Atomistic modeling of the interaction of cladding elements (Fe, Ni, Cr) with U-Zr fuel

Atomistic simulations of U-Zr fuel and its interaction with Fe, Ni, and Cr using the BFS method for alloys are presented. Results for the γU-βZr solid solution are discussed, including the behavior of the lattice parameter and coefficient of thermal expansion as a function of concentration and temperature. Output from these calculations is used to study the surface structure of γU-βZr for different crystallographic orientations, determining the concentration profiles, surface energy, and segregation behavior. The analysis is completed with simulations of the deposition of Fe, Ni and Cr on U-Zr substrates with varying Zr concentration. All results are discussed and interpreted by means of the concepts of strain and chemical energy underlying the BFS method, thus obtaining a simple explanation for the observed Zr segregation and its influence in allowing for cladding elements diffusion into the U-Zr fuel.     

Thermodynamic study of the Np-Zr system

A thermodynamic model of the Np-Zr system is developed using the CALPHAD method, and a review of previous work performed on this system is presented here. In general, results obtained are in good agreement with those proposed from experimental observations. It is found that the nature of reactivity of Np with Zr, is different from that of U and Pu: an expected elevation of melting point of Np-Zr alloys was not seen and a miscibility gap existed between the high-temperature bcc phases of Np and Zr. Formation enthalpy of the bcc phase obtained from the model is compared with results from KKR-ASA-CPA calculations. Lattice stabilities of various phases in the system are compared to values obtained from first-principles LDA and GGA calculations. The δ-NpZr₂ phase is modeled as a non-stoichiometric phase with a C32 structure, similar to what has been determined for the δ-phase in the U-Zr system. This phase is analogous to ω-phase in pure Zr, which is stabilized at high pressures. Two different possibilities for stability of the δ and ω phases have been proposed in the present work. Finally, calculated changes in enthalpy versus temperature are plotted for two alloys to guide future experimental work in resolving important issues in this system.     

Thermotransport in γ(bcc) U-Zr alloys: A phase-field model study

Atomic transport in the presence of a temperature gradient, commonly known as thermotransport or the thermomigration phenomenon, was simulated for U-Zr alloys using a phase-field model derived from irreversible thermodynamics. The free energy of the U-Zr system, a necessary ingredient for the phase-field-model, was directly incorporated from the available thermodynamic database. Kinetic parameters such as atomic mobility and heat of transport terms were obtained from experimental values reported in the literature. The model was applied to a single-phase (bcc-γ phase) alloy and to a diffusion couple consisting of two single-phase (bcc-γ phase) alloys of different compositions, both subjected to a constant temperature gradient. Constituent redistribution in the absence and presence of a compositional gradient was examined. An enrichment of Zr with a corresponding depletion of U was observed at the hot end of the initially homogeneous single-phase alloy. A similar atomic transport behavior was observed in the diffusion couple, where the magnitude and direction of the final composition gradient was dictated by the combined influence of atomic mobility and heat of transport terms.     

Role of zirconium addition on stability of UAl3 phase in AlU system

The microstructural changes of Al-22 wt%U and Al-46 wt%U alloys containing 3 wt% Zr were investigated after heat treatment at 620°C for 1 to 45 days. Though it is reported that addition of ∼ 3 wt% Zr stabilizes the (U,Zr)Al₃ phase at room temperature, the present investigation shows that the (U,Zr)Al₃ phase is not stable but slowly transforms to the U_0.9Al₄ phase. The high temperature creep curves generated for these ternary alloys showed a wavy pattern which also suggests that the (U,Zr)Al₃ phase is not stable.     

Stability of the body-centered cubic gamma phase in the uranium-zirconium-niobium system

A thermodynamic representation of the stability of the binary gamma phases has been developed for the U-Zr, U-Nb and Zr-Nb binary systems. This has permitted the calculation of the miscibility gaps in the ternary system. The three binary gaps merge to form a large three-phase region in the middle of the composition trangle. Because of the large coherency energies, the spinodal decomposition in the U-Nb and Zr-Nb gaps will be drastically modified, but a large region remains in the ternary system where the decomposition products have small coherency energies such that spinodal decomposition is probable. From these results, we have identified the four decomposition processes in Mulberry (U-16.6 at% Nb-5.6 at% Zr) as being discontinuous and spinodal decomposition within the miscibility gap, the diffusionless formation of α and the formation of (α + γ) from the supersaturated gamma phase.     

The effect of RE-rich phase on the thermal conductivity of U-Zr-RE alloys

Thermal conductivity of RE-bearing U-Zr fuels was investigated. Under an assumption of a RE-rich phase forming a macroscopic mixture with the matrix, the thermal conductivity was estimated for U-Zr-RE alloy. It was evaluated that the thermal conductivity of the U-Zr fuels would be lowered by less than 5% due to the addition of RE. The measurement of the thermal properties of U-Zr-Ce supported the present estimation.     

Investigation on the martensitic transformation and the associated intermediate phase in U-2 wt%Zr alloy

In most of the binary uranium alloy systems, different metastable phases can be retained at room temperature depending on the rate of cooling of the gamma phase. On the other hand, in U-rich U-Zr system, the gamma phase transforms into either martensitic α or Widmanstatten α depending on the cooling rate. It has not been ascertained so far if any intermediate phase is involved prior to the martensitic transformation in U-rich U-Zr system and, therefore, nature of the intermediate phase remains undetermined. Present investigation shows the existence of an intermediate phase prior to the final martensitic transformation. Although the equilibrium beta phase is known to undergo martensitic transformation, quenching from the γ phase appears to suppress the β phase formation. In fact the present experimental evidence suggests that the most likely intermediate phase is the monoclinic distortion of alpha phase (α^″), and not the beta, when gamma phase of U-2 wt%Zr alloy is quenched rapidly.     

Low silicon U(Al,Si)3 stabilization by Zr addition

Previous knowledge states that (U,Zr)Al₃ and U(Al,Si)₃ phases with Zr and Si content higher than 6 at.% (7.7 wt%) and 4 at.% (1.4 wt%), respectively, does not partially transform to UAl₄ at 600 °C. In this work, four alloys within the quaternary system U-Al-Si-Zr were made with a fixed nominal 0.18 at.% (0.1 wt%) Si content in order to assess the synergetic effect of both Zr and Si alloying elements to the thermodynamic stability of the (U,Zr)(Al,Si)₃ phase. Heat treatments at 600 °C were undertaken and samples were analyzed by means of XRD, EPMA and EDS techniques. A remarkable conclusion is that addition of 0.3 at.% Si in the (U,Zr)(Al,Si)₃ phase reduces in 2.7 at.% the necessary Zr content to inhibit its transformation to U(Al,Si)₄.     

Characterization of solidified melt among materials of UO2 fuel and B4C control blade

To predict the fundamental phase relationships in the solidified core melt of the Fukushima Daiichi Nuclear Power Plant, solidified melt samples of the various core materials [B₄C, stainless steel, Zr, ZrO₂, (U,Zr)O₂] were prepared by arc melting. Phases and compositions in the samples were determined by means of X-ray diffraction, microscopy, and elemental analysis. With various compositions, the only oxide phase formed was (U,Zr)O₂. After annealing, the stable metallic phases were an Fe-Cr-Ni alloy and an Fe₂Zr-type (Fe,Cr,Ni)₂(Zr,U) intermetallic compound. The borides, ZrB₂ and Fe₂B-type (Fe,Cr,Ni)₂B, were solidified in the metallic part. Annealing at 1773 K under an oxidizing atmosphere (Ar-0.1%O₂) resulted in the oxidation of U and Zr in the alloy and in ZrB₂, and consequently the (Fe,Cr,Ni)₂B and Fe-Cr-Ni alloy became dominant in the metallic part. The experimental phase relationships in the metallic part agreed reasonably with the thermodynamic evaluation of equilibrium phases in a simplified B₄C–Fe–Zr system. The metallic Zr content in the melt was found to be a key factor in determining the phase relationships. As a basic mechanical property, the microhardness of each phase was measured. The borides, especially ZrB₂, showed notably higher hardness than any other oxide or metallic phases.     

Characterization of zirconia-thoria-urania ceramics by X-ray and electron interactions

X-ray and electron interactions with matter were used as probes to characterize the structure and chemistry of zirconia-thoria-urania ceramics. The ceramics were prepared by coprecipitation of Zr, Th and U salts. In this study, transmission electron microscopy (TEM) techniques such as energy dispersive X-ray (EDX) analysis and electron energy loss spectroscopy (EELS) complement X-ray diffraction, extended X-ray absorption fine structure (EXAFS) and X-ray absorption near edge spectroscopy (XANES), techniques to reveal the phase structure and chemistry. The results from XRD and EDX show that these ceramics separate into a Zr-based phase and an actinide-based phase with low mutual affinity of Th and Zr, as well as partial solubility of U in Zr. The comparison of EELS spectra collected for the ceramics with spectra collected for UO₂ and U₃O₈ reference materials also allow us to assess U oxidation state independently in the two separate phases.     

A study of the structure of Zr(CrFe)2 laves phase precipitates in a ZrCrFe alloy

C15 type Zr(CrFe)₂ Laves phase precipitates have been found in Zr-1.15 wt% Cr-0.1 wt% Fe alloy. Twinned, multiple twinned and dislocation structures have been found in the precipitates. Comparison of calculated and measured precipitate size show the growth of the Zr(CrFe)₂ Laves phase is controlled by diffusion.The orientation relationships (1̄11̄)_L//(112̄0)_α, [110]_L//[0001]_α between the Zr(CrFe)₂ Laves phase precipitates and ga-Zr matrix in the ZrCrFe alloy give the same type of model for the transformation as previously suggested for Zircaloy-4.     

A TEM study of the C15 type Zr(CrFe)2 Laves phase in Zircaloy-4

The intermetallic precipitates in Zircaloy-4 have been identified as the C15 type Zr(CrFe)₂ Laves phase using high order Laue zones and series diffraction patterns from the transmission electron microscope. A model for the transformation from the α-Zr matrix to the Zr(CrFe)₂ Laves phase has been constructed using the data on orientation relationships obtained from TEM diffraction patterns. A defect structure of R-type stacking variety has been found in the Zr(CrFe)₂ particles.     

Performance of FCCI barrier foils for U-Zr-X metallic fuel

Diffusion couple tests of U-Zr or U-Zr-Ce alloys vs. ferritic martensitic steels such as HT9 or T91 were carried out in order to evaluate the performance of the diffusion barrier candidates. Elemental metal foils of Zr, Nb, Ti, Mo, Ta, V and Cr were very effective in inhibiting interdiffusion between these fuels and steels. Eutectic melting between the fuels and steels was not observed in any of the diffusion couples using these diffusion barrier foils at annealing temperatures up to 800 °C. Among the metallic foils evaluated in this study, V and Cr exhibited the most promising performances as a diffusion barrier material for eliminating the fuel cladding chemical interaction problem. However, Zr, Nb and Ti showed an active interaction with the fuel mainly due to the large U solubility.     

The effect of alloying modifications on hydrogen uptake of zirconium-alloy welding specimens during corrosion tests

The hydrogen uptake behavior during corrosion tests for electron beam welding specimens made out of Zircaloy-4 and zirconium alloys with different compositions was investigated. Results showed that the hydrogen uptake in the specimens after corrosion tests increased with increasing Cr content in the molten zone. This indicated that Cr element significantly affected the hydrogen uptake behavior. Fe and Cr have a low solubility in α-Zr and exist mainly in the form of Zr(Fe,Cr)₂ precipitates, which is extremely reactive with hydrogen in its metallic state. It is concluded that the presence of Zr(Fe,Cr)₂ second phase particles (SPPs) is responsible for the increase in the amount of hydrogen uptake in the molten zone of the welding samples after corrosion, as Zr(Fe,Cr)₂ SPPs embedded in α-Zr matrix and exposed at the metal/oxide interface could act as a preferred path for hydrogen uptake.     

The influences of Pu and Zr on the melting temperatures of the UO2–PuO2–ZrO2 pseudo-ternary system

Specimens of (U, Pu, Zr)O₂ were prepared as simulated corium debris that were assumed like debris generated in the severe accident of the Fukushima Daiichi Nuclear Power Plant and their melting temperatures were measured by the thermal arrest technique in order to evaluate the influence of plutonium and zirconium content on the melting temperature of the corium debris. From the evaluation, it was found that the influence of zirconium on the melting temperatures of both (U, Pu, Zr)O₂ and (U, Zr)O₂ was similar and that the melting temperature of (U, Pu, Zr)O₂ had a local maximum value in the Pu-content between 0 and 20 mol%. The UO₂–PuO₂–ZrO₂ pseudo-ternary phase diagram at 2900 and 3000 K was evaluated from the present experimental results and previously reported results.     

Thermodynamic model for Zr solubility in the presence of gluconic acid and isosaccharinic acid

Zr solubility in the presence of gluconic acid (GLU) and isosaccharinic acid (ISA) was investigated as a function of hydrogen ion concentration (pH_c) and the total concentration of GLU or ISA. The dependence of the increase in Zr solubility on the pH_c and GLU concentration suggested the existence of Zr(OH)₄(GLU)₂^2? in the neutral pH region and Zr(OH)₄(GLU)(GLU_-H)^3? in the alkaline pH region above pH_c 10 as the dominant species in the presence of 10^?3–10^?1 mol/dm³ (M) GLU. In the presence of ISA, the dominant species Zr(OH)₄(ISA)₂^2? and Zr(OH)₄(ISA)(ISA_-H)^3? were proposed to occur in the neutral and alkaline pH regions, similar to those found in the presence of GLU. From X-ray diffraction analysis, the solubility-limiting solid phase in the presence of GLU and ISA was considered to be Zr(OH)₄(am). The formation constants of the Zr gluconate and isosaccharinate complexes were determined by least-squares fitting analysis of the solubility data, and the obtained values were discussed in comparison with those of tetravalent actinides.     

Evolution of microstructure and second-phase particles in Zr–Sn–Nb–Fe alloy tube during Pilger processing

Evolution of microstructure and second-phase particles (SPPs) in Zr–Sn–Nb–Fe alloy tube were investigated during Pilger process using electron backscatter diffraction, secondary electron and transmission electron microscopy imaging techniques. Results show that the Pilger rolled tubes present heterogeneous structures with the C axes of less deformed grains mostly concentrated in the axial direction. During the Pilger rolling, the increase of deformation caused weakening of linear distribution of second-phase particles. The mean diameters of the precipitates are in the range of 70–100 nm in all specimens, and the growth mechanism of SPPs follows second-order kinetics. The grain growth is controlled by Zener pinning in the Pilger rolling–annealing specimens. Clusters containing the Zr(Nb,Fe)₂ and β_Nb precipitates formed in the Zr–1.0Sn–1.0Nb–0.12Fe alloy. Most of the particles located in grain boundaries are the Zr(Nb,Fe)₂ Laves phase with hexagonal structure, and stacking faults have been found in the Zr(Nb,Fe)₂ precipitates. The types, morphology and distribution of precipitates depend on the constituent and structural fluctuations of the nucleation area.     

Crud in Solvent Washing Process for Nuclear Fuel Reprocessing

In a solvent washing process for nuclear fuel reprocessing, one of the important problems is a formation of stable emulsions between organic and aqueous phases. These emulsions are called interfacial “crud”. Crud is defined as an emulsion stabilized by finely dispersed solids. These stable emulsions lead to decreased washing efficiency, lower phase separation, disturbance of the interfacial control at the settler of the extractor, and so on. Cruds formed by precipitates of Zr and tributyl phosphate (TBP) degradation products, such as di-n-butyl phosphate (HDBP), mono-n-butyl phosphate (H₂MBP), and phosphoric acid (H₃P0₄) are studied by experiments using a sodium carbonate solution as a washing reagent. Experimental results show that not only pH value of the washing reagent, but also phosphate and zirconium mole ratio (P/Zr) are important in crud formation. Moreover, it is shown that the complex of Zr and HDBP, or Zr and H₂MBP has a significant role in stabilizing emulsions. However, the complex of Zr and H₃P04 is not effective in stabilizing cruds.