Takeo Fujino

Thermodynamic study of UO2 +x by solid state emf technique

Abstract A complete set of thermodynamic parameters of UO2 +x — the relative partial molar thermodynamic quantities of oxygen: g(O2), h(O2) and s(O2) as a function of nonstoichiometry x and temperature T — have been determined with sufficient accuracy by the precise emf measurements of the solid state galvanic cell of the type Ni.NiO/Stabilized ZrO2/UO2 + x, at 0.0030 ⩽ x ⩽ 0.23 between 500 and 1100°C. Nonstoichiometry x was controlled and determined by the coulometric titration of oxide ions at 1000°C by using NiO in the Ni/NiO reference mixture as a source of oxygen. UO2 +x samples of two different preparation procedures give almost identical results and show that g(O2) versus T plots at various compositions x are not a linear function of temperature, but curve downwards with temperature in the composition and temperature ranges studied. This tendency becomes more pronounced with decreasing nonstoichiometry x and indicates that both h(O2) and s(O2) of UO2 +x increase with temperature, their temperature dependence becoming stronger with decreasing nonstoichiometry x. The statistical analysis on about forty emf versus T plots at various compositions x confirms that g(O2), h(O2) and s(O2) of UO2 +x at 0.0030 ⩽ x ⩽ 0.23 and 500 ⩽ T ⩽ 1100°C are accurately expressed by the following equations utilizing the polynomial forms of log x for the temperature independent heat capacity-, entropy- and enthalpy-parameters: cp(O2), s0 and h0 g(O2) = h(O2)−Ts(O2), s(O2) =s0 +cp(O2) In T, h(O2) = h0 +cp(O2)T, where temperature T is in Kelvin, and cp(O2), s0 and h0 are given by c(O2) = −43.4642−120.129 log x−60.9395(log x)2− 19.4064(log x)3 (J/mol·K). s0= −93.807 + 281.272 log x + 119.575(log x)2 + 72.651(log x)3 (J/mol·K). h0 = 373.411 + 277.706 log x + 23.4894(log x)2 + 654.207(log x)−1 + 198.212(log x)−2 (kj/mol). The present results are extensively discussed in comparison with the available literature data on these quantities and also in connection with the point defect model of UO2 +x recently proposed by the present authors.

Magnetic susceptibility of LiUO3

Abstract LiUO3 was prepared, and its magnetic susceptibility was measured in the 4.2–300 K temperature range. Magnetic transition occurred at 16.9 K, and below this temperature large field dependence of the magnetic susceptibility was observed. The crystal field parameters of LiUO3 were determined from the optical absorption spectrum of U5+ doped in LiNbO3. The susceptibility and the g-value of electron paramagnetic resonance were calculated and compared with the experimental results.

Studies on cubic magnesium uranate, MgyU1−yO2+x—I: Phase relations and crystal chemistry

Abstract Cubic magnesium uranate, Mg y U 1− y O 2+ x , was prepared by the reaction of the mixtures of MgUO 4 , MgU 3 O 10 and/or UO 2 in a stream of helium. The cubic region and the values of x and y in Mg y U 1− y O 2+ x were studied under various reaction conditions. Density measurements of this compound gave a strong support to a substitutional structure. The variation of lattice constant with composition was observed to follow a linear equation, a 0 = 5·4704−0·1170x−0·5677 y . Equilibrium oxygen pressure was found to increase with magnesium concentration. The results were discussed by modifying the equation to express partial molar entropy of oxygen given by Aronson and Clayton to valence control type Mg y U 1− y O 2+ x .

Magnetic susceptibilities of UO2ThO2 solid solutions

Abstract The magnetic susceptibility of UO 2 ThO 2 solid solutions has been measured from room temperature to 2.0 K. The magnetic moment and the Weiss constant have been determined in the temperature range in which the Curie-Weiss law holds. For the solid solutions showing antiferromagnetic transition, the Neel temperature has been also determined. These values decrease monotonically with increasing ThO 2 concentration. The results were analyzed using the molecular field theory which includes the interaction between next-nearest neighbor spins. The interactions between nearest neighbor spins, J 1 , and those between next-nearest neighbor spins, J 2 , both decrease with increasing ThO 2 concentration. The change of J 1 with composition is larger than that of J 2 . The effect of magnetic dilution with ThO 2 is considered to be stronger on the interaction between nearest neighbor uranium ions.

Thermodynamics of MgyU1−yO2+x by EMF measurements. I. Properties at high magnesium concentrations

Phase stability and thermodynamic properties of a solid solution, MgyU1−yO2+x, have been investigated at high magnesium concentrations. The lattice constant of this cubic solid solution varies differently withx in the two regions of positive and negativex values. The relation between the lattice constant and the composition was determined in the respective regions. Solid-state emf measurements on MgyU1−yO2+x revealed that both partial molar entropy and enthalpy of oxygen are temperature independent in the experimental range 700 ∼ 1050°C and that−ΔS¯O2 and −ΔH¯O2 increase withx andy of MgyU1−yO2+x. Least-squares calculations showed that−ΔS¯O2 and−ΔH¯O2 could be expressed as logarithmic functions ofx andy. The negative partial molar free energy,−ΔG¯O2, btained by the−ΔS¯O2 and−ΔH¯O2 formulas was found to decrease with temperature more rapidly for the solid solutions of largery values, which indicates the greater effect of divalent magnesium on the thermodynamic properties of MgyU1−yO2+x.

Thermodynamics of fluorite type solid solutions containing plutonium, lanthanide elements or alkaline earth metals in uranium dioxide host lattices

Abstract Thermodynamic data for the solid solutions M y U 1−y O 2+x ( x x ⩾ 0) were reviewed for M = Pu , Ce, La and Gd. Subsequently new experimental results were presented for M = Eu, Sr and Ba-Y. The oxygen potential of the plutonium-uranium solid solution was shown to be well-expressed by the Woodley equation. This type of equation was also obtained for Ce y U 1−y O 2+x ( x y = 0.01, shows a fairly wide range of hypostoichiometry. The increase of the oxygen potential due to changes of y from 0.01 to 0.05 is not very large. The gadolinium-uranium solid solutions, Gd y U 1−y O 2+x , exhibited the steepest increase in oxygen potential at x = 0 for both y = 0.14 and 0.27. The effect on the oxygen potential followed the order: La > Gd > Ce. Results for Eu y V 1−y O 2+x revealed a much larger effect of europium concentration together with the composition of the steepest increase in the oxygen potential at x y U 1−y O 2+x , was found to exist up to x ~ 0.3 if the oxygen partial pressure is sufficiently high. The change of the lattice parameter was linear with the change in crystal radius. Following this rule, the valence state of cerium was between Ce 3+ and Ce 4+ in Ce y U 1−y O 2.00 . Barium dissolves at least up to 5 mol% in the presence of yttrium, forming Ba 0.05 Y 0.05 U 0.9 O 2+x . The oxygen potential is increased by the effect of barium. The steepest change in the oxygen potential also occurred at x

Composition and oxygen potential of cubic fluorite-type solid solution EuyU1−yO2+x () and rhombohedral Eu6UO12+x' (x'< 0)

A fluorite-type solid solution, EuyU1−yO2+x, was formed in single phase up to y = 0.51 when heated in vacuum at 1400°C. Between y = 0.51 and 0.80, a two-phase mixture of fluorite and rhombohedral phases exists. The rhombohedral phase was of single phase in a range y = 0.8–0.9. The change of lattice parameter and x value in the fcc solid solution were determined as a function of y value in EuyU1−yO2+x. The crystal structure of the nonstoichiometric Eu6UO11.41 is the same as RE6UO12 (R3, Z = 3) with Bartrams atom parameters. The measured oxygen potentials (ΔḠO2) for fcc EuyU1−yO2+x (y = 0.1 and 0.3) are not only much higher than those of the solid solutions of the other rare-earth elements, but are also shifted to the lower x side yielding a nearly vertical change of ΔḠO2 in the hypostoichiometric range (x < 0). As a result, the partial molar entropy of oxygen (ΔSO2) and enthalphy (ΔHO2) are significantly different from those of the solid solutions of the other rare-earth elements. ΔḠO2, ΔSO2 and ΔHO2 for rhombohedral Eu0.8571U0.1429O1.7143+x (x < 0) were measured and discussed.

The crystal structure of La6UO12

The phase relations in RE-U-O systems (RE = rare-earth element) have been studied extensively. An outstanding feature in these systems is the existence of rhombohedral compounds, Re/sub 6/UO/sub 12/ which form by heating the mixture of uranium oxides and rare-earth oxides in a ratio U:RE = 1.6 (where RE = La, Pr, Nd, Sm-Ho, Tm-Lu, and Y) above 1000/sup 0/C. Bartram has determined the crystal structure of Y/sub 6/UO/sub 12/ and Lu/sub 6/UO/sub 12/. In the present paper, the crystal structure of La/sub 6/UO/sub 12/ was studied by the X-ray powder diffraction technique. The crystal system of La/sub 6/UO/sub 12/ was known to rhombohedral. Atom positional parameters are reported.

Phase relations and crystal chemistry in the ternary PrO1.5-UO2-O2 system

Phase relations and defect structures were studied in a PrO1.5-UO2-O2 ternary system in the temperature range from 1200 to 1500°C. Phases and compositions of the products heated in either air, helium or vacuum were examined by X-ray diffraction and chemical analysis, respectively. The regions of existence of solid solution having fluorite type structure, of rhombohedral phase and of A-type rare earth sesquioxide phase were determined. It was found that these phase relations could be classified by the mean valency of uranium and the type of oxygen defects. The change of cubic lattice parameter of the solid solution, PryU1−yO2+x, in single phase region (0 ≤ y ≤ 0.7) was expressed as linear equations of x and y: a = 5.4704−0.127x −0.007y, for x ≥ 0 and a = 5.4704−0.397x−0.007y, for x < 0. The change of lattice parameters was discussed in some detail.

Reaction of lithium and sodium nitrates and carbonates with uranium oxides

Abstract The reactivity and reaction conditions to form lithium and sodium uranates were studied in an attempt to grope some useful head-end processes in nuclear fuel reprocessing. In the reactions between alkali metal carbonates and U 3 O 8 in air at 800°C, the products with Na/U ratios 0.8 and 0.857 gave the same X-ray diffraction patterns in which the peaks of α-U 3 O 8 were almost not detected. The observed peaks for uranates with Li/U = 1.205, 2, 4 and Na/U = 1 are well consistent with the values reported previously. No indication of formation of Li 2 U 6 O 19 , Li 6 UO 6 and Na 4 UO 5 was observed. Thermogravimetric observations on the reactions between the carbonates and U 3 O 8 revealed that they consisted of two processes, i.e. (1) the formation of uranates and (2) the oxidation of the uranates formed. The rate of reaction (1) was higher than that of reaction (2) when the M/U ( M = Li, Na ) ratio was 0.5. When the ratio was 1, the rate of reaction (1) lowered and became rate determining. The reactions between alkali metal nitrates and UO 2 showed that the minimum M/U ratios for obtaining the uranates without U 3 O 8 were 0.667 and 0.8 for lithium and sodium uranates, respectively. They were formed by heating at 600°C for 3 h in oxygen or air. Mixing process of initial materials is not required for these reactions. The uranates formed were found to be dissolved in 1 M HNO 3 within 1 min.