Microstructural evolution of as-cast Th–U alloys

Microstructural evolution of as-cast (arc-melted) microstructures of selected compositions of Th–U alloys is investigated. Characterizations are done using XRD, optical microscopy, SEM, TEM and hardness measurement. Microstructures vary from single phase to biphasic with increase in uranium content in thorium. Uranium is present as spheroids (<1 μm) or islands (∼7 μm) in high thorium (<10.7 wt.% U) alloys. With further increase in uranium content it forms an interconnected network of uranium rich phase and gets interspersed with similar network of thorium rich phase. High uranium Th–U alloys has dendrites of thorium dispersed in uranium rich phase. Grain size refinement of thorium rich phase is effective up to ≤10.7 wt.% uranium addition. Hardness of Th–U alloys increases with increase in uranium content. Comparatively lower hardness observed in Th–96U is due to presence of softer eutectic colonies in the microstructures. High thorium alloys have potentials for being utilized as dispersion type fuel with fissile uranium spread in thorium matrix.     

Moessbauer study of the orientation of the magnetic moments in Fe—based nanocrystalline alloys

Magneto-impedance(MI) effect in Fe-based nanocrystalline Fe73 Cu1Nb1.5Mo2Si13.5B alloys has been observed by Moessbauer spectroscopy.The results show that the field dependence of the MI ratio is strongly influenced by the transverse magnetic structure in samples.     

Thermodynamic analysis of uranium–gadolinium fuel stability at high temperatures

Using the most accurate measurements of the liquidus temperature in the UO₂–Gd₂O₃ system up to 30 mol.% of Gd₂O₃, thermodynamic models of the melt and cubic solution GdO_1.5 in UO₂ are constructed. The equilibrium phase diagram of the system UO₂–GdO_1.5 in the interval 1900–3200 K is calculated in the entire composition range and the metastable diagram is calculated assuming that no cubic solid solutions are formed. The upper and lower boundaries of the melting onset temperature (solidus) of uraniumgadolinium fuel are presented. The phase composition of the pellets made from such fuel and, ultimately, the technology determine the melting onset temperature uniquely.     

Preshock-induced phase transition in spalled U–0.75 wt% Ti

Uranium–0.75 wt% Ti samples were spalled in the range of 5–24 GPa shock pressure. One sample was preshocked to a pressure of 24 GPa, `soft' recovered, and then reloaded and spalled at 10 GPa. The spall strength of U–3/4 wt% Ti was found to range from −4.1 to −2.9 GPa when the Romanchenko correction is used in the spall strength calculation. The spall morphology of the sample that was preshocked and then spalled showed a significant change in microstructure from a parent alpha' martensite to a 2-phase eutectoid. The thermodynamically calculated temperature rise resulting from the preshock at 15–24 GPa in these samples is ∼555°C. This temperature is not sufficient to induce such a phase change. However, the preshock conditions additionally increase the flow stress of the U–34 wt% Ti, and it is postulated that this additional hardening is sufficient to increase the temperature above 885°C due to the increased amount of plastic work required during spall, thereby triggering the phase change.     

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 (1&#x0304;11&#x0304;)_L//(112&#x0304;0)_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.     

Contribution to the study of the pseudobinary Zr1Nb–Oxygen phase diagram by local oxygen measurements of Zr1Nb fuel cladding after high temperature oxidation

This work is concerned with measurement of oxygen concentrations and construction of the pseudobinary Zr1Nb–O phase diagram, acceptable for diffusion models predicting the oxidation behavior of Zr1Nb fuel cladding during thermal transients such as LOCA. Oxygen concentrations were measured in existing phases in the wall of Zr1Nb nuclear fuel cladding tubes after the high-temperature oxidation. The oxygen concentrations at the α/α + β phase boundary in the α-Zr(O) layer have been determined using WDS method. Oxygen concentrations in the prior β-Zr were measured using two experimental methods (SIMS and TEA). Consequently, the ceiling of the oxygen concentration in β-Zr has been assessed based on the results of SIMS, TEA, and microhardness measurements. Eventually, the experimental results were compared to the pseudobinary Zr1Nb–O phase diagram, calculated using CALPHAD software, with satisfactory agreement. The effect of hydrogen was also examined.     

Investigation of cascade-induced re-solution from nanometer sized coherent precipitates in dilute Fe–Cu alloys

Molecular dynamics (MD) simulations have been used to investigate the re-solution of copper atoms from coherent, nanometer-sized copper precipitates in a body-centered cubic iron matrix. The molecular dynamics simulations used Finnis–Sinclair type interatomic potentials to describe the Fe–Cu system. Precipitate diameters of 1, 3 and 5 nm were studied, with primary knock-on atom (PKA) from 1 to 100 keV, although the majority of the cascade simulations and analysis of solute re-solution were performed for cascades of 10 or 20 keV. The simulation results provide an assessment of the re-solution on a per-atom basis as a function of precipitate size, cascade location and energy. Smaller sized precipitates, with a larger surface to volume ratio, experienced larger re-solution on a per-atom basis than larger precipitates. Re-solution was observed to occur predominantly in the initial ballistic stages of the cascades when atomic collisions occur at high kinetic energy. A minimum PKA energy of around 1 keV was required to produce re-solution, and the amount of re-solution appears to saturate for PKA energies above approximately 10 keV, indicating that the MD results are representative of the energy range of interest. A model for prompt, cascade induced solute atom re-solution has been derived, following the approach used to describe fission gas bubble re-solution, and the parameters for describing copper atom re-solution are provided.     

Development of thermodynamic databases in the system U–Zr–Ce–Cs–Fe–B–C–I–O–H for application to simulating phase equilibria in severe nuclear accidents

In case of severe nuclear accidents involving melt down of nuclear fuels at high temperatures, it is of considerable importance to accurately evaluate the highly-volatizing behavior of fission products (FPs) over multicomponent debris. Particularly, cesium (Cs)- and iodine (I)- bearing chemical species are regarded as notable FPs. In the present work, the authors have generated original thermodynamic databases for the system U–Zr–Ce–Cs–Fe–B–C–I–O–H featuring Cs- as well as I-bearing subsystems, which are contained in oxide, iodide, and metal (including borides and carbides) sub-databases. It has been confirmed that the phase diagrams calculated by the present set of the databases reproduce the corresponding literature data well in various kinds of subsystems of the above multicomponent system. The present set of databases has subsequently been applied to simulate phase equilibria and volatizing behavior of Cs- and I-including species, respectively, in multicomponent debris under specific temperature and atmospheric conditions corresponding to severe nuclear accidents.     

Investigations of the melting behaviour of the U–Zr–Fe–O system

During a severe nuclear accident, the UO₂ fuel rods, Zircaloy cladding, guide tubes, absorber and steel structural components inside the reactor pressure vessel overheat and a series of interactions between these elements and the steam atmosphere occur. These produce more heat in addition to the decay heat and result in a liquid corium of oxidic and metallic phases depending on the exact conditions and processes. A major systems resulting from this is the U–Zr–Fe–O system. High-temperature data for this system is important in order to be able to model these interactions. The Joint Research Centre, Institute for Transuranium Elements (JRC-ITU) has been examining the melting ranges for this system over the whole FeO range by means of a specialized laser flash technique that achieves very high temperatures and avoids crucible contamination. The melted zones were examined for their structure, composition and for estimation of the liquidus and solidus temperatures. The results showed that with FeO contents of over 20mol% there was a very large melting range that would permit long liquid cooling times and extend the relocation of fuel material within the reactor pressure vessel. Based on these results, the main phase regimes expected under severe accident conditions could be identified.     

Fundamental experiments on phase stabilities of Fe–B–C ternary systems

To understanding the control blade degradation mechanism of a boiling-water reactor (BWR), a thermodynamic database for the fuel assembly materials is a useful tool. Although the iron, boron, and carbon ternary system is a dominant phase diagram, phase relation data are not sufficient for the region in which boron and carbon compositions are richer than the eutectic composition. The phase relations of three samples such as Fe_0.68B_0.06C_0.26 (at%), Fe_0.68B_0.16C_0.16 (at%), and Fe_0.76B_0.06C_0.18 (at%) were analyzed by X-ray diffraction, scanning electron microscopy, and energy–dispersive X-ray spectrometry. The results indicate that the Fe₃(B,C) phase exists only in the intermediate region at 1273 K and that the solidus temperature widely maintains at approximately 1400 K for all three samples; these results differ from the calculated data using the previous thermodynamic database. The difference might originate from the overestimation of the interaction parameter between boron and carbon in Fe₃(B,C). Proper titling was performed using the present data.     

Cathodic polarization curves of the oxygen reduction reaction on various structural materials of boiling water reactors in high temperature–high purity water

Cathodic polarization curves of the O₂ reduction reaction were measured by using electrodes made from typical structural materials of boiling water reactors (BWRs) to evaluate the effects of kind of material on the electrochemical corrosion potential (ECP) calculation. To estimate ECPs at any region in the BWRs on the basis of the BWR environmental conditions, anodic and cathodic polarization curves should be obtained in advance under relevant conditions. The concentration of oxidants such as O₂ and H₂O₂ in coolant changes depending on the region in which they exist. As well, reduction reaction rates might differ depending on the kind of materials. In this work, the cathodic polarization curves of type 316L stainless steel (316L SS) and Alloy 182 were measured in high purity water at 553 K with different O₂ concentrations and compared with those of type 304 SS (304 SS). The results showed that the cathodic polarization curves differed depending on the kind of materials at the activation-controlled region. But, the difference in the ECP vs. O₂ concentration relationship was small when the ECPs were calculated by using both anodic and cathodic polarization curves measured on the objective material.     

Creep studies on U–7%Zr,U–7%Nb and U rich U–Nb–Zr alloys

Creep behavior of U–7%Zr, U–5%Zr–2%Nb, U–3.5%Zr–3.5%Nb, U–2%Zr–5%Nb, U–7%Nb alloys (composition in wt.%) was investigated by the impression creep technique at 630 and 700 °C at a stress of 22.2 MPa. Creep rate was found to be two orders lower in the binary U–7%Nb than the U–7%Zr alloy. In ternary U–Nb–Zr alloys, the creep rate was found decreasing drastically with the increase in Nb content.     

Application of synchrotron X-ray radiography to the study of dendritic equiaxed microstructure formation in Al–Cu alloys

The phenomena involved during equiaxed growth, which is the most common growth morphology in metal casting, are dynamic. Therefore, in situ investigation is necessary to fully analyse the microstructure formation. Synchrotron X-ray radiography has become a major tool for in situ characterization of solidification in metallic systems. This paper reports on dedicated equiaxed solidification experiments carried out at the European Synchrotron Radiation Facility (ESRF) in Grenoble-France on Al–10 wt.% Cu alloy. The analysis is based on the recorded images obtained through absorption radiography. Equiaxed growth is achieved in nearly isothermal conditions and observed continuously in real time from the early stages of solidification to the final state. The evolution of the dendrite arm lengths and the corresponding growth rates are analysed for two couples of grains in relation with their environment. This analysis suggests that the solutal interaction is dominant between equiaxed grains.     

Metallurgical characterizations of Fe–Cr–Ni–Zr base alloys developed for geological disposal of radioactive hulls

Alloy melting route is currently being considered for radioactive hulls immobilization. Towards this, wide range of alloys, belonging to Zirconium–Iron binary and Zirconium–Stainless steel pseudo-binary systems have been prepared through vacuum arc melting route. Detail microstructural characterization and quantitative phase analyses of these alloys along with interaction study between Zirconium and Stainless steel coupons at elevated temperatures identify Zr(Fe,Cr)₂, Zr(Fe,Cr), Zr₂(Fe,Cr), Zr₃(Fe,Ni), Zr₃(Fe,Cr), Zr₃(Fe,Cr,Ni), β-Zr and α-Zr as the most commonly occurring phases within the system for Zirconium rich bulk compositions. Nano-indentation studies found Zr(Fe,Cr)₂ and Zr(Fe,Cr) as extremely hard, Zr₃(Fe,Ni) as moderately ductile and β-Zr, Zr₂(Fe,Cr) as most ductile ones among the phases present. Steam oxidation studies of the alloys, based on weight gain/loss procedure and microstructural characterization of the mixed oxide layers, suggest that each of the alloys responded to the corrosive environment differently. Fe₂O₃, NiFe₂O₄, NiO, monoclinic ZrO₂ and tetragonal ZrO₂ are found to be most common constituents of the oxide layers developed on the alloys. Integrating the microstructural, mechanical and corrosion properties, ZrFeCrNi3 (Zr: 84.00, Fe: 11.20, Cr: 3.20, Ni: 1.60, in wt.%) is identified as the acceptable base alloy for disposal of radioactive hulls.     

Experimental study of the coupled thermo-hydraulic–neutronic stability of a natural circulation HPLWR

The HPLWR (high performance light water reactor) is the European concept design for a SCWR (supercritical water reactor). This unique reactor design consists of a three pass core with intermediate mixing plena. As the supercritical water passes through the core, it experiences a significant density reduction. This large change in density could be used as the driving force for natural circulation of the coolant, adding an inherent safety feature to this concept design. The idea of natural circulation has been explored in the past for boiling water reactors (BWR). From those studies, it is known that the different feedback mechanisms can trigger flow instabilities. These can be purely thermo-hydraulic (driven by the friction – mass flow rate or gravity – mass flow rate feedback of the system), or they can be coupled thermo-hydraulic–neutronic (driven by the coupling between friction, mass flow rate and power production). The goal of this study is to explore the stability of a natural circulation HPLWR considering the thermo-hydraulic–neutronic feedback. This was done through a unique experimental facility, DeLight, which is a scaled model of the HPLWR using Freon R23 as a scaling fluid. An artificial neutronic feedback was incorporated into the system based on the average measured density. To model the heat transfer dynamics in the rods, a simple first order model was used with a fixed time constant of 6 s. The results include the measurements of the varying decay ratio (DR) and frequency over a wide range of operating conditions. A clear instability zone was found within the stability plane, which seems to be similar to that of a BWR. Experimental data on the stability of a supercritical loop is rare in open literature, and these data could serve as an important benchmark tool for existing codes and models.     

Oxidation behaviour of zirconium alloys and their precipitates – A mechanistic study

The precipitate oxidation behaviour of binary zirconium alloys containing 1 wt.% Fe, Ni, Cr or 0.6 wt.% Nb was characterised in TEM on FIB prepared transverse sections of the oxide and reported in previous studies [1], [2]. In the present study the following alloys: Zr1%Cu, Zr0.5%Cu0.5%Mo and pure Zr are analysed to add to the available information. In all cases, the observed precipitate oxidation behaviour in the oxide close to the metal-oxide interface could be described either with delayed oxidation with respect to the matrix or simultaneous oxidation as the surrounding zirconium matrix. Attempt was made to explain these observations, with different parameters such as precipitate size and structure, composition and thermodynamic properties. It was concluded that the thermodynamics with the new approach presented could explain most precisely their behaviour, considering the precipitate stoichiometry and the free energy of oxidation of the constituting elements.The surface topography of the oxidised materials, as well as the microstructure of the oxide presenting microcracks have been examined. A systematic presence of microcracks above the precipitates exhibiting delayed oxidation has been found; the height of these crack calculated using the Pilling–Bedworth ratios of different phases present, can explain their origin. The protrusions at the surface in the case of materials containing large precipitates can be unambiguously correlated to the presence of these latter, and the height can be correlated to the Pilling–Bedworth ratios of the phases present as well as the diffusion of the alloying elements to the surface and their subsequent oxidation. This latter behaviour was much more considerable in the case of Fe and Cu with Fe showing systematically diffusion to the outer surface.     

Understanding the phase equilibrium and irradiation effects in Fe–Zr diffusion couples

We have studied the radiation effects in Fe–Zr diffusion couples, formed by thermal annealing of a mechanically bonded binary system at 850 °C for 15 days. After irradiation with 3.5 MeV Fe ions at 600 °C, a cross sectional specimen was prepared by using a focused-ion-beam-based lift out technique and was characterized using scanning/transmission electron microscopy, selected-area diffraction and X-ray energy dispersive spectroscopy analyses. Comparison studies were performed in localized regions within and beyond the ion projected range and the following observations were obtained: (1) the interaction layer consists of FeZr₃, FeZr₂, Fe₂Zr, and Fe₂₃Zr₆; (2) large Fe₂₃Zr₆ particles with smaller core particles of Zr-rich Fe₂Zr are found within the α-Fe matrix; (3) Zr diffusion is significantly enhanced in the ion bombarded region, leading to the formation of an Fe–Zr compound; (4) grains located within the interaction layer are much smaller in the ion bombarded region and are associated with new crystal growth and nanocrystal formation; and (5) large α-Fe particles form on the surface of the Fe side, but the particles are limited to the region close to the interaction layer. These studies reveal the complexity of the interaction phase formation in an Fe–Zr binary system and the accelerated microstructural changes under irradiation.     

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.