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Author: Adrien Couet
Publisher:
ISBN:
Category :
Languages : en
Pages :
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Although the optimization of zirconium based alloys has led to significant improvements in hydrogen pickup and corrosion resistance, the mechanisms by which such alloy improvements occur are still not well understood. In an effort to understand such mechanisms, a systematic study of the alloy effect on hydrogen pickup is conducted, using advanced characterization techniques to rationalize precise measurements of hydrogen pickup. The hydrogen pick-up fraction is accurately measured for a specially designed set of commercial and model alloys to investigate the effects of alloying elements, microstructure and corrosion kinetics on hydrogen uptake. Two different techniques to measure hydrogen concentrations were used: a destructive technique, Vacuum Hot Extraction, and a non-destructive one, Cold Neutron Prompt Gamma Activation Analysis. The results indicate that hydrogen pickup varies not only from alloy to alloy but also during the corrosion process for a given alloy. For instance Zircaloy type alloys show high hydrogen pickup fraction and sub-parabolic oxidation kinetics whereas ZrNb alloys show lower hydrogen pickup fraction and close to parabolic oxidation kinetics. Hypothesis is made that hydrogen pickup result from the need to balance charge during the corrosion reaction, such that the pickup of hydrogen is directly related to (and indivisible of) the corrosion mechanism and decreases when the rate of electron transport or oxide electronic conductivity through the protective oxide increases. According to this hypothesis, alloying elements (either in solid solution or in precipitates) embedded in the oxide as well as space charge variations in the oxide would impact the hydrogen pick-up fraction by modifying electron transport, which drives oxidation and hydriding kinetics. Dedicated experiments and modelling were performed to assess and validate these hypotheses.In-situ electrochemical impedance spectroscopy (EIS) experiments were performed on Zircaloy-4 tubes to directly measure the evolution of oxide electronic conductivity as function of exposure time. The results show that oxide electronic conductivty decreases as function of exposure time and that its variations are directly correlated to the instantaneous hydrogen pickup fraction variations. The electron transport through the oxide layer is thus altered as the oxide grows, reasons for which are yet to be exactly determined. Preliminary results also show that oxide electronic conductivty of ZrNb alloys would be much higher compared with Zircaloy-4. Thus, it is confirmed that oxide electronic conductivity is a key parameter in the hydrogen and oxidation mechanism.Because the mechanism whereby alloying elements are incorporated into the oxide layer is critical to changing [sigma]_(e^-)^ox, the evolution of the oxidation state of two common alloying elements, Fe and Nb, when incorporated into the growing oxide layers is investigated using X-Ray Absorption Near-Edge Spectroscopy (XANES) using micro-beam synchrotron radiation on cross sectional oxide samples. The results show that the oxidation of both Fe and Nb is delayed in the oxide layer compared to that of Zr, and that this oxidation delay is related to the variations of the instantaneous hydrogen pick-up fraction with exposure time. The evolution of Nb oxidation as function of oxide depth is also compatible with space charge compensation in the oxide and with an increase in oxide electronic conductivity of ZrNb alloys compared to Zircaloys.Finally, various successively complex models from the well-known Wagner oxidation theory to the more complex effect of space charge on oxidation kinetics have been developed. The general purpose of the modeling effort is to provide a rationale for the sub-parabolic oxidation kinetics and demonstrate the correlation with hydrogen pickup fraction. It is directly demonstrated that parabolic oxidation kinetics is associated with high oxide electronic conductivity and low space charges in the oxide whereas sub-parabolic oxidation kinetics is associated with lower oxide electronic conductivity and higher space charge in the oxide.All these observations helped us to propose a general corrosion mechanism of zirconium alloys involving both oxidation and hydrogen pickup mechanism to better understand and predict the effect of alloying additions on the behavior of zirconium alloys.
Author: Adrien Couet
Publisher:
ISBN:
Category :
Languages : en
Pages :
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Book Description
Although the optimization of zirconium based alloys has led to significant improvements in hydrogen pickup and corrosion resistance, the mechanisms by which such alloy improvements occur are still not well understood. In an effort to understand such mechanisms, a systematic study of the alloy effect on hydrogen pickup is conducted, using advanced characterization techniques to rationalize precise measurements of hydrogen pickup. The hydrogen pick-up fraction is accurately measured for a specially designed set of commercial and model alloys to investigate the effects of alloying elements, microstructure and corrosion kinetics on hydrogen uptake. Two different techniques to measure hydrogen concentrations were used: a destructive technique, Vacuum Hot Extraction, and a non-destructive one, Cold Neutron Prompt Gamma Activation Analysis. The results indicate that hydrogen pickup varies not only from alloy to alloy but also during the corrosion process for a given alloy. For instance Zircaloy type alloys show high hydrogen pickup fraction and sub-parabolic oxidation kinetics whereas ZrNb alloys show lower hydrogen pickup fraction and close to parabolic oxidation kinetics. Hypothesis is made that hydrogen pickup result from the need to balance charge during the corrosion reaction, such that the pickup of hydrogen is directly related to (and indivisible of) the corrosion mechanism and decreases when the rate of electron transport or oxide electronic conductivity through the protective oxide increases. According to this hypothesis, alloying elements (either in solid solution or in precipitates) embedded in the oxide as well as space charge variations in the oxide would impact the hydrogen pick-up fraction by modifying electron transport, which drives oxidation and hydriding kinetics. Dedicated experiments and modelling were performed to assess and validate these hypotheses.In-situ electrochemical impedance spectroscopy (EIS) experiments were performed on Zircaloy-4 tubes to directly measure the evolution of oxide electronic conductivity as function of exposure time. The results show that oxide electronic conductivty decreases as function of exposure time and that its variations are directly correlated to the instantaneous hydrogen pickup fraction variations. The electron transport through the oxide layer is thus altered as the oxide grows, reasons for which are yet to be exactly determined. Preliminary results also show that oxide electronic conductivty of ZrNb alloys would be much higher compared with Zircaloy-4. Thus, it is confirmed that oxide electronic conductivity is a key parameter in the hydrogen and oxidation mechanism.Because the mechanism whereby alloying elements are incorporated into the oxide layer is critical to changing [sigma]_(e^-)^ox, the evolution of the oxidation state of two common alloying elements, Fe and Nb, when incorporated into the growing oxide layers is investigated using X-Ray Absorption Near-Edge Spectroscopy (XANES) using micro-beam synchrotron radiation on cross sectional oxide samples. The results show that the oxidation of both Fe and Nb is delayed in the oxide layer compared to that of Zr, and that this oxidation delay is related to the variations of the instantaneous hydrogen pick-up fraction with exposure time. The evolution of Nb oxidation as function of oxide depth is also compatible with space charge compensation in the oxide and with an increase in oxide electronic conductivity of ZrNb alloys compared to Zircaloys.Finally, various successively complex models from the well-known Wagner oxidation theory to the more complex effect of space charge on oxidation kinetics have been developed. The general purpose of the modeling effort is to provide a rationale for the sub-parabolic oxidation kinetics and demonstrate the correlation with hydrogen pickup fraction. It is directly demonstrated that parabolic oxidation kinetics is associated with high oxide electronic conductivity and low space charges in the oxide whereas sub-parabolic oxidation kinetics is associated with lower oxide electronic conductivity and higher space charge in the oxide.All these observations helped us to propose a general corrosion mechanism of zirconium alloys involving both oxidation and hydrogen pickup mechanism to better understand and predict the effect of alloying additions on the behavior of zirconium alloys.
Author: Adrien Couet
Publisher:
ISBN:
Category : Zirconium
Languages : en
Pages : 38
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Because hydrogen ingress into zirconium cladding can cause embrittlement and limit cladding lifetime, hydrogen pickup during corrosion is a critical life-limiting degradation mechanism for nuclear fuel. However, mechanistic knowledge of the oxidation and hydrogen pickup mechanisms is still lacking. In an effort to develop such knowledge, we conducted a comprehensive study that included detailed experiments combined with oxidation modeling. We review this set of results conducted on zirconium alloys herein and articulate them into a unified corrosion theoretical framework. First, the hydrogen pickup fraction (fH) was accurately measured for a specific set of alloys specially designed to determine the effects of alloying elements, microstructure, and corrosion kinetics on fH. We observed that fH was not constant and increased until the kinetic transition and decreased at the transition. fH depended on the alloy and was lower for niobium-containing alloys. These results led us to hypothesize that hydrogen pickup during corrosion results from the need to balance the charge during the corrosion reaction such that fH decreases when the rate of electron transport through the protective oxide increases. To assess this hypothesis, two experiments were performed: (1) micro-X-ray absorption near-edge spectroscopy (?-XANES) to investigate the evolution of the oxidation state of alloying elements when incorporated in the growing oxide and (2) in situ electrochemical impedance spectroscopy (EIS) to measure oxide resistivity as a function of exposure time on different alloys. With the use of these results, we developed an analytical zirconium alloy corrosion model based on the coupling of oxygen vacancies and electron currents. Both modeling and EIS results show that as the oxide electric conductivity decreases the fH increases. These new results support the general hypothesis of charge balance. The model quantitatively and qualitatively predicts the differences observed in oxidation kinetics and hydrogen pickup fraction between different alloys.
Author: Sylvester Brian Setiadinata
Publisher:
ISBN:
Category :
Languages : en
Pages :
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Author: Aditya Shivprasad
Publisher:
ISBN:
Category :
Languages : en
Pages :
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Hydrogen pick-up of zirconium-based fuel cladding and structural materials duringin-reactor corrosion can degrade fuel component performance in existing light waterreactors (LWRs) and advanced nuclear reactors, such as the LWR-like supercriticalwater reactors (SCWRs), as the ingress of corrosion hydrogen can lead to the formationof brittle hydrides. In the boiling water reactor (BWR) environment, Zircaloy-2 fuelcladding and reactor core components, such as water rods and channel boxes, canexperience accelerated hydrogen pick-up (higher pickup fraction) at high burnup whenexposed for one extra 24-month cycle, while Zircaloy-4 components under similarconditions do not. Because the principal difference between the two alloys is thatZircaloy-2 contains nickel, this accelerated hydrogen pick-up has been hypothesizedto result from the presence of nickel and its role in the corrosion process whenincorporated into the protective oxide layer.Zircaloy-2 and Zircaloy-4 sister samples were corroded in 360 _C water and anadditional set of Zircaloy-2 samples was corroded in 400 _C steam. Total weightgain, assumed to be due mostly to oxygen, and hydrogen content were measured asfunctions of exposure time. The results indicate that Zircaloy-2 samples absorbed morehydrogen than did Zircaloy-4 samples on the basis of total weight gain (hydrogen pickupfraction), though both exhibited similar corrosion kinetics parameters. Microbeamsynchrotron radiation X-ray absorption near-edge spectroscopy (XANES) of selectedZircaloy-2 samples at the Advanced Photon Source (APS) was used to probe theoxidation states of nickel and iron in these materials and understand the evolutionof the oxidation states of these alloying elements as functions of distance from theoxide/metal interface. Result showed that a significant fraction of nickel atomsremained metallic upon incorporation in the oxide layer. In contrast, iron atomsoxidized much earlier than did nickel atoms and, in most cases, fully oxidized withinseveral micrometers from the oxide/metal interface. A general hypothesis was madethat metallic nickel in contact with the coolant may catalyze the surface reactionsinvolved in the hydrogen pick-up mechanism.To understand accelerated hydrogen pick-up of certain Zircaloy-2 samples at highburn-up, additional XANES examinations were performed on Zircaloy-2 water rodsexposed in-reactor to high burn-up in commercial BWRs. The first set of samples wascorroded in the Limerick-1 reactor, while the second set was corroded in the Dresden-2reactor. Within each set of samples, fluences, oxide thicknesses, and sample elevationswere similar, but hydrogen pick-up fractions were vastly different. In the first setof samples, oxide thicknesses ranged from 28 - 35 m, but hydrogen pick-up rangedbetween 15 and 51%. In the second set of samples, oxide thicknesses ranged between3.5 m and 16 m, but hydrogen pick-up ranged from 28 - 69%. All samples wereirradiated to fluences between 9.4 and 13.1 1021 n/cm2 for neutron energies above1 MeV. Results of XANES examinations showed a similar correlation between thedelayed oxidation of nickel and higher hydrogen pick-up of Zircaloy-2 at high burn-up.A significant fraction (greater than 30%) of nickel atoms were found to be in themetallic state in the porous oxide layer. It was hypothesized that this metallic nickelis responsible for enhancing hydrogen pick-up by catalyzing the surface reactions thataffect the overall hydrogen pick-up reaction. This would allow for easier absorptionof hydrogen into the protective oxide layer from the coolant. Ab initio modeling ofXANES of selected iron- and nickel-containing compounds was also performed andcompared to experimental results to help understand how different populations ofalloying elements oxidized upon incorporation into the oxide layer.A concurrent study of the microstructure of oxide layers formed on these sameirradiated water rods was performed to understand if there was a characteristicmicrostructure associated with accelerated hydrogen pick-up. Microbeam X-raydiffraction (XRD) at the APS was performed on water rod samples to study oxidetexture, phase content, and grain size. A similar examination was performed onsteam-corroded Zircaloy-2 to serve as a comparison. Results showed that the oxidelayers formed on these samples consisted primarily of highly-oriented monoclinic phasezirconium oxide with a small fraction of tetragonal phase oxide. Monoclinic phasegrains were shown to grow as a function of distance from the oxide/metal interface,while tetragonal phase grains remained a constant size, indicating a tetragonal-to-monoclinic phase transformation above a critical grain size of approximately 10 nm.The tetragonal phase fraction was also calculated and observed to maximize nearthe oxide/metal interface, coinciding with the appearance of the (002)-tetragonalphase diffraction reflection, which appeared to be highly-oriented and strained, butdisappeared away from the oxide/metal interface. Findings were consistent withprevious microbeam XRD examinations of oxide layers formed on Zircaloy-4 underautoclave conditions. Transmission XRD examinations were also performed on aselected steam-corroded sample to serve as an additional comparison.The observations presented in this study helped to propose a mechanism foroxidation of different populations of iron and nickel upon incorporation into theZircaloy-2 oxide layer and the effect on the hydrogen pick-up mechanism.
Author: Warren E. Berry
Publisher:
ISBN:
Category : Zirconium alloys
Languages : en
Pages : 24
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Author: Mattias Thuvander
Publisher:
ISBN:
Category : Alloys
Languages : en
Pages : 25
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The ability of a zirconium alloy to resist corrosion relies on a compromise between two opposing strategies. Minimizing the hydrogen pickup fraction (HPUF) by invoking metallic electron conduction in the barrier oxide results in rapid parabolic oxide growth. On the other hand, slow sub-parabolic barrier oxide growth, as reflected in rate limiting electron transport, may result in a high HPUF. The objective of the present study is to offer mechanistic insights as to how low concentrations of different alloying elements become decisive for the overall corrosion behavior. Combining atomistic microanalysis with first principles modeling by means of density functional theory, the speciation and redox properties of Fe and Ni towards hydrogen evolution are firstly explored. Complementary atom probe microanalysis at the metal-oxide interface provides evidence for Fe and Ni segregation to grain boundaries in Zircaloy-2 that propagates into the ZrO2 scale. Descriptors for how alloying elements in ZrO2 control electron transport as well as catalytic electron-proton recombination in grain boundaries to form H2 are determined by means of theory. The findings are generalized by further atomistic modeling, and are thus put in the context of early reports from autoclave experiments on HPUFs of zirconium with the alloying elements Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, and Nb. A shunting mechanism which combines inner and outer hydrogen evolution mechanisms is proposed. Properties of the transient zirconium sub-oxide are discussed. A plausible atomistic overall understanding emerges.
Author: George P. Sabol
Publisher: ASTM International
ISBN: 0803124996
Category : Microstructure
Languages : en
Pages : 953
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Author: Gerry D. Moan
Publisher: ASTM International
ISBN: 0803128959
Category : Nuclear fuel claddings
Languages : en
Pages : 891
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Annotation The 41 papers of this proceedings volume were first presented at the 13th symposium on Zirconium in the Nuclear Industry held in Annecy, France in June of 2001. Many of the papers are devoted to material related issues, corrosion and hydriding behavior, in-reactor studies, and the behavior and properties of Zr alloys used in storing spent fuel. Some papers report on studies of second phase particles, irradiation creep and growth, and material performance during loss of coolant and reactivity initiated accidents. Annotation copyrighted by Book News, Inc., Portland, OR.
Author: K. Kakiuchi
Publisher:
ISBN:
Category : Corrosion
Languages : en
Pages : 18
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The latest PIE results of Zry-2 and HiFi alloy (0.4 %Fe-Zry2) showed that iron addition reduces the hydrogen pick-up ratio. In order to clarify this lower hydrogen absorption mechanism, three types of experiments were carried out for both alloys: (1) Measurements of the hydrogen pick-up ratio in the pre-transition region using an autoclave. (2) CPD (Contact Potential Difference) and FBP (Flat-band Potential) measurements of oxide film using a High Temperature Kelvin system and Photocurrent system. (3) Investigation of hydrogen absorption properties by Sieverts system and corrosion properties by autoclave for intermetallic compounds simulating SPP's Fe/Cr and Fe/Ni ratio in Zry-2 and HiFi.
Author: A. M. Garde
Publisher: ASTM International
ISBN: 0803120117
Category : Nuclear fuel claddings
Languages : en
Pages : 805
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