Terminal Solid Solubility For Dissolution Research Articles

Hydride reorientation in fuel cladding under interim storage conditions with low hoop stress

Abstract. The restart of the search for a final repository for high-level nuclear waste in Germany and the resulting so-called white map lead to a reassessment of the safety of interim storage as well. Due to the resulting increase in interim storage periods with a prospected timescale until 2068, among other things, renewed consideration of cladding tube integrity is required for the handling of the fuel assemblies during repackaging into the final storage containers. Besides oxide formation during operation, hydrides can have a negative influence on stability during interim storage and handling later on. The hydrides, which are initially precipitated tangentially in the tube material during operation, can reorient during interim storage as a result of the temperature and load history and precipitate again in a radial direction during cooling under load. In recent years, a number of investigations have already been carried out with respect to this topic, with cladding conditions that are present in high burn-up fuel assemblies with corresponding high hydrogen contents. In these cases, reorientation effects did occur with stresses that can be encountered in interim storage, but the proportions of radial hydrides were very low and were classified as not being harmful. In addition to the tests with cladding tubes containing high hydrogen contents, sporadic tests with low hydrogen contents were also carried out, for example, by Hardie and Shanahan (1975) and later by Chu et al. (2008). These indicated that strong reorientation effects could occur because of the complete dissolution of hydrogen under high temperature and tensile stress. In recent years, Kaufholz et al. (2018) also formulated a hypothesis that describes this effect as being the consequence of free nucleation without the presence of precipitates. This results in the possible formation of extremely pronounced radial hydrides, which can even extend through the tube wall. This is possible because residual hydrides that can hinder free precipitation or represent preferential points for nucleation are absent. The results of the present investigations confirm this behaviour. They show that massive reorientations can already occur at stresses at the lower end of the literature values and that, until now, these reorientations were interpreted as being uncritical. Considering the hysteresis in solubility between terminal solid solubility for dissolution (TSSD) and precipitation (TSSP), it becomes apparent that precipitation at hydrogen contents of 100 ppmw (parts per million by weight) already starts at approx. 538 K and that the hydrides precipitate radially under a stress of 52 MPa and form extremely pronounced hydrides. This results in a need for further research, since the thermal and mechanical loads under which the tests were carried out are well below the permissible limit loads in interim storage and below the values expected so far. This applies in particular to materials and cladding tubes that either absorb less hydrogen during operation or have only low burn-ups. The study provides an overview of the status quo, presents part of the investigations carried out during the project MUDZ (Modellierung und Untersuchung der Degradation von Hüllrohrmaterialien aus Zr-Legierungen durch Hydridbildungs- und Hydridverteilungsprozesse im Hinblick auf die Langzeitzwischenlagerung) in the KEK (Kompetenzerhalt in der Kerntechnik) initiative and the results obtained, and discusses the ramifications of the results.

Sensitivity analysis for characterizing the impact of HNGD model on the prediction of hydrogen redistribution in Zircaloy cladding using BISON code

Hydrogen in zirconium cladding is able to precipitate into zirconium hydrides which impacts cladding integrity. The Hydride Nucleation-Growth-Dissolution (HNGD) model in the BISON code accounts for the precipitation and dissolution kinetics of hydride in Zircaloy material. This paper presents global sensitivity analyses of the HNGD model aiming to enhance our understanding of the hydride precipitation phenomena by quantifying the variance that key parameters have on the prediction of hydrogen behavior under various environmental conditions. Model predictions are compared to experimental data obtained under two different conditions: 1) with uniformly precharged specimens subjected to a linear thermal gradient, and 2) specimens precharged with a cathodically applied hydride rim at one end of the sample and subjected to an asymmetric thermal gradient. The Sobol sensitivity analysis identifies the key parameters in the HNGD model for both types of specimens. For linear temperature cases, the heat of transport dominates the accuracy of predictions when no precipitation occurs at the cold end, while Terminal Solid Solubility for Dissolution (TSSD) is the most important parameter when precipitation occurs. A large variation in the predicted hydrogen concentration profiles is found in the range of high TSSD due to the occurrence of precipitation. For asymmetric temperature cases, the solubility coefficient gives the largest impact on the predicted hydrogen distribution, as it determines the amount of solute hydrogen dissolved from the initially applied hydride rim. A large discrepancy in hydrogen distribution between simulations and experiments exists with the asymmetric specimens because BISON simulations fail to predict the precipitation of hydride at the cooler end. Comparative studies using former and updated models verifies the significant impact of the hydride growth mechanism on predicted hydrogen concentration profiles. In particular, when hydride initially exists, changes in TSSD generate a large variation in the predicted amount of precipitation by hydride growth, giving large uncertainty in predicting the hydrogen distribution over the sample length. The outputs characterize the significant impact of the hydride growth mechanism in the HNGD model on predicting hydrogen behavior, and improve the understanding of the precipitation of hydride in Zircaloy cladding within a range of expected environmental conditions. The analyses indicate work is still needed to improve the hydride solvus models in the BISON code to accurately predict experimentally observed hydride concentrations and distributions.

Study of variables that affect hydrogen solubility in α + β Zr-alloys

Zr–2.5Nb and Excel pressure tubes, both containing α and β phases were submitted to different heat treatments. Then, hydrogen Terminal Solid Solubility for Dissolution (TSSD) and Precipitation (TSSP) curves were measured by Differential Scanning Calorimetry (DSC). The solvus of Excel heat treated at 380 °C–24 h or 750 °C–0.5 h exceeds the solvus of Zr–2.5Nb in standard conditions of CANDU pressure tubes. Aging at 500 °C–168 h decreases the limit of solubility. The lowest solubility was obtained in Excel aged at 500°C–168 h. In DSC measurements the effect of maximum temperature and hold time at such temperature on solubility curves were studied. The TSSD decreases when thermal cycle causes decomposition of the β phase, and is recuperated when α → α + β transformation temperature is exceeded. The TSSP is affected not only by βZr phase decomposition but also by the relief of defects produced during hydride precipitation.

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.

Initiation and Arrest of Delayed Hydride Cracking in Zr–2.5Nb Tubes

Using Kim’s delayed hydride cracking (DHC) model, this study reanalyzes the critical temperatures for DHC initiation and arrest in Zr–2.5Nb tubes that had previously been investigated with the previous DHC models. At the test temperatures above 180°C, DHC initiation temperatures fell near the terminal solid solubility for precipitation temperatures, requiring some undercooling or ΔT from the terminal solid solubility for dissolution (TSSD) temperatures, and increased toward TSSD with the number of thermal cycles. At the test temperatures below 180°C, DHC initiation occurred at temperatures near TSSD with little ΔT. DHC arrest occurred on heating toward TSSD where the hydrogen concentration difference between the bulk region and a crack tip ΔC decreased to a minimum ΔCmin, under which nucleation of the hydrides was restrained. ΔCmin after the first thermal cycle increased with increasing temperature, demonstrating that nucleation of the hydrides becomes more difficult with increasing temperatures. Different DHC initiation and arrest temperatures with the test temperatures or hydrogen concentrations are discussed in view of a supersaturation of hydrogen (ΔC) for nucleation of hydrides in the zirconium matrix.

Determination of the Hydrogen Concentration in Zr-2.5Nb Alloy by a Resonant Ultrasound Spectroscopy

Zirconium alloys are used for many applications in nuclear components, such as the pressure tube material in a pressurized heavy water reactor, nuclear fuel cladding, etc. One of the problems during the operation of a nuclear reactor is the degradation of the zirconium alloys, which is due to an increase of the hydrogen content in the zirconium alloy. Therefore a non-destructive determination of the hydrogen concentration in zirconium alloy is one of the important issues that need to be addressed. The resonant ultrasound spectroscopy (RUS) technique is evaluated for a characterization of the hydrogen concentration in Zr-2.5Nb alloy. Referring to the terminal solid solubility for dissolution (TSSD) of Zr-2.5Nb alloy, the plot of the mechanical damping coefficient (Q-1) versus the temperature or the deviation of the resonant frequency for the temperature (df/dT) versus the temperature was correlated for the hydrogen concentration in Zr-2.5Nb alloy. It was found that the temperature at an abrupt change of the slope can be correlated with the hydrogen concentration of the Zr-2.5Nb alloy.

Critical Temperatures for Initiating and Arresting Delayed Hydride Cracking in a Zr-2.5Nb Pressure Tube

The hydrogen concentration limit and critical temperatures for a delayed hydride cracking (DHC) in zirconium alloys have been reanalyzed using Kim’s DHC model that a driving force for DHC is not the stress gradient but the supersaturated hydrogen concentration or ∆C arising from a hysteresis of the terminal solid solubility on a heating and on a cooling. The DHC initiation occurs generally at the temperatures corresponding to the terminal solid solubility for precipititation (TSSP), demonstrating that the supercooling from the terminal solid solubility for dissolution (TSSD) is required to initiate the DHC. The DHC arrest temperatures correspond to the temperatures where the ∆C is reduced to zero. Therefore, we conclude that the ∆C is the driving force for the DHC and that the Kim’s DHC model is feasible.

Review of the initiation and arrest temperatures for delayed hydride cracking in zirconium alloys

Based on Kim's delayed hydride cracking (DHC) model, this study reanalyzes the critical temperatures for DHC initiation and arrest in zirconium alloys that had previously been investigated with Puls DHC model. In an unratcheting thermal cycle where DHC crack tip hydrides were dissolved fully at the peak temperature, the DHC initiation was suppressed, which required a supercooling or ΔT from the terminal solid solubility for dissolution (TSSD) temperatures. At a hydrogen concentration as 7 ppm H, the DHC initiation temperatures coincided with the TSSD, which is in conflict with Puls' DHC model. In a ratcheting thermal cycle, where the hydrides precipitated at the DHC crack tip were not fully dissolved, the DHC initiation was enhanced, so as to require a lesser ΔT, compared to that of the unratcheting thermal cycle. Therefore, the DHC initiation temperatures are concluded to depend upon at what temperatures the hydrides can nucleate in the zirconium matrix with the supersaturated hydrogen concentration. The DHC arrest temperatures were governed by the critical supersaturated hydrogen concentration or ΔC regardless of the thermal cycle treatment, providing further supportive evidence that Kim's DHC model is feasible.

Driving force for delayed Hydride cracking of zirconium alloys

The effect of hydrogen concentration on the delayed hydride cracking velocity (DHCV) and the threshold stress intensity factor, KIH of a Zr-2.5Nb tube were examined at test temperatures ranging from 100 to 280°C by subjecting compact tension specimens with a hydrogen concentration of 12 to 100 ppm H to an overtemperature cycle. The DHCV and KIH increased and decreased, respectively, with an increase in the supersaturated hydrogen concentration over the terminal solid solubility for dissolution (TSSD) or ΔC. They then leveled off to constant values at ΔC in excess of the ΔCmax corresponding to a difference of the terminal solid solubility of the hydrogen on cool-down and on heat-up. Further, intentional introduction of an undercooling by 0 to 40°C at the test temperature decreased the DHCV of the Zr-2.5Nb tube, indicating that ΔC between the bulk region and the crack tip governs the DHCV. A new DHC model is proposed where the driving force for DHC is the difference in the hydrogen concentration between the bulk region and the crack tip by preferentially nucleating the hydrides only at the crack tip under an applied tensile stress, due to a hysteresis in the TSS of hydrogen on heat-up and on cool-down. A supplementary experiment was conducted to validate the feasibility of the proposed DHC model.

Delayed hydride cracking in Zr–2.5Nb tube with the cooling rate and the notch tip shape

The objective of this study is to demonstrate the feasibility of the Kim’s delayed hydride cracking (DHC) model. To this end, this study has investigated the velocity and incubation time of delayed hydride cracking (DHC) for the water-quenched and furnace-cooled Zr–2.5Nb tubes with a different radius of notch tip. DHC tests were carried out at constant KI of 20MPa√m on cantilever beam (CB) specimens subjected to furnace cooling or water quenching after electrolytic charging with hydrogen. An acoustic emission sensor was used to detect the incubation time taken before the start of DHC. The shape of the notch tip changed from fatigue cracks to smooth cracks with its tip radius ranging from 0.1 to 0.15mm. The DHC incubation time increased remarkably with the increased radius of the notch tip, which appeared more strikingly on the furnace-cooled CB specimens than on the water-quenched ones. However, both furnace-cooled and water-quenched CB specimens indicated little change in DHC velocity with the radius of the notch tip unless their notch tip exceeded 0.125mm. These results demonstrate that the nucleation rate of hydrides at the notch tip determines the incubation time and the DHC velocity becomes constant after the concentration of hydrogen at the notch tip reaches terminal solid solubility for dissolution (TSSD), which agrees well with the Kim’s DHC model. A difference in the incubation time and the DHC velocity between the furnace-cooled and water-quenched specimens is attributed to the nucleation rate of reoriented hydrides at the notch tip and the resulting concentration gradient of hydrogen between the notch tip and the bulk region.

Effect of β-Zr decomposition on the solubility limits for H in Zr–2.5Nb

The zirconium alloy Zr–2.5Nb (Zr–2.5 wt.% Nb) is used to fabricate pressure tubes for CANDU®11CANDU® (CANada Deuterium Uranium) is a registered trademark of AECL (Atomic Energy of Canada Limited). nuclear reactors. The microstructure of the as-extruded tubes consists of elongated h.c.p. α-Zr grains surrounded by a network of b.c.c. β-Zr containing at least 20 wt.% Nb. The β-Zr phase is meta-stable at temperatures below 900 K and gradually dissociates into its constituent elements α-Zr and β-Nb. The aim of this study was to determine the effect of β-Zr decomposition on the terminal solid solubility for dissolution (TSSD) for H in Zr–2.5Nb. Specimens of Zr–2.5Nb were charged to different hydrogen concentrations and aged at temperatures ranging from 673 to 773 K for 30 to 6000 min, to decompose their β-Zr component by different amounts. Hydride dissolution temperature and hydrogen concentration were determined using differential scanning calorimetry and hot vacuum extraction mass spectrometry, respectively. The results show that the as-extruded pressure tube structure, with the β-Zr almost intact, has the highest TSSD and that the TSSD for structures with fully decomposed β-phase is the lowest and approaches that of unalloyed Zr.