Kinya Nakamura

Thermodynamic assessment of the Fe–U, U–Zr and Fe–U–Zr systems

Abstract In an attempt to understand the phase formation mechanism at the interface of metal fuel and cladding, the isothermal phase diagrams and chemical potential diagrams of the Fe–U–Zr ternary system were calculated using the optimized interaction parameters of three binary subsystems. The Gibbs energies of solution phases and compounds in the Fe–U and U–Zr systems were calculated through an optimization procedure based on both the experimental thermochemical and phase diagram data. The obtained ternary phase diagrams were consistent with the experimental data, when the Gibbs energies of formation of ternary compounds; Fe 0.06 U 0.71 Zr 0.23 and Fe 0.3 U 0.3 Zr 0.4 , were assumed to be −3.7∼−4.3 and −16∼−17.5 kJ per g-atom, respectively. The calculated chemical potential diagrams described satisfactorily the experimental diffusion path for the U 0.8 Zr O.2 /Fe couple at 923 K.

Reactions of U–Zr alloy with Fe and Fe–Cr alloy

Abstract The reaction zones formed in two kinds of diffusion couples: U–23at.%Zr/Fe and U–23at.%Zr/Fe–12at.%Cr, at 908, 923, 953, 973 and 988 K have been examined using the electron-probe microanalysis. In the U–Zr/Fe–Cr couples, diffusion of Cr to the U–Zr side is slower than that of Fe, and the Cr-rich phase is formed adjacent to the unreacted Fe–Cr alloy. Except for the Cr-rich phase, the measured compositions of the phases in the reaction zones in both U–Zr/Fe and U–Zr/Fe–Cr couples have corresponded well to those in the U–Zr–Fe ternary system. Each reaction zone can be divided to several layers. For the U–Zr side of the reaction zone, the configurations of the schematic diffusion paths, which are the curves connecting the average compositions of these layers on the U–Zr–( Fe + Cr ) composition triangle, are independent of the annealing temperature and the Cr addition to Fe. For the Fe(–Cr) side, however, the paths depend on the annealing temperature and the Cr addition to Fe. Some of the phases that are expected to emerge considering the schematic diffusion path and the U–Zr–Fe phase diagram have not been found at 988 K.

Reactions of Uranium-Plutonium Alloys with Iron

In metallic U-Pu-Zr fuel for fast reactors, metallurgical reactions occur between the fuel alloy and the stainless steel cladding, and a liquid phase may be formed in the reaction zone at a higher temperature. In order to clarify the condition for liquefaction at the fuel-cladding interface, the reactions of U-Pu alloys with Fe have been examined at 923 and 943 K. The test results confirmed that the liquid phase is not formed at 923 K in any region of the reaction zone when the maximum Pu content in the (U,Pu)6Fe phase is less than the Pu solubility limit in this phase. Comparison of the present test results with the liquefaction data from the various tests on metallic fuel-cladding compatibility suggested that the liquefaction condition is independent of the Zr content in the fuel alloy and can be expressed as a function of the atom fraction ratio of Pu/(U+Pu) in the fuel alloy and the reaction temperature. At 923 K, liquefaction will occur when the Pu/(U+Pu) ratio is larger than 0.25.

Thermodynamic evaluation of the quaternary U–Pu–Zr–Fe system – assessment of cladding temperature limits of metallic fuel in a fast reactor

Abstract The quaternary U–Pu–Zr–Fe system was assessed using thermodynamic and phase diagram data in order to evaluate fuel-cladding chemical interactions (FCCI) of metallic fuel in a fast reactor. The Gibbs energy of mixing for solution phases and the Gibbs energy of formation of compounds in the binary sub-systems were calculated using an optimization procedure. The use of such data in optimizing the binary sub-systems enabled appropriate calculations for the thermodynamic properties of the systems, which were also important when extrapolating to higher-order systems. Isotherms of ternary sub-systems were calculated by using the optimized parameters of the binary sub-systems. Based on the phase relation data measured in regions of the ternary systems, the isotherms were then modified by adding ternary interaction parameters. The calculation results agreed well with the experimental data points. Finally, the quaternary system was assessed. The phase relationship observed experimentally in the diffusion couple of U–Pu–Zr–Fe was in reasonable agreement with the calculated phase diagrams.

Reactions between U-Pu-Zr Alloys and Fe at 923 K

In metallic U-Pu-Zr fuel, metallurgical reactions occur between the fuel slug and the cladding, and a phase of which the melting point is relatively low is formed in the reaction zone. If a liquid phase is formed, it can degrade cladding integrity. The potential for liquid phase formation near the cladding, therefore, should be excluded during normal reactor operation. In order to clarify the mechanism of liquefaction, the authors have conducted diffusion experiments at 923 K using two couples: U-13 at%Pu-22 at%Zr/Fe and U-22at%Pu- 22at%Zr/Fe, and examined the influence of the Pu content in the fuel alloy on the phases formed in the reaction zones. The liquid phase has been observed in the U-22 at%Pu-22 at%Zr/Fe couple. An assessment of the diffusion paths for these couples has indicated that the Pu content in the (U, Pu)6Fe-type phase in the reaction zone is a crucial factor in determining the conditions that lead to liquefaction. The Pu content in the (U, Pu)6Fe-type phase increases with that in the initial U-Pu-Zr alloy. The fuel design specifications to exclude the potential for liquefaction will be clarified by further examination of the Pu content in the (U, Pu) 6Fe-type phase for various U-Pu-Zr/Fe couples annealed at different temperatures.

Phase relations in the quaternary Fe–Pu–U–Zr system

Abstract The phase diagram in the quaternary Fe–Pu–U–Zr system was established at 923 K in the uranium-rich region to understand better the compatibility between the metal fuel and stainless steel cladding in a fast reactor. The experimental phase relation data obtained in this study was applied to the thermodynamic methodology for construction of the phase diagram. The calculated phase diagram was consistent and well within the experimental data. The applicability of the thermodynamic model to other temperatures was confirmed by comparing the present results of differential thermal analysis with the calculated phase diagrams. These consistencies mean that both the thermodynamic model and the assessed parameters in the binary and ternary subsystems developed so far are reasonable. The calculated phase diagram established in this study was also in good agreement with the analysis of the diffusion zone in tests on Pu–U–Zr/Fe couples. This suggests that the diffusion zone formed at the fuel–cladding interface in the reactor system can be assessed using the phase diagram in the quaternary Fe–Pu–U–Zr system.

Vaporization study on lanthanum–cerium alloys by mass-spectrometric method

Abstract The partial vapor pressures of La(g) and Ce(g) over La x Ce 1− x alloys ( x =0.00, 0.05, 0.29, 0.50, 0.80, 1.00) were measured with a time-of-flight mass-spectrometer equipped with a tungsten Knudsen cell over the temperature range of 1592–1781 K. The thermodynamic activities of lanthanum and cerium in the liquid alloys were determined by comparing the partial vapor pressures of La(g) and Ce(g) over the alloys with those over the pure metals, respectively. The thermodynamic activities were also calculated from the ion intensity ratios of two components (lanthanum and cerium) in the alloys using the equation derived by Belton and Fruehan. Both activities for each element, thus obtained, were in good agreement with each other and almost obeyed Raoults law. The partial molar Gibbs free energy, and the Gibbs free energy of formation were calculated from the thermodynamic activity values.

Vapor pressure measurements of LaGd alloys

Abstract The vapor pressures of La(g) and Gd(g) over La x Gd 1− x alloys ( x = 0.00, 0.12, 0.22, 0.45, 0.70, 0.74, 0.85, 1.00) were measured with a time-of-flight mass spectrometer equipped with a tungsten Knudsen cell over the temperature range 1588 to 1797 K. The chemical activities of lanthanum and gadolinium in the alloys were determined by comparing the vapor pressures of La(g) and Gd(g) over the alloys with those over the pure metals. The chemical activities, thus obtained, showed positive deviations from Raoults law over the entire compositional range. The interatomic force between gadolinium and lanthanum was thought to be repulsive. The partial molar Gibbs free energy and the Gibbs free energy, enthalpy and entropy of formation were calculated from the activity values.

Phase relations in the Fe-Pu-U ternary system

An isothermal section of the Fe-Pu-U ternary system at 650 °C was assessed in a previous study. In the present study, the predictions of the phase relations in the Fe-Pu-U system to higher and lower temperatures were performed by applying the interaction parameters determined at 650 °C. Differential thermal analysis (DTA) for the Fe-Pu-U alloys was also carried out to confirm the phase relations in the temperature region of 500 to 800 °C. Both results agreed well. On the basis of the predicted ternary phase diagram, the phase relations for a region surrounded by Fe2Pu, Fe2U, U, and Pu were described by a reaction scheme and a projection of the liquidus surface.

Thermal analysis of pseudo-binary system: LiClKCI eutectic and lanthanide trichloride

Copyright (c) 1997 Elsevier Science B.V. All rights reserved. The phase diagrams of the pseudo-binary systems of LiCl-KCl eutectic and lanthanide trichloride (LnCl 3 : Ln=La, Ce, Pr, Nd, Sm, and Gd) were determined for compositions less than 25 mol% LnCl 3 by means of thermal analysis, visual observation, electromotive force measurement and powder X-ray diffraction. The existence of K 2 LnCl 5 compounds was confirmed in all the systems, but K 3 LnCl 6 was observed only in the systems containing Sm and Gd. In the region up to 17 mol% LnCl 3 , the eutectic temperature and composition were observed to be close to those of the LiCl-KCl eutectic system (625±1 K, LiCl-41 at.% KCl). For compositions greater than 17 mol% LnCl 3 , on the other hand, the eutectic points were 701±1, 691±1, 681±1, 656±1, 635±1 and 649±1 K for La, Ce, Pr, Nd, Sm, and Gd, respectively. The liquidus surface of the LiCl-KCl-LaCl 3 system was also determined at compositions up to 40 mol% LnCl 3 .