Oxidation Of Zircaloy-4 Research Articles

A thermogravimetric comparative study of the high temperature steam oxidation of bare and pre-oxidized Zircaloy-4, M5Framatome and Optimized ZIRLO™

For Loss of Coolant Accident (LOCA) analysis, high-temperature oxidation kinetics in steam of Zircaloy-4 and the two Nb-bearing alloys used in French Pressurized Water Reactors (PWRs), M5Framatome11M5 and M5Framatome are trademarks or registered trademarks of Framatome or its affiliates, in the USA or other countries. and Optimized ZIRLO™22Optimized ZIRLO is a trademark or registered trademark of Westinghouse Electric Company LLC, its affiliates and/or its subsidiaries in the United States and may be registered in other countries throughout the world. All rights reserved. Unauthorized use is strictly prohibited. Other names may be trademarks of their respective owners., have been studied using Thermo-Gravimetric Analysis (TGA) technique in the temperature range of 900–1200 °C. A specific test procedure allowing the use of a high steam flow rate and good steam supply to the whole specimen surface, as well as fast heating of the specimen, has been developed. Clean kinetic data, not affected by steam starvation, were obtained on the bare, as-received alloys. For as-received claddings, oxidation kinetics are comparable and close to parabolic for the three alloys at 1100 °C and above. Below 1100 °C, a significant deviation from the parabolic kinetic regime was observed, and M5 Framatome oxidizes slower than Zircaloy-4 and Optimized ZIRLO. Thanks to the fast-heating protocol, the influence of pre-oxidation, simulating corrosion in normal reactor operation, could be investigated, avoiding evolution of the pre-oxidation scale during a slow, long heating phase. Pre-oxidized Zircaloy-4 and M5 Framatome were investigated. A strong protective effect is observed below 1200 °C, while at 1200 °C, the pre-oxidation layer appears to be much less protective, possibly due to the monoclinic to tetragonal zirconia phase transformation.

High Temperature Oxidation of Zircaloy-4 Under Conditions Simulating a Loss of Cooling Accident in Spent Fuel Pools Examined with Raman Imaging and 18o Tracer Techniques

Since the Fukushima accident, an increased attention is continuously paid to the vulnerability of the Spent Fuel Pools (SFPs). In SFPs, for different reasons, degradation of the fuel rods induced by oxidation in air-steam mixtures after dewatering is a major safety concern: (i) the cladding material is the only barrier left against fission product dissemination, (ii) the heat released by the oxidation reactions can overcome the residual decay heat of the fuel and may become the main driving force to the accident escalation, (iii) hydrogen production is a strong concern because of the presence of steam, (iv) presence of air in the atmosphere is known to be an aggravating factor because of the “catalytic” role of nitrogen on the oxidation mechanism.In this study, high temperature oxidation tests in oxygen, air and air - steam atmospheres were performed with Zircaloy-4 cladding tubes and plates specimens. Because spent fuels are concerned, the presence of the corrosion oxide scale formed in reactor during in-service operation was also considered. The tests described in this study were restricted to the 700 – 950°C range, and mostly performed at 850°C. Raman imaging was used to examine the specimens after the oxidation tests, either directly at the oxide surface, or on metallographic preparations for cross-section examinations.We first used new opportunities given by Raman imaging to analyze, at least qualitatively, the structural and mechanical phenomena involved in this particular corrosion phenomena. Indeed, this method can provide information on the nature of phases that are present in the scale and on the stress level with sub-micron spatial resolution. The different methods used to extract this information that is present in the Raman data will be described and discussed.Moreover, isotopic substitution was also considered. Actually, Raman spectroscopy has also the ability to quantitatively extract the 18O distribution in a zirconia oxide scale after a two stage experiment with a fair accuracy. Raman imaging gave clear evidence for different characteristic distributions of 18O in the scales. Some of them have been correlated with the development of cracks and porosity in the oxide which allows the corroding medium to penetrate locally in the scales. Nitrogen appears to have no or limited influence on the oxygen diffusion, but is observed to reach the metal/oxide interface much faster than oxygen. Thus, we also emphasize the potential of this method to investigate the 18O distribution in such complex corrosion scales.

Study of the evolution of stresses and associated mechanisms in zirconia growing at high temperature on Zircaloy-4 by use of synchrotron radiation

The present work studied the evolution of stresses and associated mechanisms, related to viscoplastic behavior during relaxation, in the zirconia layer formed during the oxidation of Zircaloy-4 under a He/O2 mixture, at temperatures of 700 °C, 800 °C and 900 °C. Measurements by X-ray diffraction are performed in-situ under synchrotron radiation during oxidation to determine the evolutions of phases and stresses in the oxide layer with time. The results show that the zirconia formed contains a mixture of monoclinic and tetragonal phases. The proportion of the tetragonal phase depends on the oxidation temperature and decreases during oxidation. In order to better understand the influence of this evolution and localize the tetragonal phase, measurements by Raman spectroscopy have also been performed. These two phases are subjected to compressive stresses in directions perpendicular to the oxide layer growth direction. These stresses depend on temperature and relax during oxidation. Two mechanical models either considering the zirconia phases independently or considering an equivalent homogeneous oxide are proposed to describe the evolution of these stresses, considering that it is due to oxide viscoplasticity. The model parameters are analysed to discuss the mechanisms of viscoplastic flow in the oxide. Numerical values for the viscoplastic parameters of the model as well as for the corresponding activation energy are therefore provided.

Oxidation properties and microstructure of a chromium coating on zircaloy-4 fuel cladding material applied by atmospheric plasma spraying

To achieve a full protective coat on welding joints at both ends of zircaloy cladding tubes, we studied the rapid deposition of a thick chromium coating on zircaloy-4 cladding tubes by atmospheric plasma spraying. Microstructure, mechanical properties and high-temperature oxidation tests of chromium coatings were also studied. The initial screening oxidation test of the specimen was carried out at 1100 °C, 1200 °C and 1300 °C in dry air, and the oxidation properties of the specimen were explained. The nanohardness and modulus of the sprayed chromium coating were higher than those of the zircaloy-4 substrate. The nitride phase of Cr2N was formed after the high-temperature oxidation test. After oxidation at 1300 °C, there were multilayer structures: surface Cr2O3, residual Cr coating, thin (Zr,Cr)O2 layer, Cr-Zr layer and zircaloy substrate. There were bubble folds on the surface of the chromium coating and voids in the Cr2O3 layer. The oxidation test showed that the chromium coating can effectively slow the severe oxidation of zircaloy-4 cladding tubes. In summary, an APS-thick chromium coating has the potential to improve the oxidation resistance of zircaloy-4 cladding tubes in light water reactors.

Plasma electrolytic oxidation of zircaloy-4 in a mixed alkaline electrolyte

Characteristics of coating resulted from plasma electrolytic oxidation (PEO) of zircaloy-4 in an electrolyte containing phosphate, silicate, and hydroxide were investigated. The mixed electrolyte is proposed to reduce power consumption. PEO was conducted in a DC mode at a current density of 30, 50, and 70 mA/cm2. Plasma was generated at a relatively low breakdown voltage of 150 V, independent of the current density. The resulting coating was composed of amorphous SiO2 and crystalline monoclinic (m) and tetragonal (t) ZrO2 phases. XRD analysis revealed that the porous outer layer developed at 30 mA/cm2 was dominated by the m phase. The percentage of the m phase decreased while the t phase increased with increasing current density. Therefore, the coating formed at 70 mA/cm2 showed the highest abrasive wear resistance, three times higher than the substrate. Similarly, the polarization curve and electrochemical impedance spectroscopy showed that the highest corrosion resistance, more than two orders of magnitude higher than the substrate, was obtained in the coated-70 mA/cm2 specimen. The use of low power and a mixed electrolyte is beneficial in producing PEO coating with high wear resistance and excellent anti-corrosion characteristic.

High temperature Zircaloy-4 oxidation in water vapour-containing environments examined with Raman imaging and labelled oxygen

High Temperature oxidation of Zircaloy-4 was studied in N2–O2–H2O gas mixtures using labelled 18O2, thermogravimetry, Raman imaging and mass spectrometry. Samples were either exposed to N2–18O2–H2O gas mixtures, or two-stage oxidised first in 18O2, then in H216O - N2 or 16O2 - N2 atmospheres. In the pre-transition, diffusion-controlled regime, mostly steam contributes to the zirconia formation, while in the nitrogen-catalysed post transition regime both oxygen and steam contribute to the oxidation. These results were tentatively explained by competitive adsorption of oxygen and steam at the gas/solid interfaces, and the formation of porosity in the oxide layers.

Breakaway oxidation of zirconium alloys exposed to steam around 1000 °C

The breakaway oxidation of Zircaloy-4 and M5Framatome alloys oxidized in flowing steam around 1000 °C was studied. The influences of oxidation temperature between 950 and 1050 °C, one-side versus two-side oxidation and chemical variations around the nominal composition of M5Framatome were investigated. Breakaway oxidation is sensitive to the oxidation temperature, the material and the oxidation type. A phenomenological mechanism for breakaway oxidation is postulated on the basis of microstructural and microchemical analyses.

High Temperature Oxidation of Zircaloy-4 Under Conditions Simulating a Loss of Cooling Accident in Spent Fuel Pools Examined with Raman Imaging and 18O Tracer Techniques

Since the Fukushima accident, an increased attention is continuously paid to the vulnerability of the Spent Fuel Pools (SFPs). In SFPs, for different reasons, degradation of the fuel rods induced by oxidation in air-steam mixtures after dewatering is a major safety concern: (i) the cladding material is the only barrier left against fission product dissemination, (ii) the heat released by the oxidation reactions can overcome the residual decay heat of the fuel and may become the main driving force to the accident escalation, (iii) hydrogen production is a strong concern because of the presence of steam, (iv) presence of air in the atmosphere is known to be an aggravating factor because of the “catalytic” role of nitrogen on the oxidation mechanism. In this study, high temperature oxidation tests in oxygen, air and air - steam atmospheres were performed with Zircaloy-4 cladding tubes and plates specimens. Because spent fuels are concerned, the presence of the corrosion oxide scale formed in reactor during in-service operation was also considered. The tests described in this study were restricted to the 700 – 950°C range, and mostly performed at 850°C. Raman imaging was used to examine the specimens after the oxidation tests, either directly at the oxide surface, or on metallographic preparations for cross-section examinations. We first used new opportunities given by Raman imaging to analyze, at least qualitatively, the structural and mechanical phenomena involved in this particular corrosion phenomena. Indeed, this method can provide information on the nature of phases that are present in the scale and on the stress level with sub-micron spatial resolution. The different methods used to extract this information that is present in the Raman data will be described and discussed. Moreover, isotopic substitution was also considered. Actually, Raman spectroscopy has also the ability to quantitatively extract the 18O distribution in a zirconia oxide scale after a two stage experiment with a fair accuracy. Raman imaging gave clear evidence for different characteristic distributions of 18O in the scales. Some of them have been correlated with the development of cracks and porosity in the oxide which allows the corroding medium to penetrate locally in the scales. Nitrogen appears to have no or limited influence on the oxygen diffusion, but is observed to reach the metal/oxide interface much faster than oxygen. Thus, we also emphasize the potential of this method to investigate the 18O distribution in such complex corrosion scales.

Effect of a pre-oxide on the high temperature steam oxidation of Zircaloy-4 and M5Framatome alloys

The effect of a pre-existing oxide layer formed at low temperature on the subsequent oxidation in steam at high temperature was studied for Zircaloy-4 and M5Framatome alloys. Various pre-oxidation conditions (air at 550 °C, steam at 415 °C, static and flowing water at ∼350 °C), pre-oxide thicknesses (3–60 μm) and high temperature oxidation conditions (850–1200 °C, 55–9500s) were investigated. Pre-oxide has an effect on the oxidation kinetics and the hydrogen uptake at high temperature, depending on the pre-oxide thickness, the pre-oxidation conditions, the alloy and the high temperature oxidation conditions.

Oxidation of Zircaloy-4 in steam-nitrogen mixtures at 600–1200 °C

High-temperature oxidation of zirconium alloys in steam-nitrogen atmospheres may be relevant during various nuclear accident scenarios. Therefore, isothermal oxidation tests with Zircaloy-4 in steam-nitrogen mixtures have been performed at 600, 800, 1000, and 1200 °C using thermogravimetry. The gas compositions were varied between 0 and 100 vol% nitrogen including 0.1 and 90 vol%. The strong effect of nitrogen on the oxidation kinetics of zirconium alloys was confirmed in these tests in mixed steam-nitrogen atmospheres. Even very low concentrations of nitrogen (starting from less than 1 vol%) strongly increase reaction kinetics. Nitrogen reduces transition time from protective to non-protective oxide scale (breakaway). The formation of zirconium nitride, ZrN, and its re-oxidation is the main reason for the highly porous oxide scales after transition. The results of this study have shown the safety relevant role of nitrogen during severe accidents and, more generally, suggest the need of using well controlled gas atmospheres for experiments on oxidation of zirconium alloys.

High-Temperature Oxidation of Zircaloy-4 in Air Studied with Labeled Oxygen and Raman Imaging

High-temperature oxidation of zirconium-based cladding materials is expected to be the primary cause of the fuel assemblies’ degradation in spent fuel storage pool loss-of-cooling accidents. Here, high-temperature Zircaloy-4 oxidation has been studied through two-stage oxidation experiments using 18O isotope. Particular attention is paid to the effect of a low-temperature pre-oxidation scale which aims to simulate the corrosion scale existing on spent fuel cladding. Raman imaging was used to investigate the 18O distribution in the scales exposed either to 18O2 or to 18O2 + N2 atmosphere at high temperature. Results were assessed against more conventional SIMS mapping. The use of the 18O isotope tracer technique associated with micro-Raman mapping of the scales is demonstrated to be a powerful method to investigate the transport properties in the scales and will help to gain understanding of the kinetic differences between different pre-oxides.

Study of the response of Zircaloy- 4 cladding to thermal shock during water quenching after double sided steam oxidation at elevated temperatures

This study investigates the failure of embrittled Zircaloy-4 cladding in a simulated loss of coolant accident condition and correlates it with the evolved stratified microstructure. Isothermal steam oxidation of Zircaloy-4 cladding at high temperatures (900–1200 °C) with soaking periods in the range 60–900 s followed by water quenching was carried out. The combined oxide + oxygen stabilized α-Zr layer thickness and the fraction of the load bearing phase (recrystallised α-Zr grains + prior β-Zr or only prior β-Zr) of clad tube specimens were correlated with the %ECR calculated using Baker-Just equation. Average oxygen concentration of the load bearing phase corresponding to different oxidation conditions was calculated from the average microhardness using an empirical correlation. The results of these experiments are presented in this paper. Thermal shock sustainability of the clad was correlated with the %ECR, combined oxide+α-Zr(O) layer thickness, fraction of the load bearing phase and its average oxygen concentration.

The initial oxidation of Zircaloy-4 in lithiated water under autoclave conditions as examined by TEM

TEM thin foil specimens prepared using Zircaloy-4 coarse-grained samples were corroded by autoclaving at 250°C and 280°C in lithated water for a short time exposure, and the initial oxidation process before the formation of ZrO2 was investigated by TEM. The results show a variation of the crystal structure along with the increase of oxygen contents at the initial oxidation stage. When the Zr/O atomic ratio reached 6–7, 3 and 1, a commensurable long period super-lattice 9a0-2H and sub-oxides with hcp and fcc ordered structure were formed. When the Zr/O atomic ratio was 0.8, monoclinic ZrO2 was detected.

High-Temperature Oxidation of Zircaloy-4 in Oxygen–Nitrogen Mixtures

Isothermal oxidation experiments with cladding tube segments of Zircaloy-4 (Zr-1.3%Sn) in oxygen–nitrogen model mixtures were performed at 800, 1000, and 1200 °C for 6, 1 h, and 15 min, respectively. The gas compositions varied between 0 and 100 vol% nitrogen including 1 and 99 vol%. A strong accelerating effect of nitrogen on the oxidation kinetics was seen for a wide range of boundary conditions. At 800 °C, oxidation in all mixtures with 1–99 % nitrogen resulted in higher reaction rates compared to the pure gases, especially after transition from protective to non-protective oxide scales. At 1000 and 1200 °C, only starvation of oxygen in mixtures with low oxygen contents resulted in lower rates compared to pure oxygen. The oxide scales formed in the mixtures were very porous due to the formation of zirconium nitride at the metal-oxide interface and its oxidation during continuing reaction. The extension of the oxide-nitride zone increased with temperature and with nitrogen content in the gas mixture. Nitrogen seems also to affect the pre-transition reaction kinetics. The mechanism of the faster oxidation kinetics of zirconium alloys in atmospheres containing nitrogen will be discussed in this paper.

Observation of the effect of surface roughness on the oxidation of Zircaloy-4

Zirconium alloys are used as fuel cladding in light water reactors. Surface preparation techniques that might impair oxidation resistance would limit the lifetime of the cladding and therefore prevent higher burn-up of fuels. The work presented here investigates the impact of surface roughness on the oxidation kinetics of Zircaloy-4 in 400°C steam for exposures up to 275 days. The work shows well defined differences in oxidation kinetics between sample preparations following ∼84 days oxidation. Scanning electron microscopy (SEM) and image analysis of samples in cross section allows for an assessment of the variation in surface roughness between samples at different stages of oxidation.

In situ high temperature oxidation analysis of Zircaloy-4 using acoustic emission coupled with thermogravimetry

Zircaloy-4 oxidation behavior at high temperature (900°C), which can be reached in case of severe accidental situations in nuclear pressurised water reactor, was studied using acoustic emission analysis coupled with thermogravimetry. Two different atmospheres were used to study the oxidation of Zircaloy-4: (a) helium and pure oxygen, (b) helium and oxygen combined with slight addition of air. The experiments with 20% of oxygen confirm the dependence on oxygen anions diffusion in the oxide scale. Under a mixture of oxygen and air in helium, an acceleration of the corrosion was observed due to the detrimental effect of nitrogen. The kinetic rate increased significantly after a kinetic transition (breakaway). This acceleration was accompanied by an acoustic emission activity. Most of the acoustic emission bursts were recorded after the kinetic transition (post-transition) or during the cooling of the sample. The characteristic features of the acoustic emission signals appear to be correlated with the different populations of cracks and their occurrence in the ZrO2 layer or in the α-Zr(O) layer. Acoustic events were recorded during the isothermal dwell time at high temperature under air. They were associated with large cracks in the zirconia porous layer. Acoustic events were also recorded during cooling after oxidation tests both under air or oxygen. For the latter, cracks were observed in the oxygen enriched zirconium metal phase and not in the dense zirconia layer after 5h of oxidation.

Oxidation of Zircaloy-4 during in situ proton irradiation and corrosion in PWR primary water

Abstract

A study into stress relaxation in oxides formed on zirconium alloys

Pressurised and boiling water reactors contain zirconium alloys, which are used as nuclear fuel cladding. Oxidation of these alloys, and the associated hydrogen pick-up, is a limiting factor in the lifetime of the fuel. To extend the burn-up of nuclear fuel requires control of the oxidation, and therefore development of a mechanistic understanding of the cladding corrosion process. Synchrotron X-ray diffraction (S-XRD) has been used to analyse oxide layers formed during in-situ air oxidation of Zircaloy-4 and ZIRLO™. Analysis shows that as the oxide thickness increases over time there is a relaxation of the stresses present in both the monoclinic and meta-stable tetragonal phases, and a reduction in the tetragonal phase fraction. To better understand the mechanisms behind stress relaxation in the oxide layer, finite element analysis has been used to simulate mechanical aspects of the oxidation process. This simulation was first developed based on stress relaxation in oxides formed in autoclave, and analysed ex-situ using S-XRD. Relaxation mechanisms include creep and hydrogen-induced lattice strain in the metal substrate and creep in the oxide layer. Subsequently the finite element analysis has been extended to stress relaxation observed by in-situ S-XRD oxidation experiments. Finite element analysis indicates that the impact of creep in the oxide is negligible, and the impact of both creep and hydrogen-induced lattice strain in the metal substrate metal is small. The implication is that stress relaxation must result from another source such as the development of roughness at the metal–oxide interface, or fracture in the oxide layer.

In-reactor oxidation of zircaloy-4 under low water vapor pressures

Complementary in- and ex-reactor oxidation tests have been performed to evaluate the oxidation and hydrogen absorption performance of Zircaloy-4 (Zr-4) under relatively low partial pressures (300 and 1000Pa) of water vapor at specified test temperatures (330 and 370°C). Data from these tests will be used to support the fabrication of components intended for isotope-producing targets and provide information regarding the temperature and pressure dependence of oxidation and hydrogen absorption of Zr-4 over the specified range of test conditions. Comparisons between in- and ex-reactor test results were performed to evaluate the influence of irradiation.

Growth Kinetics of Zr-4 in Sodium Bisulphite: Solvent Studies

Anodization of zircaloy-4 in 0.1 M sodium bisulphite has been carried out. Kinetics of anodic oxidation of zircaloy-4 has been studied at a constant current density of 8 mA.cm -2 and at room temperature. The plots of formation voltage vs. time, reciprocal capacitance vs . time and reciprocal capacitance vs . formation voltage were drawn. From these plots, formation rate, current efficiency and differential field were calculated. The addition of solvent (Ethylene glycol) showed better kinetic results. For 20, 40, 60 and 80% aquo-glycolic media, the dielectric constant values are low leading to the marked improvement in the kinetics.