (δ-phase), and y = 0.939 (β-phase) with i = 2 or 3. The phase boundaries of the β-phase at 868 K were delineated by a continuous monitoring of the intensity of Te 2 + as a function of time, starting with a sample having 72.9 mass per cent of Te. Activities of Te were determined and those of Fe computed as a function of composition of the β-phase, using Gibbs-Duhem integration. Chemical potential differences of Te and Fe and, thus, the Gibbs free energy of formation of the β-phase, were deduced across the homogeneity range. Enthalpies or Gibbs free energies of formation of the phases β, e, δ′ and δ were derived.

Thermal conductivity of rare earth–uranium ternary oxides of the type RE6UO12

The knowledge of thermophysical properties of the rare earth uranium ternary oxides of the type RE6UO12 (RE=La, Gd and Dy) is essential to understand the fuel performance during reactor operation and for modeling fuel behavior. Literature on the high temperature properties of this compound is not available and there is no report at all on the thermal conductivity of these compounds. Hence a study of thermal conductivity of this compound has been taken up. The compounds were synthesized by a solution combustion method using metal nitrates and urea. Thermal diffusivity of these compounds was measured by the laser flash method in the temperature range 673–1373 K. The specific heat data was computed using Neumann–Kopp’s law. Thermal conductivity was calculated using the measured thermal diffusivity value, density and specific heat data for different temperatures. The temperature dependence of thermal conductivity and the implication of structural aspects of these compounds on the data are discussed here.

On the phase relationships and electrical properties in the CaCl2CaH2 system

Abstract The phase diagram of the CaCl2CaH2 system was investigated by using a differential scanning calorimeter and is reported. The hydride-ion conducting compound, CaHCI, is found to be peritectically melting. The total conductivity of CaCl2-5 mol% CaH2 solid electrolyte was measured by impedance spectroscopy and the electronic conductivity was determined by the DC polarisation technique in the temperature range of 673 to 796 K and hydrogen pressure range of 0.03–5 Pa. The activation energy for the ionic conduction was found to be 1.02 eV.

Appearance potential and electron impact ionisation cross-section of C60

A Knudsen cell mass spectrometric technique was employed to determine the electron impact ionisation cross-section of C60. Ion intensities were measured as a function of time, and weight loss due to evaporation was determined. Similar experiments were carried out with silver to obtain calibration factors. From the weight loss and the ion intensity integrated over the entire duration of the experiment for both siver and C60, the ratio of the ionisation cross-section of C60 to that of silver was obtained. Taking the value for silver from the literature, the ionisation cross-section of C60 at 38 eV was calculated to be (53.5 ± 5.6) × 10−16 cm2. This experimental value (determined for the first time) is discussed in relation to those derived by using various empirical formulae generally applied for obtaining molecular cross-sections. Ion intensities were measured as a function of electron energy to obtain ionisation efficiency curves for C60, and its appearance potential was determined by linear extrapolation of these curves