Zircaloy Research Articles

PROTECTIVE VACUUM-ARC COATINGS ON ZIRCONIUM ALLOY FUEL CLADDING TO PREVENT CATASTROPHIC ACCIDENTS AT NUCLEAR REACTORS

The processes of vacuum arc deposition of multicomponent coatings based on Cr, Ti, Al, Fe, Y, Si, and their nitrides with a thickness of 5…9 µm for the protection of zirconium alloy fuel rods were developed. The composition, structure, and mechanical properties of the coatings were determined by electron microscopy, energy dispersive X-ray spectroscopy, X-ray diffraction analysis, and nanoindentation. Comparative studies of the protective properties of coatings during high-temperature oxidation of Zr1Nb tubes were carried out. It was found that metal coatings with a high chromium content have excellent protective properties during tests under simulated accident conditions. They are simultaneously resistant to high-temperature corrosion in air at 750…1100 °C and in water steam during thermal cycling up to 1020 °C. Hard nitride coatings, which significantly strengthen the surface of the zirconium alloy and provide good protection against oxidation in air, are less effective in water steam under thermal cycling. The coatings and processes of synthesis developed at NSC KIPT can be useful for improving fuel cladding made of zirconium alloys of the world's leading nuclear fuel producer Westinghouse Electric Company.

A novel type of core-shell structured secondary phase particles in Ge-addition modified Zircaloy-4 alloy subjected to β-phase region solution treatment

Secondary phase particles (SPPs) in a Ge-addition modified Zircaloy-4 alloy subjected to β-phase region solution treatment was investigated in detail by transmission electron microscopy to acquire a better understanding on the microstructure of this new zirconium alloy. Result revealed that a novel type of SPPs with core-shell structure were frequently observed in this alloy, in which the “core” and “shell” structures were separately identified as primitive tetragoanl Zr3Ge phase and Zr(Fe,Cr)2 Laves phase with a hexagonal close-packed (HCP) structure. The orientation relationship between the Zr3Ge core, Zr(Fe,Cr)2 shell structures and the surrounding matrix were investigated and can be approximately described as [115]Zr3Ge||[112¯6¯]Zr(Fe,Cr)2||[112¯3]α−Zr. It was strongly suggested that the gradient of Ge concentration occurred during the subsequent β-phase region annealing affected the phase stability of Zr(Fe,Cr,Ge)2 nanophase in the as-cast Ge-addition modified Zr-4 alloy, thereby causing the formation of Zr3Ge/core-Zr(Fe,Cr)2/shell microstructures. The knowledge attained from this study, shedding new lights on the formation mechanism of these novel core-shell SPPs, would also greatly benefit the understanding on the formation of similar composite precipitates in other alloys.

A study of Ni–P electroless coating modified by rare earth oxide (La2O3, Y2O3 and Ce2O3) on pre-phosphorized zirconium alloy

Electroless Ni–P coatings are successfully deposited on the surface of pre-phosphorized zirconium alloy from plating baths containing various concentrations of rare earth oxide (Re Y2O3, Ce2O3 and La2O3). The surface morphology, elementary composition, and structure of coatings are characterized by scanning electron microscopy, energy dispersive X-ray spectrometer and X-ray diffraction, respectively. Gravimetrical method is used to calculate the deposition rate. Electrochemical corrosion resistance of coatings is analyzed using electrochemical workstation. Results revealed that a small amount addition of rare earth oxides does not change the original cauliflower-like morphology and amorphous structure of coatings. When the amount of addition is moderate, the deposition rate and microhardness increase with the increment of the amount of addition, while P content decreases. However, excessive addition can lead to poisoning of the plating bath. Electrochemical corrosion data indicates that the addition of rare earth oxide in the plating solution has little influence on the corrosion resistance of the coating.

Predictive machine learning approaches for the microstructural behavior of multiphase zirconium alloys

Zirconium alloys are widely used in harsh environments characterized by high temperatures, corrosivity, and radiation exposure. These alloys, which have a hexagonal closed packed (h.c.p.) structure thermo-mechanically degrade, when exposed to severe operating environments due to hydride formation. These hydrides have a different crystalline structure, than the matrix, which results in a multiphase alloy. To accurately model these materials at the relevant physical scale, it is necessary to fully characterize them based on a microstructural fingerprint, which is defined here as a combination of features that include hydride geometry, parent and hydride texture and crystalline structure of these multiphase alloys. Hence, this investigation will develop a reduced order modeling approach, where this microstructural fingerprint is used to predict critical fracture stress levels that are physically consistent with microstructural deformation and fracture modes. Machine Learning (ML) methodologies based on Gaussian Process Regression, random forests, and multilayer perceptrons (MLP) were used to predict material fracture critical stress states. MLPs, or neural networks, had the highest accuracy on held-out test sets across three predetermined strain levels of interest. Hydride orientation, grain orientation or texture, and hydride volume fraction had the greatest effect on critical fracture stress levels and had partial dependencies that were highly significant, and in comparison hydride length and hydride spacing have less effects on fracture stresses. Furthermore, these models were also used accurately predicted material response to nominal applied strains as a function of the microstructural fingerprint.

A machine learning microstructurally predictive framework for the failure of hydrided zirconium alloys

Hydride precipitation within zirconium alloys affects ductility and fracture behavior. The complex distribution of hydrides and their interaction with defects, such as dislocations, have a significant role in crack nucleation and failure. Hence, there is substantial variability in the microstructural behavior of hydrided zirconium. A deterministic fracture model coupled to a dislocation-density based crystalline plasticity approach was used to predict failure. Deterministic simulations were used to develop a database of crack initiation for representative microstructural characteristics, such as texture, crystalline structure, hydride orientations and spacing, and hydride geometry. The machine learning (ML) analysis is based on Extreme Value Theory (EVT) and a Bayesian based Gaussian Process Regression (GPR). Fracture probability is significantly influenced by hydride orientation and dislocation-density interactions. Furthermore, surrogate reduced order models (ROM) models were used to predict the likelihood of failure. This approach provides a ML framework to predict failure at different physical scales.

Investigations of Microstructure and Mechanical Properties in Wire + Arc Additively Manufactured Niobium–Zirconium Alloy

Herein, the feasibility of the gas tungsten arc welding‐based wire + arc additive manufacturing process for fabricating thin wall structures of niobium‐1 wt% zirconium (NbZr1) alloy is investigated. Three different heat input conditions (low, medium, and high) have been selected for fabricating it. The microstructure is characterized by using optical microscopy, scanning electron microscopy, X‐ray diffraction, energy‐dispersive spectroscopy, and electron backscattered diffraction (EBSD). The microstructure shows the columnar dendritic structure elongated in the build direction. No cracks or porosity are observed in the structure. Average Vickers hardness for low, medium, and high heat input conditions are 146.6, 162.1, and 163.5 HV, respectively. There is an increasing trend of microhardness value along the deposition height, which can be attributed to the difference in secondary dendritic arm spacing and the formation of precipitates. The tensile strength of the specimen is comparable to the conventional and additively manufactured structures. EBSD analysis confirms that possible subgrains are responsible for good mechanical properties at room temperature. In the majority of the tensile samples, the failure mechanism has been identified as a ductile fracture. The mechanical characteristics fluctuate with locations in each of the thin walls, suggesting anisotropy in the deposits.

Long-Term Oxidation of Zirconium Alloy in Simulated Nuclear Reactor Primary Coolant—Experiments and Modeling

Oxidation of Zr-1%Nb fuel cladding alloy in simulated primary coolant of a pressurized water nuclear reactor is followed by in-situ electrochemical impedance spectroscopy. In-depth composition and thickness of the oxide are estimated by ex-situ analytical techniques. A kinetic model of the oxidation process featuring interfacial reactions of metal oxidation and water reduction, as well as electron and ion transport through the oxide governed by diffusion-migration, is parameterized by quantitative comparison to impedance data. The effects of compressive stress on diffusion and ionic space charge on migration of ionic point defects are introduced to rationalize the dependence of transport parameters on thickness (or oxidation time). The influence of ex-situ and in-situ hydrogen charging on kinetic and transport parameters is also studied.

Microstructural and mechanical property evolution of a nuclear zirconium-4 alloy fabricated via laser powder bed fusion and annealing heat treatment

ABSTRACT Zirconium (Zr) alloys are widely used in nuclear energy because of their excellent mechanical properties and low thermal neutron absorption cross-section. This work investigated the printability, microstructure, and mechanical properties of Zr-4 alloy additively manufactured by laser powder bed fusion (LPBF) for the first time. The effect of annealing temperature on the microstructural and the mechanical property evolution of the printed Zr-4 alloy was studied. The results exhibited that the Zr-4 alloy with a high relative density of 99.77% was obtained using optimised printing parameters. With an increase in the annealing temperature, the formed α phase of the Zr-4 alloy changed from an acicular shape to a coarse-twisted shape, and finally to an equiaxed shape. Such microstructure change endowed the alloy with a high compressive strength of 2130 MPa and compressive strain of 36%. When the annealing temperature exceeded 700°C, Zr x (Fe2Cr) compounds were precipitated, strengthening the alloy by pinning effect. These findings provide valuable guidance for the manufacture of geometrically complex Zr alloy parts for nuclear power applications.

Effects of Cr/Zircaloy-4 coating qualities for enhanced accident tolerant fuel cladding

Cr-coated zirconium alloys represent a modern approach to enhance cladding safety during accident scenarios. Two high-power impulse magnetron sputtered Cr-coated Zry-4 systems were subjected to simulated loss-of-coolant accident conditions to investigate cladding performance. The first Cr-coating (4.8 µm thick) was deposited onto Zry-4 cladding and exhibited through-thickness cracking while the second Cr-coating (6.8 µm thick) was deposited with improved deposition parameters onto polished Zry-4 and exhibited no cracking. During burst testing, the coating with a higher density of defects failed to consistently reduce oxidation and exhibited similar burst behavior as Zry-4. In contrast, the second Cr-coating reduced ZrO2 formation through formation of Cr2O3 and displayed enhanced burst temperatures by ∼80 °C compared to Zry-4. Utilizing an empirical relation for burst behavior of zirconium alloys, the 6.8 µm Cr/Zry-4 system displayed enhanced burst temperatures equivalent to an effective 0.464 mm increase in Zry-4 wall thickness, highlighting the value of continuous Cr coatings for accident scenarios.

Comparison of Raman imaging assessment methods in phase determination and stress analysis of zirconium oxide layer

This work describes Raman imaging and its data evaluation methods by using the software’s original features: built-in fitting function and K-means cluster analysis (KMC) followed by fitting in an external environment. For the first time, these methods were compared in terms of their principles, limitations, versatility, and process duration. The performed analysis showed the indispensability of Raman imaging in terms of phase distribution, phase content calculation, and stress determination. Zirconium oxide formed on different zirconium alloys under various oxidation conditions was selected as an exemplary material for this analysis. The reason for the material choice is that it is an excellent example of the application of this type of Raman analysis since both phase distribution and stress analysis in zirconium oxide are of crucial importance for the development of zirconium alloys, especially for nuclear applications. The juxtaposition of the results showed advantages and limitations of both procedures allowing a definition of the criteria for selecting the evaluation method for different applications.

Oxidation-Induced Amorphization of Niobium Precipitates in Zirconium Alloys

This article is a commentary on our previous work entitled “Reassessment of oxidation-induced amorphization and dissolution of Nb precipitates in Zr-Nb nuclear fuel cladding tube”, published in Acta Materialia in 2017. The highlight of this 2017 paper is a discovery that solid-state amorphization can occur even when the motion of atoms (or more precisely, all constituent elements of the amorphous material) is not frozen-in. Although judging from the highlight, this work is a fundamental study of amorphous materials, it was originally performed under an engineering research program on nuclear materials, irrelevant to amorphous. The inside story is revealed: how such a fundamental discovery was born out of engineering research.

Kazakhstan Study of the influence of large shear deformations and vortex flow of metal on the formation of an equiaxed ultrafine‑grained structure of the E110 zirconium alloy by the method of RSP

In this work, the task was to investigate the processes of microstructure change occurring in relatively large bulk bars under the action of large plastic deformations. Such large levels of deformation are usually achievable in high pressure twisting of small flat disks, but are difficult to achieve in large bulk bars. The method of radial shear rolling makes it possible to achieve comparable ultrahigh degrees of deformation (~45 mm/mm) in combination with the vortex flow of the metal. Sequential rolling of the E110 zirconium alloy was carried out under extreme conditions on 2 radial shear rolling mills with a total diameter reduction ε = 185 % and a maximum accumulated strain = 46 mm/mm. To assess the level of deformation and its distribution over the section, FEM modeling was carried out in Deform‑3D. The resulting structure was studied by electron microscopy (TEM/SEM). A detailed cross‑sectional study of the EBSD structure was performed with a resolution of 1 mm. A gradient structure with a predominance of an equiaxed ultrafine‑grained structure was found, which was not very pronounced compared to the use of smaller deformations.

Evolution of Cr/Cr2O3 interface in Cr-coated zirconium alloy in high temperature steam

<100> textured Cr-coating was deposited on Zr-based nuclear fuel cladding and went through steam oxidation at 1200 °C. The formed Cr2O3 layer exhibited thickening-thinning transition characteristic accompanying undulation of the Cr/Cr2O3 interface with oxidation progress. Electron microscopy characterization revealed that the convex tips of Cr/Cr2O3 interface invariably linked to grain boundaries of residual Cr-coating, which channeled the outward diffusion of Zr atoms consequently reducing Cr2O3-grains into Cr-grains. The reduction reaction at the Cr/Cr2O3 interface accounts for the interface undulation and consumption of protective oxides. This study is expected to shed light on failure mechanism of protective oxides on ATF Cr-coatings.

Effects and quantitative analyses of mechanical properties on strain limit during plastic deformation of zirconium alloys

Effects and quantitative analyses of mechanical properties on strain limit during plastic deformation of zirconium alloys

Dislocation density transients and saturation in irradiated zirconium

Zirconium alloys are widely used as the fuel cladding material in pressurized water reactors, accumulating a significant population of defects and dislocations from exposure to neutrons. We present and interpret synchrotron microbeam X-ray diffraction measurements of proton-irradiated Zircaloy-4, where we identify a transient peak and the subsequent saturation of dislocation density as a function of exposure. This is explained by direct atomistic simulations showing that the observed variation of dislocation density as a function of dose is a natural result of the evolution of the dense defect and dislocation microstructure driven by the concurrent generation of defects and their subsequent stress-driven relaxation. In the dynamic equilibrium state of the material developing in the high dose limit, the defect content distribution of the population of dislocation loops, coexisting with the dislocation network, follows a power law with exponent α≈2.2. This corresponds to the power law exponent of β≈3.4 for the distribution of loops as a function of their diameter that compares favourably with the experimentally measured values of β in the range 3≤β≤4.

Effect of irradiation damage on the tensile deformation of [formula omitted]-zirconium systems: A molecular dynamics study

Zirconium alloys are widely used as structural materials in the core of nuclear reactors. In such conditions, the continuous exposure to neutron irradiation of these alloys can have considerable effects on their microstructure and mechanical properties, which significantly impact their overall performance. In this work, we investigate the cumulative effects of irradiation damage in α-Zr using molecular dynamics (MD) simulations. To this end, displacement cascades are approximated using the computationally efficient stochastic core–shell model. Application of this approach results in structural damage which is representative of those generated using the more computationally demanding simulations of explicit collision cascades. Therefore, the examination of the damage at the atomic-scale, and the effects of long-term irradiation exposure on the material properties of zirconium can be assessed. We first evaluate the influence of radiation damage on the mechanical response of single crystal α-Zr. Subsequently, the effects in α-Zr systems including symmetric tilt grain boundaries are investigated.

Jk-integral applied to mixed-mode fatigue crack propagation and life prediction in metal welding interface

In this paper, an innovative fatigue crack propagation model of metal welding interface is investigated based on the concept of Jk-integral (k = 1, 2) in elastic–plastic material, including the prediction of crack propagation path and life. In order to validate the effectiveness of the present fatigue model, the experiments of mixed-mode interface fatigue cracks are carried out on compact tensile shear specimens of N36 zirconium alloy using DIC methods. The results demonstrate that the fatigue crack only propagates along the interface under mode I loading where the relationship between the crack growth rate da/dN and ΔJ1 (the J1-integral range per loading cycle) is logarithmic linear. As for mode I/II mixed-mode loading, interface cracks start to propagate along the interface when J1 reaches Jint-C (the interface fracture toughness). When Jtotal (the vector sum of J1 and J2) reaches Jsub-C (the substrate fracture toughness), the crack deflects to the substrate in the Jtotal direction. In this case, da/dN shows a two-stage logarithmic linear relation with ΔJtotal (the Jtotal-integral range per loading cycle) under mixed-mode loading, and the critical Jtotal at the beginning of crack deflection is approximately matching the specific value. The theoretical deflection angle (i.e. the Jtotal direction) calculated from the corresponding J1 and J2 at the beginning of crack deflection is wellconsistentwiththe actual deflection angle observed in experiments. Moreover, the model can clearly describe the two-stage process from the crack propagation along the interface to the crack deflection to the substrate under mixed-mode loading. It is concluded that the proposed fatigue model based on Jk-integral can accurately predict the fatigue crack propagation path and lifetime of metal welding interface.

Simulating the carbon distribution in zirconium alloy spent fuel cladding

The diffusivity values of C in pure Zr and in Zr containing dissolved O (Zr(O)) were determined in a temperature range of 623–1023 K for Zr and 923–1123 K for Zr(O) from depth profiles of C obtained by glow-discharge emission spectroscopy. The C diffusivity in Zr(O) decreased as the O concentration in Zr increased and that for Zr containing 15 at % O was two orders of magnitude smaller than that for pure Zr. One-dimensional numerical calculations of Fick’s diffusion equation with a Soret effect indicated various non-uniform distributions of C in a 5-mm-thick Zr matrix under a temperature gradient of 573 to 773 K for 3 years, assuming a heat of transport of − 1.5 to + 1.5 eV. Isothermal annealing at 773 K for 10 years could result in a uniform distribution, whereas dissolution of O in the interstitials of the Zr matrix would hinder C transport through the interstitials.Graphical As concentration of O increased in HIP Zr by 15 at%, the diffusivity of C decreased more than two orders of magnitude.For the positive {{Q}^{*}}_{c}, concentration of C slightly segregated at the surface of the cooler side but had maximum peaks at a middle to a higher temperature zone, and depleted at the surface of the hotter side.For the negative {{Q}^{*}}_{c}, concentration of C depleted at the surface of the cooler side and at the middle to the higher temperature zone, and highly segregated at the surface of the hotter side.

Mechanism of wear in zircaloy-4 under different loading conditions

Core components of nuclear reactor are generally made of zirconium alloys and some of them are prone to low amplitude vibration leading to material wear at contact interface. Reciprocating wear tests were conducted between zircaloy-4 (Zr-4) under dry and water-submerged conditions. To confirm the material transfer due to adhesion, wear tests were also conducted between Zr-4 and stainless steel. To understand the wear mechanism, worn surfaces were analyzed using scanning electron microscopy (SEM), field emission scanning electron microscopy (FESEM), energy dispersive spectroscopy (EDS), Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). Coefficient of friction (COF) increases with increasing fretting duration and decreases with increasing frequency and displacement amplitude. Greater COF is observed under dry condition than that under submerged condition. The wear mechanism changes from adhesion to abrasion with increasing displacement amplitude.

In-Vitro Evaluation of Photofunctionalized Implant Surfaces in a High-Glucose Microenvironment Simulating Diabetics

The present study aimed to assess the efficacy of photofunctionalization on commercially available dental implant surfaces in a high-glucose environment. Discs of three commercially available implant surfaces were selected with various nano- and microstructural alterations (Group 1-laser-etched implant surface, Group 2-titanium-zirconium alloy surface, Group 3-air-abraded, large grit, acid-etched surface). They were subjected to photo-functionalization through UV irradiation for 60 and 90 min. X-ray photoelectron spectroscopy (XPS) was used to analyze the implant surface chemical composition before and after photo-functionalization. The growth and bioactivity of MG63 osteoblasts in the presence of photofunctionalized discs was assessed in cell culture medium containing elevated glucose concentration. The normal osteoblast morphology and spreading behavior were assessed under fluorescence and phase-contrast microscope. MTT (3-(4,5 Dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide) and alizarin red assay were performed to assess the osteoblastic cell viability and mineralization efficiency. Following photofunctionalization, all three implant groups exhibited a reduced carbon content, conversion of Ti4+ to Ti3+, increased osteoblastic adhesion, viability, and increased mineralization. The best osteoblastic adhesion in the medium with increased glucose was seen in Group 3. Photofunctionalization altered the implant surface chemistry by reducing the surface carbon content, probably rendering the surfaces more hydrophilic and conducive for osteoblastic adherence and subsequent mineralization in high-glucose environment.