Ewald Veleckis

Uranium release and secondary phase formation during unsaturated testing of UO2 at 90°C

Abstract Experimental results indicate that UO2 will readily react after being exposed to dripping oxygenated ground water at 90°C. A pulse of rapid U release, combined with the formation of dehydrated schoepite characterizes reactions between one to two years. Rapid dissolution of intergrain boundaries and spallation of UO2 granules appears to be responsible for the rapid U release. Less than 5% of the U is released in a soluble or suspended form. After two years, U release rates decline and a more stable assemblage of uranyl silicate phases form by incorporating cations from the leachant. Uranophane, boltwoodite, and sklodowskite are the final solubility-limiting phases for U in these tests. This observed paragenetic sequence (from uraninite to schoepite to uranyl silicates) is identical to those observed in weathered uraninite deposits. Dispersion of particulate matter may be an important release mechanism for U and other radionuclides in spent nuclear fuel.

Solidus and liquidus temperatures in the uranium-plutonium-zirconium system☆

Abstract Renewed interest in metallic fuel for nuclear reactions has prompted study of the solidus and liquidus for the uranium- plutonium-zirconium system. These temperatures are of importance in assessing the possibility of fuel melting during abnormal reactor conditions. Data obtained in previous work in this area were found to be inadequate for the needs of the current reactor development effort. A dual effort was undertaken to provide the needed data: (1) thermodynamic phase diagram analysis and calculation of the ternary solidus and liquidus surfaces and (2) experimental determination of solidus and liquidus temper- atures for selected alloys. The methods used and results obtained are described.

Decomposition pressures in the (α + β) fields of the Li-LiH, Li-LiD, and Li-LiT systems

Abstract Decomposition (“plateau”) pressures of H 2 , D 2 , and T 2 over Li-LiH, Li-LiD, and Li-LiT alloys were determined between 600 and 850°C, for pressures ranging from 3 to 460 Torr, and for alloy compositions falling within the (α + β) miscibility gaps. The measurements were carried out separately for each hydrogen isotope using the same lithium sample and experimental procedures. For each system the ln P vs. 1/ T data form a pair of linear segments, the intersection of which represents the monotectic temperature (694°C for Li-LiH, 690°C for Li-LiD, and 688°C for Li-LiT). For a given temperature plateau pressures were in the order P T 2 > P D 2 > P H 2 . The D H and T H isotope effects in pressures varied from 1.48 and 1.74 at 600°C to 1.34 and 1.45 at 850°C. The results were used to calculate the standard free energies of formation of solid LiH, LiD, and LiT. The tritium gas used in this study had significant amounts of hydrogen and deuterium. A method for correcting the plateau pressures of this mixture to those of pure tritium is presented.

Thermodynamic investigation of the LiAl and LiPb systems by the hydrogen titration method

Abstract The hydrogen titration method (HTM) constitutes a new technique for investigating the thermodynamic properties and phase relations in binary A-B alloys. It is based on the reaction A ( in alloy ) + ( x 2 ) H 2 ( g ) ⇌ AH x ( s ) in which constituent A in the alloy is the material being titrated, hydrogen is the titrant and AHx is the titration product. Equivalence points of the titration are indicated by the steeply rising segments in the isotherms of hydrogen pressure versus alloy composition and correspond to the stoichiometries of the intermediate phases in the alloy. For a given temperature and composition the chemical potential of constituent A can be determined from the corresponding hydrogen pressure and the standard free energy of formation of AHx. The method was tested on two lithium-based systems, LiAl and LiPb. The results verified the validity of the HTM by showing that (1) the hydriding reaction (with A  Li, x = 1) is reversible, (2) the anticipated intermediate phases are correctly identified and (3) there is a good agreement with the available e.m.f. data on the chemical potentials of lithium in these alloys. For the α + β field of the LiAl system the lithium activity, calculated from the combined HTM and e.m.f. data, is represented by In aLi = 2.662−5302T−1. For the LiPb system, HTM results indicated the existence of a new phase having a stoichiometry between Li22Pb5 and Li7Pb2. Criteria for applying the HTM to other alloy systems are discussed.

Application of the hydrogen titration method to a thermodynamic investigation of solid Al-Ca alloys

Abstract The hydrogen titration method (HTM) was employed to investigate the condensed phases in the Al-Ca system. The method is based on the reaction Ca(in alloy) + H 2 (g) ⇄ CaH 2 (c) in which hydrogen acts as the titrant and CaH 2 acts as the titration product. The calcium activity in the alloy is calculated from the known equilibrium constant K for this reaction and the experimental hydrogen pressure P via a simple relationship, a Ca = 1 KP . Hydrogen plateau pressures, determined for the Al 4 Ca + Al and Al 2 Ca + Al 4 Ca phase fields in the ranges 400–630 °C and 0.02–1.1 MPa, were used to derive temperature-dependent expressions for the corresponding calcium activities and electromotive forces in Ca-Ca 2+ -(Al-Ca)-type galvanic cells. Integration of these expressions has produced 1. (1) the standard free energies of formation for Al 4 Ca(c) ΔG f o (kJ (g atom) −1 ) = −(20.18 ± 0.38) + (4.29 ± 0.78) × 10 −3 T and for Al 2 Ca(c) ΔG f o (kJ (g atom) −1 ) = −(31.28 ± 0.46) + (5.67 ± 0.93) × 10 −3 T and 2. (2) the standard free energies accompanying the reaction of Al 4 Ca(c) or Al 2 Ca(c) with H 2 (g) to produce Al(c) and CaH 2 (c). Favorable log P versus 1 T relationships and no significant alloy degradation during cycling suggest a potential utility for Al 2 Ca as a “high temperature” hydrogen storage medium having an absorption capacity of 2.13 wt.% H and an operation range of 450–650 °C. The potential for hydrogen storage of other calcium-containing binary systems undergoing HTM reactions is discussed.

Solubility of lithium deuteride in liquid lithium

Abstract The solubility of LiD in liquid lithium between the eutectic and monotectic temperatures was measured using a direct sampling method. Solubilities were found to range from 0.0154 mol.% LiD at 199 °C to 3.32 mol.% LiD at 498 °C. The data were used in the derivation of an expression for the activity coefficient of LiD as a function of temperature and composition and an equation relating deuteride solubility and temperature, thus defining the liquidus curve. Similar equations were also derived for the Li-LiH system using the existing solubility data. Extrapolation of the liquidus curves yielded the eutectic concentrations (0.040 mol.% LiH and 0.035 mol.% LiD) and the freezing point depressions (0.23 °C for Li-LiH and 0.20 °C for Li-LiD) at the eutectic point. The results are compared with the literature data for hydrogen and deuterium. The implications of the relatively high solubility of hydrogen isotopes in lithium just above the melting point are discussed with respect to the cold trapping of tritium in fusion reactor blankets.

Phase relations for reactions of hydrogen with sodium oxide between 500 and 900°C☆

The equilibrium pressure technique, commonly used for investigating the phase relations in binary gas-condensed phase equilibria, was applied to the Na-Na2O-NaOH-NaH corner of the ternary Na-O-H system. Measured amounts of hydrogen were reacted with Na2O (sealed in vacuo in a thin-walled nickel crucible) and the equilibrium hydrogen pressure (0.03 < P < 100 kPa) was determined as a function of temperature(510 < T < 879°C) and hydrogen atom fraction(0 < XH < 0.275). Discontinuities encountered in the isothermal(√P vs XH) or fixed-hydrogen-concentration (InP vs 1/T) plots were used to delineate the condensed-phase boundaries that occur within theP-T-XH ranges studied. Data were fitted to analytical equations which permitted construction of a portion of the phase diagram and yielded pertinent thermodynamic information. Limitations of the technique in applications to other similar systems are discussed.