Takanari Ogata

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.

Development and validation of ALFUS: An irradiation behavior analysis code for metallic fast reactor fuels

An irradiation behavior analysis code for metallic fast reactor fuel, ALFUS, has been revised so that it can be applied to stress-strain analysis of U-Pu-Zr ternary fuel pins. The stress-strain cal...

Interdiffusion in uranium-zirconium solid solutions

Abstract Interdiffusion coefficients in the (γ-U, β-Zr) solid solutions have been measured in the temperature range of 973 to 1223 K and in the Zr concentration range of 0.1 to 0.95 atomic fraction. The data measured at lower temperature than 1223 K have shown notable depression in the Zr concentration range of 0.2 to 0.4 atomic fraction. The dependencies of the obtained data on alloy composition and temperature are consistent with variation of the thermodynamic factor which is calculated based on the evaluation of the activity coefficient in the U Zr system. The interdifusion coefficient in the neighborhood of the Zr atomic fraction of 0.3 cannot be expressed by a single set of a frequency factor and an activation energy because of the significant influence of the thermodynamic factor on the interdiffusivity in the U Zr solid solutions.

Constituent Migration Model for U-Pu-Zr Metallic Fast Reactor Fuel

A model for constituent migration behavior in U-Pu-Zr metallic fast reactor fuel is proposed. It is based on diffusion equations for the ternary system under a radial temperature gradient, and it takes into account the alloy phase decomposition, assuming a quasi-binary U-Zr phase diagram with a constant plutonium content. Parametric simulations of Experimental Breeder Reactor II irradiation data with appropriate transport properties of the alloy system showed that the model can predict the experimentally observed radial three-zone structure and zirconium and uranium redistribution, although the predicted radial location of zirconium-depleted middle zone disagreed with the experimental result. Accumulation of basic experimental data on transport properties and a ternary phase diagram of the system are needed for a better understanding of the behavior.

Development of an Innovative Electrorefiner for High Uranium Recovery Rate from Metal Fast Reactor Fuels

To increase the uranium recovery rate of molten salt electrorefining step in pyrometallurgical reprocessing of metallic fast reactor fuels, tests were carried out using electrorefiners equipped with mechanisms for scraping cathode deposits. After the modifications in the design of the anode basket and scraper mechanism, no stalling of the anode and scraper rotation due to interference by cathode deposits occurred. Under the condition that codissolution of zirconium and uranium was allowed in order to obtain maximum throughput, a current of 400–450A was maintained until 82% of the initially loaded uranium was recovered. The uranium recovery rate for the same duration reached 789 g U/h (32.9 g U/h_L per electrode volume). On the assumption that an electrorefiner operates for 20 h/d and 200 d/y in an actual pyrometallugical reprocessing facility, this result corresponds to a uranium recovery rate of 3.16 t U/y using only one electrode assembly of about 30 cm diameter, which should be a sufficiently high performance for practical use. From these results, the engineering feasibility of uranium recovery using an electrorefiner with cathode deposit scraper mechanism has been demonstrated.

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.