Terminal Solid Solubility Of Hydrogen Research Articles

Effect of radial hydrides on fracture behavior of Zr-2.5Nb pressure tube material

The fracture behavior of Zr-2.5Nb pressure tube (PT) material containing radial hydrides with hydrogen concentrations between 30 and 300wppm was evaluated between 25 and 300 °C covering lower-shelf, transition and upper-shelf temperatures. The fracture toughness vs. temperature curves showed S curve behavior. In the lower-shelf region, specimens failed in a brittle manner with unstable crack growth, whereas in the upper-shelf region, specimens showed ductile fracture with stable crack growth. The S curve shifted to higher temperatures with hydrogen concentration, with saturation of lower-shelf, transition and upper-shelf temperatures between 100 and 300wppm. The fracture behavior was rationalized in terms of the strength of the Zr-2.5Nb PT material, hydrogen terminal solid solubility during dissolution, stress in the hydride during hydride dissolution, the hydride continuity coefficient and the strength of the embedded hydrides.

Behavioral Analysis of Hydrogen in Metals under the Effect of H2S Corrosion Using a Layer-stripping Micro-hardness Technique

In order to study the distribution behavior of hydrogen in metals under the condition of H2S corrosion, a layer-stripping micro-hardness test was designed to analyze the hydrogen distribution along the depth of hydrogen-charged 45 high-quality structural carbon steel at three different hydrogen sulfide concentrations and four corrosion periods in this study. The results show that there is a terminal solid solubility of hydrogen in the metal for hydrogen sulfide solutions over various concentrations and corrosion periods. A hydrogen-saturated layer is produced by hydrogen diffusing through the metal from an unsaturated state to a fully saturated state. The hydrogen-saturated layer is not affected by the concentration of the corrosion, but its thickness increases as the corrosion period increases. In this way, we established a new hydrogen diffusion model in metals.

Terminal solid solubility of hydrogen of optimized-Zirlo and its effects on hydride reorientation mechanisms under dry storage conditions

TSSD, TSSP, and TSSP2 of hydrogen for optimized-Zirlo (Zirlo™) alloy were measured by DSC in the range of 53–457 wppm. Solvus curves of the TSSs are derived and proposed in this study. The results show that the temperature gap between TSSD and TSSP solvus lines of Zirlo™ are similar to those of other zirconium alloys, but another gap between the TSSD and TSSP2 line differs significantly. In particular, the TSSP2 solvus line becomes closer to the TSSD solvus line than to TSSP unlike Zircaloy-4, so ΔTTSSD-TSSP2 of Zirlo™ decreases with decreasing temperature. This implies that hydride reorientation can take place more significantly in Zirlo™ than in Zircaloy-4, and the limited temperature variation of 65 °C during the vacuum drying and the cooling-down process may not be sufficient to prevent the triggering of hydride reorientation in Zirlo™ cladding under long-term dry storage.

Variance-based sensitivity analysis applied to hydrogen migration and redistribution model in Bison. Part I: Simulation of historical experiments

In a light water nuclear reactor environment, zirconium-alloy cladding undergoes corrosion that introduces hydrogen into the cladding. Hydrogen being slightly soluble in zirconium means that the terminal solid solubility of hydrogen is quickly reached. This leads to hydrogen precipitation into crystals of zirconium hydride. These hydrides decrease the ductility of the cladding, which increases the chance of cladding failure, especially during dry storage or transportation events, depending on the temperature and stress conditions. In this work, we tested the existing hydrogen migration and redistribution model implemented in the Bison fuel performance code by comparing Bison predictions of hydrogen distribution in various temperature profiles against historical published experimental data.Using 1D simulations of hydrogen evolution in zirconium-alloy, we demonstrated the accuracy of the existing model in the case of a sharp temperature gradient profile. We also performed a sensitivity analysis to investigate the impact of the model parameters on Bison predictions of hydrogen migration and redistribution to understand the discrepancies in the case of a low temperature gradient profile. In both cases, the Bison prediction of hydrogen distribution is accurate in the regions where the temperatures are higher. In these areas, the relative error on average does not exceed 1%. On the other hand, the error grows increasingly higher in the cold regions where hydrides are more likely to form and a steep gradient in the hydrogen concentration profile develops. The potential for the uncertainties in the model parameters to explain this behavior is investigated through the sensitivity analysis, including the impact of a clamping factor that artificially reduces the maximum allowable volume fraction of hydrides. This study also provides a literature review on hydrogen migration and redistribution experiments and a systematic comparison of the relative experimental data.

Influence of kinetic effects on terminal solid solubility of hydrogen in zirconium alloys

The integrity of irradiated zirconium based nuclear fuel cladding is related to the precipitation of hydrides which is closely connected with the solubility of hydrogen. A review on the development of measurement technologies is given and the resulting terminal solid solubilities are reflected with respect to the established model of hydrogen solubility in zirconium alloys. The results often allow for a different interpretation than the established model. An alternative qualitative approach is proposed in which the fixed TSSp for the precipitation of zirconium hydrides is replaced by a kinetic model based on thermal history, total hydrogen content and the cooling rate. The influence of the modification of the model from a fixed TSSp to a kinetically limited TSSd is discussed with respect to the understanding of hydride embrittlement processes in long term storage.

Kinetics of the isothermal decomposition of zirconium hydride: terminal solid solubility for precipitation and dissolution

The hydrogen permeation technique in the surface-limited regime (SLR) was first used to study the isothermal decomposition of zirconium hydride. It is shown that under isothermal conditions, the hydrogen terminal solid solubility in the α-phase for hydride precipitation (TSSp) and dissolution (TSSd) differ only by 6%, in contrast to the 20–30% indicated in the available literature. It is demonstrated that even the minimum heating/cooling rate (1 C/min) used in the traditional methods of studying TSSp and TSSd is too high to exclude the effect of kinetics on the results obtained.

Discrete Dislocation Plasticity Modeling of Hydrides in Zirconium under Thermal Cycling

ABSTRACTUnderstanding the ratcheting effect of hydrogen and hydride accumulation in response to thermal cycling is important in establishing a failure criterion for zirconium alloy nuclear fuel cladding. We propose a simple discrete dislocation plasticity model to study the evolution of the dislocation content that arises as a micro-hydride repeatedly precipitates and dissolves over a series of thermal cycles. With each progressive thermal cycle, we find a steady growth in the residual dislocation density in the vicinity of the hydride nucleation site; this corresponds to a gradual increase in the hydrogen concentration and, consequently, the hydride population. The simulated ratcheting in the dislocation density is consistent with experimental observations concerning the hysteresis in the terminal solid solubility of hydrogen in zirconium, which can be correlated to the plastic relaxation of hydrides.

Hydride reorientation in Zircaloy-4 examined by in situ synchrotron X-ray diffraction

The phenomenon of stress-reorientation has been investigated using in situ X-ray diffraction during the thermomechanical cycling of hydrided Zircaloy-4 tensile specimens. Results have shown that loading along a sample’s transverse direction (TD) leads to a greater degree of hydride reorientation when compared to rolling direction (RD)-aligned samples. The elastic lattice micro-strains associated with radially oriented hydrides have been revealed to be greater than those oriented circumferentially, a consequence of strain accommodation. Evidence of hydride redistribution after cycling, to α-Zr grains oriented in a more favourable orientation when under an applied stress, has also been observed and its behaviour has been found to be highly dependent on the loading axis. Finally, thermomechanical loading across multiple cycles has been shown to reduce the difference in terminal solid solubility of hydrogen during dissolution (TSSD,H) and precipitation (TSSP,H).

Development of tantalum–zirconium alloy for hydrogen purification

Terminal solid solubility of hydrogen in Ta–Zr alloys has been studied in connection with the development of tantalum based metallic membrane for hydrogen/tritium purification. The alloys were prepared by vacuum arc melting technique and subsequently cold rolled to 0.2mm thickness. The terminal solid solubility of hydrogen in these cold rolled samples was investigated in a modified Sieverts apparatus. The terminal solid solubility of hydrogen was marginally increased with zirconium content. The change in the lattices parameter of tantalum upon zirconium addition and the higher affinity of zirconium for hydrogen as compared to tantalum could be the possible reasons.

Terminal solid solubility determinations in the H–Ti system

The terminal solid solubility of hydrogen in titanium was measured by differential scanning calorimetry in the concentration range of 0.3–4.1 at.% which practically corresponds to the whole solubility range of hydrogen in α-Ti. The solvus enthalpy obtained in this range from the overall data set was 22.8 ± 0.5 kJ/molH. However, a more careful analysis of the experimental results shows that the solubility curve has two different behaviors as a function of concentration. In the high concentration range 1.4–4.1 at.% a solvus enthalpy of 29.0 ± 1.5 kJ/molH was obtained representing the α/α + δ equilibrium boundary. In the low concentration range, 0.3 at.% to 1.4 at.%, the slope was noticeably lower with 24.2 ± 1.5 kJ/molH for the solvus enthalpy. This last value should correspond to the [α]/[α + γ] equilibrium curve. Although it is possible this value might be influenced by the presence of tiny amounts of the now metastable δ phase–as its presence is revealed by X-ray diffraction analysis – anyway it is consistent with a α + δ ↔ γ peritectoid reaction temperature of 168 °C obtained from the literature.The eutectoid α + δ ↔ β decomposition temperature was determined using samples of high hydrogen contents, ranging from 9 to 11.0 at.%. This temperature was determined to be 319.9 ± 1 °C from the analysis of the DSC diagrams. The solubility limit [α]/[α + δ] at this eutectoid reaction was estimated to be 5.44 ± 0.27 at.%.The present results are believe to provide a closer approximation to the solubility values of H in α-Ti as presently reported in the literature.

Cooldown-induced hydride reorientation of hydrogen-charged zirconium alloy cladding tubes

Radial hydride precipitation behaviors of Zr-Nb alloy cladding tubes were investigated using 250 and 500 ppm hydrogen-charged Zr-Nb alloy cladding tubes, cooldown processes from 400 to 300, 200°C and room temperature with five kinds of cooling rates of 0.3, 2.0, 4.0, 7.0 15.0 °C/min under a tensile hoop stress of 150 MPa, which can simulate various cooldown processes during an interim dry storage of PWR nuclear fuel. The slower cooling rate and the lower terminal cooldown temperature generated the more hydrides precipitated during the cooldown as well as the larger fraction and the longer length of radial hydrides. These phenomena can be explained by the difference in the terminal solid solubility of hydrogen for dissolution and precipitation occurring during the heatup and cooldown processes and the cooling rate-dependent hydride nucleation and growth rates. In addition, a drastic decrease in ultimate tensile strength and plastic strain of the tensile tested specimens experiencing the cool-down processes appear to be correlated with the amount of the radial hydrides precipitated during the cooldown.

Effects of hydride morphology on the embrittlement of Zircaloy-4 cladding

Spent nuclear fuel claddings discharged from water reactors contain hydrogen up to 800wppm depending on the burn-up and power history. During long-term dry storage, the cladding temperature slowly decreases with diminishing decay heat and absorbed hydrogen atoms are precipitated in Zr-matrix according to the terminal solid solubility of hydrogen. Under these conditions, hydrides can significantly reduce cladding ductility and impact resistance, especially when the radial hydrides are massively present in the material. In this study, the effects of hydride morphology on the embrittlement of Zircaloy-4 cladding were investigated using a ring compression test. The results show that circumferentially hydrided Zircaloy-4 cladding is brittle at room temperature but its ductility is regained substantially as the temperature goes above 150°C. On the other hand, radially hydrided cladding remains brittle at 150°C and micro-cracks developed in the radial hydrides can act as crack propagation paths. Fracture energy analysis shows that ductile to brittle transition temperature is low in between 25°C and 100°C in the former case, whereas it lies in between 200°C and 250°C in the latter case.

Effect of thermal history on the terminal solid solubility of hydrogen in Zircaloy-4

The terminal solid solubility (TSS) of hydrogen in zirconium alloys has a hysteresis. The TSS of hydrogen in Zircaloy-4 during cooling and heating were studied using differential scanning calorimetry (DSC) with a hydrogen content of 40–731 wppm. A significant hysteresis gap was observed between the TSS for dissolution (TSSD) and precipitation (TSSP). It was confirmed that the hydrogen dissolution temperature was unaffected by the previous thermal history in comparison with the hydride precipitation temperature. The TSSP temperature increased with a decrease in the maximum temperature, but a significant temperature gap remained even when the maximum temperature was equal to the TSSD temperature. The terminal solid solubility of hydrogen in Zircaloy-4 can be represented by the following equations.TSSD: C = 2.255 × 105 exp (−39101/RT).TSSP: C = 4.722 × 104 exp (−26843/RT).TSSP2: C = 8.612 × 105 exp (−30583/RT).Based on the experimental results hydrogen solubility path depending on the previous thermal history was proposed.

Terminal solid solubility of hydrogen in V–Al solid solution

Hydrogen solubility was studied in vanadium and series of vanadium aluminum alloys at varying temperatures and pressures. The alloys were prepared through aluminothermy followed by electron beam refining and consolidation through vacuum arc melting. The consolidated buttons were hot rolled into 1.2mm strips. The strips were cut into small pieces. The specimens so obtained were annealed to release the stresses developed during hot-rolling. Hydrogen gas was charged in the specimens at present temperatures and pressures. It was observed that hydrogen solubility decreases linearly with aluminum contents. The decrease in the hydrogen solubility with the aluminum content has been explained on the basis of increase in the Fermi energy level arising from the contribution of electrons by the solute aluminum atom to the valence band of the host matrix vanadium.

PREDICTION OF CRITICAL TEMPERATURE FOR DELAYED HYDRIDE CRACKING IN IRRADIATED N18 ZIRCONIUM ALLOY

Delayed hydride cracking (DHC) has been recognized as a potential threat to the structural integrity of zirconium alloy components in water–cooled nuclear reactors since the early 1970 s. Although DHC has been studied for a long time, most of the investigations were focused on unirradiated Zr–Nb and Zr–Sn alloys, no information was found on Zr–Sn–Nb alloy. Currently, several new Zr–based alloys have been developed from the point of view of enhancement of corrosion resistance and hydrogen pickup properties. In China, N18 (Zr–Sn–Nb) alloy has been developed based on Zr–Sn and Zr–Nb systems to meet the requirements of higher fuel burn–up. Therefore, it is necessary to investigate the DHC behavior of N18 alloy systematically. The objective of this study is to predict the critical temperatures for initiating and arresting DHC in irradiated N18 alloy. A model used to forecast DHC critical temperature of irradiated zirconium alloys was established based on the terminal solid solubility of hydrogen in irradiated zirconium alloys and stress–induced hydrogen diffusion. By using the model, the critical temperatures for irradiated N18 alloy were predicted. The result showed that the irradiated N18 alloy has high sensitivity for DHC initiation, because neutron irradiation increased the solubility of hydrogen and yield strength of N18 alloy. Compared with unirradiated N18 alloy, the critical temperature * $! AE# JJXM–QNJJ–09–04–04 % : 2009–12–29, &% : 2010–03–31 $%' : ! , !, 1981 , # , DOI: 10.3724/SP.J.1037.2009.00868

On the Precipitation of Hydrides in Metals under Stress - Finite Deformation Approach

An analytic relation for the terminal solid solubility of hydrogen in a stressed metal is derived, based on finite deformation theory. Phase transformation is assumed to be a reversible process, which occurs under local chemical equi- librium among hydride, metal and hydrogen in solid solution. Hydrogen terminal solid solubility depends on stress due to the interaction of the applied stress field with the field of the expanding hydride as well as due to the reduction of hydro- gen chemical potential in solid solution, caused by hydrostatic stress. The present analysis is complementary to hydride- induced embrittlement studies in metals, under conditions, which require the use of finite deformations.

The terminal solid solubility of hydrogen in irradiated Zircaloy-2 and microscopic modeling of hydride behavior

Differential scanning calorimetry (DSC) has been applied to elucidate the terminal solid solubility (TSS) of hydrogen in Zircaloy-2 cladding tubes and spacer bands irradiated in commercial BWRs. While recovery of irradiation defects during the first heating stage of as-irradiated specimens made the DSC peak of hydride dissolution dull or broader, no significant difference was detected in the TSS between unirradiated and irradiated Zircaloy-2, irrespective of fast neutron fluence. The effect of post-irradiation annealing on TSS was also examined. The results suggest almost no interaction between irradiation defects and dissolved hydrogen or hydrides at temperatures around 300 °C. Using the present TSS data and reported hydrogen- and hydride-related properties, a microscopic analysis code HYMAC for analyzing hydride behavior in cladding tube with textured grains was constructed. Stress-induced preferential precipitation and dissolution of hydrides were reproduced by adopting a TSS sub-model in which the solubilities decrease in proportion to stress normal to the habit plane in grains and to grain faces. Analyzed results by the code were consistent with typical experimental results of hydride behavior.

Hydrogen terminal solid solubility determinations in Zr–2.5Nb pressure tube microstructure in an extended concentration range

In the present work the temperature of terminal solid solubility of hydrogen in the zirconium alloy Zr–2.5Nb was measured for material taken from a CANDU type pressure tube. The measurements were made in a wide concentration range (50–463 wppm) using two techniques, differential scanning calorimetry and differential dilatometry, with the additional purpose of checking the coincidence of the TSS points in both techniques. The work was complemented by microstructural studies pursuing the (β-Zr) → (α-Zr + β-Zr enr + β-Nb) transformation. This transformation is accelerated if the material remains at temperatures higher than 400 °C, which corresponds to a hydrogen solubility of about 180 wppm. Thus, microstructural studies were made by transmission electron microscopy and energy dispersive analysis, which allowed to relate changes in the solubility with these microstructural transformations.

Effect of Nb addition on the terminal solid solubility of hydrogen for Zr and Zircaloy-4

The terminal solid solubility of hydrogen (TSS) for pure Zr, Zr–Nb binary alloys with different Nb concentrations, and Nb added Zircaloy-4 was examined from the view point of the integrity of new-type nuclear fuel cladding. These alloys were hydrogenated by a modified UHV Sieverts’ apparatus at 973 K. The hydrogen concentration and the hydride dissolution temperature of specimen were measured by using a hydrogen analyzer and a differential scanning calorimeter (DSC), respectively, and then the terminal solid solubility of hydrogen was determined. The TSS of the α single-phase Zr–0.3Nb (Zr–0.3 wt.% Nb) specimen appeared to be almost same as that of pure Zr. On the contrary, the TSS of the Zr–1.0Nb and Zr–2.5Nb alloys, which were α + β biphasic specimens, were larger than that of pure Zr and slightly increased with Nb concentration. The increment of TSS by Nb addition was slightly larger than that by the traditional additive elements of Sn, Ni, and Cr in Zircaloys. The Nb added Zircaloy-4 had higher TSS than the Zircaloy-2 and -4, which was attributed to the further additive effect by βZr precipitation in Zircaloy besides the traditional additive element effects.

Influence of additive elements on the terminal solid solubility of hydrogen for Zirconium alloy

The terminal solid solubility of hydrogen for pure Zr, Zircaloy-2, Zr–M (M = Fe, Sn, Cr, Ni) was measured, whose dependency on additive element was elucidated in relation to the integrity of fuel cladding. The present experimental method was in good accordance with the previous reports. The terminal hydrogen solid solubility for Zircaloy-2 was larger than that for pure Zr, the reason for which was considered that the solute Sn leads to decrease the free energy of α Zr phase.