Zr Alloy Cladding Research Articles

High-temperature oxidation of accident tolerant Cr-coated Zr alloy cladding: Model development and validation

High-temperature oxidation of accident tolerant Cr-coated Zr alloy cladding: Model development and validation

Study on Nb interlayer as Cr[sbnd]Zr diffusion barrier layer at high-temperature

The study examined the diffusion mitigation effect and mechanical properties of the Nb interlayer prepared by magnetron sputtering as a CrZr diffusion barrier through annealing experiment, high-temperature air oxidation experiment and ring compression tests. The results show that the presence of an Nb interlayer significantly alleviates the CrZr interdiffusion, especially prevents the CrZr interdiffusion in the 1200 °C air environment and stabilizes the interface structure in the 1400 °C air environment. After undergoing air oxidation at 1200 °C, the Cr/Nb-coated samples showed the structure of Cr2O3, Cr, Cr2Nb Laves phase, Nb, (Zr, Nb) miscibility gap, β-Zr, α-Zr(O), and ZrO2 layers. During a certain period of argon annealing at 1332 °C, the increase in the thickness of the interlayer of the Cr/Nb coating, especially for samples containing micron sized Nb layer, is smaller than that of Cr-coated samples, indicating that the presence of the Nb layer is effective in alleviating the CrZr contact reaction to a certain extent. During the ring compression tests, the Cr/Nb coating is still firmly bonded to the Zr alloy cladding tube even under large deformation, and the micron-scale Nb layer releases part of the stress, resulting in short crack clusters on the surface of the coating instead of a large number of through-wall cracks. In summary, the Nb layer has the potential to be used as a diffusion barrier for CrZr interface, and the details of Nb as a diffusion barrier are discussed.

Micro-mechanical evaluations of adhesion properties for Cr-coated accident tolerant fuel cladding

The development of accident tolerant fuel (ATF) cladding has been encouraged to satisfy increased nuclear safety demands under accident conditions since Fukushima accident. Coating techniques using oxidation-resistant materials, such as Cr and Al, on existing Zr-alloy claddings have been widely applied as short-term solutions. However, coating delamination due to poor adhesion may reduce its benefits, such as low waterside corrosion/oxidation rates and fretting resistance, which necessitates accurate evaluation of coating adhesion suitability. In this study, micro-mechanical tests, micro-cantilever bending test and surface and interfacial cutting analysis system test, were conducted to qualitatively and quantitatively evaluate adhesion properties on a coated ATF cladding. In situ scanning electron microscopy-based micro-cantilever bending tests were performed using a pico-indenter equipped with a diamond flat tip. We present the results for magnetron-sputtered Cr-coated ATF claddings in detail. Micro-cantilever tests on the Cr-Zr interface revealed that deformation and failure primarily occurred in the Zr substrate, rather than at the interface. And the Cr-coated cladding showed highly adherent mechanical bonds, as evidenced by the interfacial strength and elastic modulus. SAICAS tests confirmed that peeling of the coating layer occurred only when the applied force exceeded the cutting force of the coating layer. The experimental results suggest that the Cr-coated cladding exhibited a sufficiently adherent mechanical bond to prevent delamination of the coating.

Autoclave grid-to-rod fretting corrosion behaviors of the Zr alloy fuel cladding with and without Cr coating through advanced characterization

The fretting corrosion behaviors and the associated mechanisms of Zr alloy claddings with and without Cr coating in the primary water (310 °C/15.5 MPa) of the simulated pressurized water reactor were studied and compared. The results demonstrated that the fretting corrosion resistance of Zr alloy cladding was improved significantly by the introduction of Cr coating. The increased fretting corrosion resistance of the Cr-coated Zr alloy cladding was mainly attributed for two reasons. Firstly, the metallic Cr had much greater hardness than the Zr alloy and the formation of the ultra-fine grains layer under the worn scar further increased its hardness. Secondly, the metallic Cr had much better plasticity than the ZrO2 layer, resulting in better resistance to fatigue failure.

Chromium Coatings Applied to Zr Alloy Claddings by Cathodic Arc Ion Plating: Effect of Nitrogen Inclusion on Limiting Columnar Defects

The effect of nitrogen on an anticorrosive Cr coating for the Zr alloy cladding of nuclear fuel rods is investigated. It is aimed to reduce the number of typical columnar defects generated in Cr‐based coatings when using the cathodic arc ion plating method through reactive nitrogen inclusion and improve the oxidation resistance. Under a working pressure of 5 Pa and an arc current of 60 A in a chrome target, Cr‐based coatings are fabricated under varying N2/Ar gas flow ratios and substrate biases. The growth structures, crystallography, and corrosion behavior of the coatings are characterized using scanning electron microscopy, X‐ray diffraction, and potentiodynamic polarization tests. The Cr coating exhibits a typical columnar structure with voids, pores, and columnar defects. In comparison, through nitrogen inclusion, when the nitrogen content of the Cr‐based coating is less than 15 at%, the Cr(N) coating shows a “featureless structure” similar to an amorphous structure, resulting in fewer voids and defects. Potentiodynamic polarization tests reveal that the Cr(N) coating, featuring a distorted body‐centered cubic‐Cr structure, exhibits a significantly low corrosion current in the passive region. The protection efficiency of the Cr(N) coating is calculated to be 96.9%, confirming its superior substrate protection capabilities compared with chromium nitride coatings.

Isothermal experiments on eutectic and oxidation reactions of Cr-coated Zr alloy cladding in steam at 1350 ℃: Behavior, mechanism and kinetics

The isothermal steam oxidation experiments of Cr-coated Zr alloy cladding at 1350 ℃ were conducted, and the influence of Cr-Zr eutectic reaction on the oxidation behavior was investigated. The oxidation duration, ranging from 50 s to 3600 s, covers the cases from the initial formation of eutectic liquid to the complete oxidation of the cladding. The microstructures of the coating and the substrate were characterized. The eutectic-oxidation degradation mechanism of the coating is summarized. The migration of the eutectic liquid and the evolution of Cr-rich phases in the post-oxidation eutectic structure are studied and discussed, confirming the process of ZrCr2 reacting with α-Zr(O) to form Zr3Cr3O. The degraded Cr coating and the Cr retained in ZrO2 during the migration of the eutectic liquid can hardly affect the oxidation of cladding substrate. However, the Cr-rich phases, which eventually aggregated in the radial center of the cladding, can reduce the oxidation rate.

Temperature-time dependence and mechanisms of redox reaction in Cr-coated Zr alloy cladding during steam oxidation at 900–1250 ℃

The structural integrity of the Cr coating can be significantly affected with increasing temperature during steam oxidation. Herein, the elemental diffusion, layered structure, and kinetics evolution of Cr coating in steam environment of 900–1250 ℃ for 0.5 to 3 h were studied. The oxidation behavior of Cr-coated Zr claddings associated with redox reaction between Cr2O3 and Zr were investigated in detail. Cr2O3 layer forms on Cr coating for steam oxidation at 900 ℃ and Cr-Zr diffusion layer appears at interface between Cr coating and Zr substrate when the samples are subjected to steam oxidation at 1000–1150 ℃ for 3 h. The starting time for redox reaction decreases dramatically when the temperature is higher than 1150 ℃. A prediction modal based on Fick's Law was put forward to predict the starting time of redox reaction for 1100 to 1250 ℃. The prediction is consistent with the experimental results. Furthermore, the initial process of redox reaction was investigated and analyzed in detail. Predominated by ion transportation within residual Cr layer, O2- from Cr2O3 could diffuse along the ZrO2 particles at Cr coating boundaries to the Cr-Zr layer, resulting in the reduction of Cr2O3 and partial oxidation of diffusion layer.

The effect of nanometre-scale kinetic competition on the phase selection in Zr/Si superstructure

Nano structured coating system demonstrates outstanding performance stemming from unique structure evolution. Zr/Si coating was proposed as an ideal candidate for the protection of Zr alloy cladding. However, the phase selection remains a challenge to coordinate the complex requirement under the service and accident conditions of reactor core. In this work, a phase selection mechanism for amorphous Zr-Si-O (abbr. ZSO) was elucidated by the competitive consumption of Zr layer by silicidation and oxidation, which was also scale-dependent. The energy-loss near-edge fine structure and nanoindentation were also employed to characterize the unique scale effect in the two processes. Both the oxides kinetics and elastic modulus gain indicated an immediate passivation process under the subcritical condition after the in situ phase selection of the amorphous ZSO. Ab initio molecular dynamics (AIMD) was performed to disclose the origination of depressed O migration in ZSO. The multilayer structure also shows accident tolerance potential. The interfacial phase selection provided a strategy to regulate mesoscale structure originating from nanomter interface and meets the diversified requirement for “structure-performance relationship” under the service and accident conditions.

Structure and Oxidation Behavior of a Chromium Coating on Zr Alloy Cladding Tubes Deposited by High-Speed Laser Cladding

Structure and Oxidation Behavior of a Chromium Coating on Zr Alloy Cladding Tubes Deposited by High-Speed Laser Cladding

Model development for oxidation and degradation behavior of accident tolerant Cr coating on Zr alloy cladding under high temperature steam atmosphere

The oxidation and degradation mechanisms of Cr coating on Zr alloy cladding under high temperature steam atmosphere are summarized. Six basic phenomena are considered for model development, including: oxidation of Cr, growth of ZrCr2, formation of Zr(Cr) solid solution, outward diffusion of Zr, reduction of Cr2O3, and volatilization of Cr2O3. The mathematical analysis model is established and validated, demonstrating its good capacity to predict the increase and subsequent decrease in the thickness of Cr2O3 scale, as well as the thickness changes in other individual layers. The developed model is applied in the analysis of the structure evolution of Cr coating under steady state, transient heating conditions and postulated accident scenarios.

High temperature oxidation of cold spray Cr-coated accident tolerant zirconium-alloy cladding with Nb diffusion barrier layer

Cr-Nb bilayer coatings on Optimized ZIRLO™ light water reactor (LWR) fuel cladding tubes were developed using cold spray deposition technology to increase the limiting operational temperature (i.e., binary Cr-Zr eutectic temperature, 1332 °C) of Cr-coated Zr-alloy system. The properties of the dual-layered coatings were improved by varying the type of Cr powder and post-annealing of the cold sprayed Nb layer. High temperature oxidation behavior of Cr-Nb-coated cladding at 1200 °C and 1425 °C in flowing steam were compared with those of Cr-coated cladding and uncoated cladding. At 1200 °C, both Cr-coated cladding and Cr-Nb coated cladding exhibited outstanding oxidation resistance and a continuous intermetallic compound layer (1 – 3 μm thickness) of predominantly Zr-Cr and Cr2Nb formed at the Cr/Zr and Cr/Nb interface, respectively. At 1425 °C, the Cr-coated Zr-alloy cladding showed enhanced oxidation due to rapid consumption of Cr coating by interdiffusion and liquid phase formation, and consequent destabilization of the outer protective Cr-oxide layer. However, the Nb diffusion barrier layer between the Cr coating and Zr-alloy substrate prevented the melting and the outer Cr coating provided excellent oxidation resistance. For this bi-layered system, a diffusion layer devoid of intermetallic phases was observed at the Nb/Zr interface due to the miscibility of the two metals. Reaction pathways that include both surface oxidation and interdiffusion effects within the coating-substrate system have been proposed based on experimental results.

Oxidation behavior and kinetics of magnetron-sputtered Cr-coated Zr alloy cladding in 1000–1300 ℃ steam environment

The isothermal oxidation behavior of magnetron-sputtered Cr-coated Zr-1Nb alloy cladding in 1000–1300 ℃ steam environment is studied. The phenomena at each temperature are summarized and the oxidation kinetics are discussed. Cr coating can maintain the oxidation resistance for more than 6 h at 1000 ℃ and 1100 ℃ under double-sided oxidation condition, while suffered degradation during long-duration oxidation at 1200 ℃ and 1300 ℃. After coating degradation, Zr substrate was oxidized prior to residual metallic Cr and the metallic Cr was not completely oxidized until the ZrO2 layer beneath the coating had grown to a certain thickness. Mainly due to the redox reaction between Zr and Cr2O3 and the preferential oxidation of Zr substrate, the thickness of Cr2O3 film decreased after a certain oxidation duration. The Cr coating after degradation at 1200 ℃ can still hinder the oxidation of Zr substrate to a certain extent, while that at 1300 ℃ almost lost the protective effect.

Fretting wear behavior of Zr alloy cladding tube mated with Zr alloy dimple under mixed fretting regime in simulated primary water of PWR

The fretting wear behavior of Zr alloy cladding tube under mixed fretting regime in a high-temperature pressurized water was investigated. The main wear mechanism is adhesive wear, with characters of small-scale delamination at the center of worn area and serious delamination on the worn edge. A long crack throughout the worn area and other cracks propagated towards the substrate are observed. The cross-sectional microstructure of worn area can be divided into a thick third-body layer, thin inner oxide layer and thick tribologically transformed structure layer, and their formation mechanisms are analyzed in detail. Finally, the mixed fretting regime process and the microstructural evolution during fretting wear are discussed.

LOCA advanced failure criteria: Implementation in the mechanistic fuel performance code and practical applications

The advanced Chung-Kassner criteria for the capability of fuel rods to withstand thermal shock during LOCA reflooding and to withstand post-quench fuel handling, as well as the complementary cladding embrittlement criteria of KAERI and KIT, are compared to the traditional LOCA safety criterion of 17% ECR oxidation. The advanced failure criteria were implemented in the mechanistic thermomechanical code SVECHA/QUENCH (S/Q) and successfully applied to predict fuel rod failure behaviour in postulated LOCA scenarios and high temperature (1400 °C) pre-oxidation and quenching tests performed with fresh and irradiated fuel rod simulators. The extension of the advanced failure criteria to coated Zr alloy claddings is discussed.

Establishment of fretting maps of Zr alloy cladding tube mated with Zr alloy dimple in simulated primary water of pressurized water reactor

Fretting maps of Zr alloy tube under grid-to-rod configuration in high temperature pressurised water were established. Running condition fretting map shows three fretting regimes, namely, partial slip regime, mixed fretting regime and gross slip regime. According to material response fretting map, the wear mechanism under partial slip regime is adhesive wear with the character of delamination of local regions, both adhesive wear and cracking are found for mixed fretting regime, while abrasive wear and severe delamination for gross slip regime. Compared with the results at room temperature, the mixed fretting regime range increases obviously at high temperature, while friction coefficient at high temperature is lower. In addition, fretting wear behaviour of Zr alloy tube under different fretting regimes is discussed.

Fretting wear behaviour of Zr alloy cladding tube under partial slip regime with different duration in simulated primary water of PWR

The fretting wear behaviour of Zr alloy cladding tubes under partial slip regime in a high temperature pressurised water was investigated. The fretting regime remained unchanged with the increasing fretting time and hence both the wear volume and depth of the worn scars formed on Zr cladding tubes changed slightly. The wear mechanism was delamination at some local regions, resulting in a slight damage. Third-body layer, oxide layer, tribologically transformed structure layer, and general deformation layer were observed form the cross-section of the worn scar and their formation mechanisms were analysed in detail. Moreover, from the metal/oxide interface to surface, the main oxide transitioned from tetragonal-ZrO2 to monoclinic-ZrO2. Finally, the partial slip regime process and the microstructural evolution during fretting wear were discussed.

Isothermal experiments on steam oxidation of magnetron-sputtered chromium-coated zirconium alloy cladding at 1200 ℃

The isothermal steam oxidation behavior of chromium-coated zirconium alloy cladding at 1200 ℃ is studied. The cladding substrate is Zr-1 Nb alloy with outer diameter of 9.5 mm and thickness of 0.57 mm. The Cr coating is prepared by magnetron sputtering method with thickness of about 11 µm. The oxidation durations are 300 s~14,400 s. After oxidation, blisters are formed on the surface due to the compressive stress in coating, and the coating surfaces are porous or exhibit a morphology with ravines and granular crystals. A four-layer structure (Cr2O3, Cr, Zr(Cr,Fe)2 and Zr from outside to inside) is formed in the early stage of oxidation. However, Zr4+ can migrate into Cr coating through grain boundaries and reacts with Cr2O3. The redox reaction between Zr and Cr2O3 leads to the coating failure, resulting in the oxidation of Zr alloy substrate. Due to selective oxidation, a certain thickness of Cr coating is retained during the thickening of outer ZrO2. The emergence of outer ZrO2 layer is significantly delayed by Cr coating, and Cr coating is able to reduce the growth rate of outer ZrO2 layer. Therefore, Cr coating has a protective effect in 1,200 ℃ steam environment and can improve the oxidation resistance of Zr alloy cladding.

Study of structural stability at high temperature of pseudo-single tube with double layer as an alternative method for accident-tolerant fuel cladding

The multilayered swaging and drawing process using raw Zr-alloy cladding is very simple, can be performed at room temperature (15-25 ℃), and enables mass productivity. In this study, we developed a pseudo-single tube with double layers (PSTD), using a thin stainless-steel (SS316L) tube and raw Zr-alloy cladding, which is not easily accessible to a coating process. The PSTD specimens were oxidized at 900 ℃, 1000 ℃, 1100 ℃, and 1200 ℃ for 60 min at each temperature in dry air. The interface between the SS316L tube and Zr-alloy cladding was investigated using optical microscopy and scanning electron microscopy. The oxidation resistance was proven excellent through energy dispersive spectroscopy and mass change results of the oxidized PSTD specimens even though 5-6 µm oxide layer was formed at the interface. Based on these results, it was concluded that an improved accident tolerant fuel (ATF) cladding can be produced using the swaging and drawing process.

An experimental study related to axial constraint of fuel rod under LOCA conditions

The fracture threshold of the fuel decreases if the oxidized Zr alloy cladding is strongly constrained by the spacer grid during quenching in a loss-of-coolant accident. Therefore, the estimation of realistic levels of the axial constraint has been a subject of significant interest on fuel safety. In this study, a test assembly consisting of a PWR-type simulated fuel segment and a 3 × 3 grid piece was heated in steam, cooled, and quenched, and the axial constraint force on the fuel segment was measured. The constraint force of the Zircaloy grid gradually decreased with temperature. Once the Zircaloy grid was heated to > 1060 K, the reduced constraint force had difficulty recovering, and thus the maximum constraint force during cooling and quenching was < 10 N. The constraint force was clearly reduced at > 1070 K during the tests with the Inconel grid. However, the reduced constraint force partially recovered during cooling. As a result, the maximum constraint force during cooling and quenching was 20–50 N for the Inconel grid. In conclusion, oxidation, ballooning, rupture, or eutectic formation would not generally cause an extremely strong constraint, as predicted by previous studies, at the grid position.

Enhanced thermal transport and corrosion resistance by coating vertically-aligned graphene on zirconium alloy for nuclear reactor applications

Owing to the requirement of higher efficiency, safety reliability and lower maintenance cost for nuclear reactors, the performance improvement of zirconium (Zr) alloy cladding coating materials is becoming increasingly significant. Graphene, with its inherent properties like ultrahigh thermal conductivity, excellent chemical stability, and super-hydrophobicity, has demonstrated enormous potential in the field of corrosion prevention and thermal management, indicating its promising application in cladding materials. Herein, plasma-enhanced chemical vapor deposition (PECVD) was employed for growing vertically-aligned graphene (VG) array on Zr alloy (ZA) substrates. In the deposition process, C2H4 gas was introduced into the reactor as the carbon source and the reactor was kept at a growth temperature range of 650–800 °C, which is 300–400 °C lower than that for traditional graphene synthesis using thermal chemical vapor deposition (CVD). The effects of other growth experimental parameters such as gas flow ratio, radio-frequency power (RF-power), growth temperature, reaction pressure and deposition time were systematically investigated. The corrosion resistance and thermal transport performance of vertical graphene enhanced Zr alloy (VGZA) were studied by using electrochemical corrosion method and heat dissipation test system. The results showed that the samples loaded with vertical graphene could effectively improve the corrosion resistance and heat transport property.