D.Darwin Albert Raj

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.

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.

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.

Homogeneity ranges and thermodynamic properties of the Te-rich phases in the CrTe system

The Te-rich region of the CrTe system was investigated by using high temperature mass spectrometry. The composition range from ∼60 to 82.7 at.% Te was covered by preferentially evaporating off tellurium at T < 700 K from the alloys of starting compositions 66.9, 70.1, 80.0 and 82.7 at.% Te. Evidence for the existence of a new polytelluride (77.0 ± 0.3 to 78.3 ± 0.2 at.% Te) was obtained while the existing uncertainty about the homogeneity range of CrTe3, hitherto considered as the most Te-rich phase in the CrTe system, was removed. This phase exists from 70.5 ± 0.8 to 74.4 ± 0.5 at.% Te. In the case of Cr5Te8, only the Te-rich boundary (63.6 ± 0.6 at.% Te) could be deduced. The activities of tellurium across the single-phase regions were obtained from the p(Te2) measured as a function of composition and those of chromium computed by a Gibbs-Duhem integration. Subsequently Gibbs free energies of formation, ΔfGmo at T = 650 K were also deduced. The values (in kJ mol−1, given in brackets) for the formulae (given in parentheses) corresponding to the Te-rich and/or Cr-rich boundary compositions are: CrTe4−y phase: − [90.7 ± 0.4] (CrTe3.61) and − [90.5 ± 0.4] (CrTe3.35); CrTe3 phase: − [90.4 ± 1.0] (CrTe2.91) and − [89.5 ± 1.5] (CrTe2.39); Cr5Te8 phase: − [86.2 ± 2.2] (CrTe1.75, the Te-rich boundary).

Laser induced vaporization mass spectrometric studies on UO2 and graphite

Abstract A Nd-YAG laser system operated in the Q-switched mode was used to generate vapour plume on UO 2 and graphite targets. The vapour species formed were analysed by a quadrupole mass filter. The time resolved spectrum was obtained for each of the observed vapour species, namely, O, U, UO, UO 2 and UO 3 over UO 2 and C 1 to C 5 over graphite. Time of arrival (TOA) spectra were obtained as a function of laser power density. These TOA data are fitted by the shifted Maxwell Boltzmann distributions so as to obtain the translational temperatures and velocities of the vapour species present in the plume.

Mass spectrometric studies on irradiated (U, Pu) mixed carbide fuel of FBTR

Mass spectrometry was employed to characterise the irradiated mixed carbide fuel of fast breeder test reactor (FBTR) for assessing its performance: thermal ionisation mass spectrometry to determine the isotopic composition, concentrations of U, Pu and Nd in the dissolver solutions and to deduce the burn-up, and a quadrupole mass spectrometric system to obtain the percentage release of fission gases (Kr + Xe). A summary and analysis of the results obtained with the fuel at 25,000 and 50,000 MWd/t in these studies is given in this paper.

A mass spectrometric study of the dissociation of the gaseous diatomic tellurium molecule

Abstract The dissociation equilibrium, Te 2 (g) = 2Te(g), was evaluated from the partial pressures of Te(g) and Te 2 (g) over different metal-tellurides (where the metal = Fe, Cr, Ni or Mo). The equilibrium constants, deduced as a function of temperature (820–1285 K), correspond to a range of monomer-to-dimer ratios, i.e. p (Te)/ p (Te 2 ) varying from ~ 0.02 to 1. They are consistent with the available results corresponding to higher temperatures (1195–1467 K), and therefore, the two sets of data were combined into the following dissociation constant-temperature relation (820–1467 K): The resulting third-law dissociation enthalpy of Te 2 (g), 257.6 ± 4.1 kJ mol −1 , which we recommend, is in excellent agreement with photoionization or spectroscopie results. The pressure-independent reaction, Te(g) + Te (in condensed phase) = Te 2 (g) was also evaluated and the following relation was obtained: This evaluation confirmed the mutual consistency of the p (Te) and p (Te 2 ) values employed to deduce the dissociation constant-temperature relation as well as the dissociation energy reported here. Furthermore, a correlation between the monomer-to-dimer ratio in the vapour phase to the activity of tellurium in the condensed phase is brought out.