Delayed Hydride Cracking Velocity Research Articles

Effect of radial hydride on delayed hydride cracking behaviour of Zr-2.5Nb pressure tube material

Delayed Hydride Cracking (DHC) behaviour of Zr-2.5Nb Pressure Tube (PT) material used in Indian 200MWe Pressurised Heavy Water Reactor (PHWR) containing radial hydrides and circumferential hydrides was studied in the temperature range of 200–250 °C. A section of the PT was charged with 70wppm of hydrogen and successfully subjected to re-orientation treatment to form radial hydride. The radial hydride in the reoriented tube was characterised by the hydrogen continuity co-efficient (HCC). It was observed that the average DHC velocity (VDHC) for the radial hydrided sample was 2.5 and 1.85 times faster than the circumferential hydrided sample at 200 and 225 °C, respectively. The VDHC at the temperature of 250 °C, for both the radial hydrided and the circumferential hydrided sample were comparable. The average VDHC for circumferential hydrided sample varies linearly with 1/T while it was observed to vary non-linearly with 1/T for radial hydrided sample. The VDHC of the radial hydrided sample was observed to increase with increase in the stress intensity factor, whereas for circumferential hydrided samples the VDHC was independent of stress intensity factor in the range of 18–45MPa.m1/2.

Multi-physics modeling of delayed hydride cracking in zirconium alloys

Delayed hydride cracking (DHC) threatens the safety and integrity of Zircaloy components. The typical feature of DHC is presented as the intermittent fracture with the striations on the fracture surface. To reproduce the multi-field coupling process and reflect the essential features of DHC, an improved cohesive model is developed considering the hydride embrittlement and irradiation effects, based on which a multi-field coupling computation scheme is proposed. The model is validated by comparing the predictions of DHC velocity and striation spacing with some experimental results. A comprehensive study of multi-field coupling DHC behavior is performed The evolution and interplay of the mechanical and chemical fields are obtained together with the effect of temperature. The results indicate that: (1) the stress concentration around the crack tip is an important factor to drive the process of multi-field coupling DHC; (2) the subsequent incubation periods are much shorter than the first incubation period, because the formed hydrides during the previous cracking provide an effective source of hydrogen for a new process of cracking. This work lays a foundation for the future research on DHC.

Theoretical models of threshold stress intensity factor and critical hydride length for delayed hydride cracking considering thermal stresses

Abstract Delayed hydride cracking (DHC) is an important failure mechanism for Zircaloy tubes in the demanding environment of nuclear reactors. The threshold stress intensity factor, K IH , and critical hydride length, l C , are important parameters to evaluate DHC. Theoretical models of them are developed for Zircaloy tubes undergoing non-homogenous temperature loading, with new stress distributions ahead of the crack tip and thermal stresses involved. A new stress distribution in the plastic zone ahead of the crack tip is proposed according to the fracture mechanics theory of second-order estimate of plastic zone size. The developed models with fewer fitting parameters are validated with the experimental results for K IH and l C . The research results for radial cracking cases indicate that a better agreement for K IH can be achieved; the negative axial thermal stresses can lessen K IH and enlarge the critical hydride length, so its effect should be considered in the safety evaluation and constraint design for fuel rods; the critical hydride length l C changes slightly in a certain range of stress intensity factors, which interprets the phenomenon that the DHC velocity varies slowly in the steady crack growth stage. Besides, the sensitivity analysis of model parameters demonstrates that an increase in yield strength of zircaloy will result in a decrease in the critical hydride length l C , and K IH will firstly decrease and then have a trend to increase with the yield strength of Zircaloy; higher fracture strength of hydrided zircaloy will lead to very high values of threshold stress intensity factor and critical hydride length at higher temperatures, which might be the main mechanism of crack arrest for some Zircaloy materials.

A theoretical model for delayed hydride cracking velocity considering the temperature history and temperature gradients

Delayed hydride cracking (DHC) threatens the safety of Zircaloy components. The DHC velocity in the stage of stable crack growth needs to be studied. Based on the recent research, with a new idea the theoretical model is developed for the DHC velocity considering the temperature history and temperature gradient. The stress field ahead of the crack tip is calculated based on the fracture mechanics theory of second-order estimate of plastic zone size, and the redistributed stresses are statically equivalent with the stresses obtained from Linear Elastic Fracture Mechanics. It is confirmed that a simple linear function of the equivalent axisymmetric temperature distribution can be effectively and easily calibrated for predictions of the DHC velocity at different temperature gradients along the cracking direction. The developed model is validated to be effective because the predicted velocities agree well with some experimental results for the cases with and without temperature gradients, whether the test temperature is approached by heating or cooling. The theoretical results indicate that (1) with a higher positive temperature gradient along the cracking direction, the DHC velocity increases and a higher crack arrest temperature will occur; while a negative temperature gradient will remarkably depress DHC process; (2) The total hydrogen concentration in the hydrogen source has an important effect on the DHC velocity, and the crack arrest temperature can be evidently lowered for the cases with a small quantity of hydrogen atoms there;(3) At higher temperatures, negative flux contribution from the hydrogen concentration gradient in the hydrogen source is found to be the main mechanism of crack arrest.

Effect of specimen thickness on DHC velocity for Zr-2.5Nb alloy pressure tube material

Zr-2.5Nb alloy pressure tubes used in pressurized heavy water reactors (PHWR) are susceptible to failure by Delayed Hydride Cracking (DHC), which is a form of localized hydride-embrittlement phenomenon manifested in the presence of a hydrostatic stress gradient and hydrogen concentration above a threshold limit as a sub-critical crack growth process in hydride forming metals. The DHC parameters used for safety assessment are DHC velocity (VDHC) and a threshold stress intensity factor (KIH). In this work DHC velocity was determined for Zr-2.5Nb pressure tube material at 250 and 280 °C using specimens of thickness between 1 mm and 4.5 mm. The DHC velocity was found to increase with increase in specimen thickness at 250 °C and 280 °C. Significant amount of tunneling of the crack was observed for 1-mm and 2-mm thick specimens at 250 °C and 280 °C, with the degree of tunneling increasing with decrease in thickness and increase in test temperature.

Delayed hydride cracking behavior of Zr-2.5Nb alloy pressure tubes for PHWR700

In order to attain improved in-reactor performance few prototypes pressure tubes of Zr-2.5Nb alloy were manufactured by employing forging to break the cast structure and to obtain more homogeneous microstructure. Both double forging and single forging were employed. The forged material was further processed by employing hot extrusion, cold pilgering and autoclaving. A detailed characterization in terms of mechanical properties and microstructure of the prototype tubes were carried for qualifying it for intended use as pressure tubes in PHWR700 reactors. In this work, Delayed Hydride Cracking (DHC) behavior of the forged Zr-2.5Nb pressure tube material characterized in terms of DHC velocity and threshold stress intensity factor associated with DHC (KIH) was compared with that of conventionally manufactured material in the temperature range of 200–283 °C. Activation energy associated with the DHC in this alloy was found to be ∼60 kJ/mol for the forged materials.

Comparison of delayed hydride cracking behavior of two zirconium alloys

Delayed hydride cracking (DHC) is an important failure mechanism that may occur in Zr alloys during service in water-cooled reactors. Two conditions must be attained to initiate DHC from a crack: the stress intensity factor must be higher than a threshold value called KIH and, hydrogen concentration must exceed a critical value. Currently the pressure tubes for CANDU reactor are fabricated from Zr–2.5Nb. In this paper the critical hydrogen concentration for DHC and the crack velocity of a developmental pressure tube, Excel, was evaluated and compared with that of Zr–2.5Nb. The DHC velocity values measured in Excel were higher than usually reported in Zr–2.5Nb. Due to the higher hydrogen solubility limits in Excel, its critical hydrogen concentration for DHC initiation is 10–50wppm over that of Zr–2.5Nb in the range of 150–300°C.

Evaluation of variables affecting crack propagation by Delayed Hydride Cracking in Zr–2.5Nb with different heat treatments

Delayed Hydride Cracking (DHC) is a failure mechanism that may occur in zirconium alloys used in nuclear reactor core components. The knowledge of the direct effects of the variables affecting the cracking velocity could be used to minimize the risk of crack propagation. In practice, most of these variables – as for example the alloy yield stress and hydrogen diffusion coefficient – are coupled and vary during reactor operation, leading to a complex variable dependence of the cracking mechanism. In order to get an insight into the relative effect of these variables, experimental data and a theoretical approach using a generally accepted DHC model were used in this work. A series of DHC velocity measurements were made in Zr–2.5Nb tube with different heat treatments. The yield stress, the Nb concentration in β phase, and hydrogen solvus of the alloy were measured for different heat treatments. Niobium concentration in β phase gave an indirect indication of β-phase continuity and, with a proper correlation, of the hydrogen diffusion coefficient. The obtained values were used as inputs in a theoretical calculation of cracking velocity. Good agreement between experimental data and predicted values was obtained, showing that hydrogen diffusion coefficient was the most relevant variable affecting DHC velocity cracking. Furthermore, this approach has been demonstrated to be useful in estimating DHC velocity in irradiated materials.

Temperature dependence of the velocity of delayed hydride cracking in Zr-2.5% Nb alloy

A delayed hydride cracking (DHC) in Zr-2.5 wt % Nb alloy in various structural-phase states at temperatures of 144–283°C using compact specimens for tension tests after a preliminary hydrogenating to 0.0038–0.0072 wt % hydrogen contents was investigated. The temperature dependences and energies of activation of the DHC velocities and spacings between the striations in the hydride crack rupture are determined. Specimens that are characterized by a higher yield strenght and the presence of extended β-Zr phase intermediate layers in the structure have a higher DHC velocity and a smaller interstriation spacing. The obtained results are compared with the previously published data and are considered from the positions of the available DHC theoretical models.

Threshold stress intensity factor for delayed hydride cracking in Zr–2.5%Nb pressure tube alloy

Delayed hydride cracking (DHC) velocity was determined at 203, 227, 250 and 283 °C using 17 mm width curved compact toughness specimens machined from an unirradiated Zr–2.5 wt.% Nb pressure tube spool, gaseously charged with 60 ppm of hydrogen by weight. Single CT specimen was used to determine DHC velocity at a constant temperature for a range of stress intensity factor ( K I) obtained by load drop method. For a given temperature and K I > 15 MPa m 1/2, DHC velocity was found to be practically independent of K I. For 15 > K I > 10 MPa m 1/2, DHC velocity decreased significantly with decrease in stress intensity factor and extrapolation of the data suggested the threshold stress intensity factor to be about 9–11 MPa m 1/2 in the aforementioned temperature range. The activation energy associated with DHC was observed to be 35.1 kJ/mol.

Optimization of stress relief heat treatment of PHWR pressure tubes (Zr–2.5Nb alloy)

The micro-structure of cold worked Zr–2.5%Nb pressure tube material consists of elongated grains of α-zirconium enclosed by a thin film of β-zirconium phase. This β-Zr phase is unstable and on heating, progressively decomposes to α-Zr phase and β-phase enriched with Nb and ultimately form β Nb. Meta-stable ω-phase precipitates as an intermediate step during decomposition depending on the heat treatment schedule, β Zr → α + β enr → α + ω + β enr → α + β enr → α + β Nb Morphological changes occur in the β-zirconium phase during the decomposition. The continuous ligaments of β Zr phase turn into a discontinuous array of particles followed by globulization of the β-phase. The morphological changes impose a significant effect on the creep rate and on the delayed hydride cracking velocity due to reduction in the hydrogen diffusion coefficient in α Zr. If the continuity of β-phase is disrupted by heat treatment, the effective diffusion coefficient decreases with a concomitant reduction in DHC velocity. The pressure tubes for the Indian PHWRs are made by a process of hot extrusion followed by cold pilgering in two stages and an intermediate annealing. Autoclaving at 400 °C for 36 h ensures stress relieving of the finished tubes. In the present studies, autoclaving duration at 400 °C was varied from 24 h to 96 h at 12 h-steps and the micro-structural changes in the β-phase were observed by TEM. Dislocation density, hardness and the micro-structural features such as thickness of β-phase, inter-particle spacing and volume fraction of the phases were measured at each stage. Autoclaving for a longer duration was found to change the morphology of β-phase and increase the inter-particle spacing. Progressive changes in the aspect ratio of the β-phase and their size and distribution are documented and reported. These micro-structural modifications are expected to decrease DHC velocity during reactor operation.

Delayed hydride cracking velocity in Zr-2.5Nb: detection by acoustic emission and theoretical model testing

Acoustic emission (AE) detects elastic waves generated during delayed hydride cracking (DHC). The detection of the first acoustic signal is used to measure the propagation time. This time and the crack length measured on the broken specimen are used to determine the DHC velocity. In this work, DHC tests were carried out on Zr-2.5Nb alloy from CANDU pressure tubes. Linear relationship between cumulative count rate and cracking velocity was corroborated. It is generally accepted that the hydrides crack when they reach a critical length; nevertheless, in this work the number of signals generated during typical DHC test were higher than expected from that assumption. Dutton and Puls DHC model with some parameters calculated by Shmakov et al. was used to test out against experimental data. This modification is an original approach that makes more rational the DHC velocity calculation, avoiding arbitrary parameter selection. Good agreement was obtained for two different CANDU pressure tubes.

Measurement of Rates of Delayed Hydride Cracking (DHC) in Zr-2.5Nb Alloys—An IAEA Coordinated Research Project

Abstract Values of DHC rates in Zr alloys are sensitive to measurement procedures. A standard method has been developed at the laboratories of AECL and evaluated in a round-robin involving ten IAEA member states. Two test materials were used—Zr-2.5Nb pressure tubes in the cold-worked (CANDU™) and heat-treated (RBMK) conditions. Cracks were grown from fatigued starter cracks in the axial direction on the axial-radial plane of the original tubes. To obtain the maximum value of Vc, specimens were heated to dissolve all their hydrogen, then cooled at 1 to 3°C/min to the test temperature before loading at 15 MPa√m. Although the start of cracking was detected by potential drop, the extent of cracking was measured directly on the crack faces. The values of incubation time to the start of cracking were highly variable but Vc was well behaved. The values of Vc were normally distributed with a range varying from a factor of 1.2 to 5.2. At 250°C the mean value of Vc from 80 specimens of cold-worked material was 8.9(±1.12)×10-8 m/s and from 41 specimens of heat-treated material the mean value of Vc was 3.3(±0.64)×10-8 m/s. Tests were also done at six other temperatures between 144 and 283°C, using up to 22 specimens at each temperature. Both materials had an Arrhenius-type temperature dependence, Vc= A exp(-Q/RT). The use of strictly defined and coordinated experimental procedures gave a consistent set of Vc values, allowing effective comparison of results obtained in different national laboratories and resulting in good correlations between the DHC velocity values and differences in strength, crystallographic texture, and distribution of β-phase in the test materials.

Crack tip stress effect on delayed hydride cracking velocity of Zr–2.5Nb tubes

Using our new delayed hydride cracking (DHC) model, we scrutinized Pan's DHC test results of a cold-worked Zr–2.5Nb tube after neutron irradiation. DHC velocity at 240 °C of the Zr–2.5Nb tube increased gradually at neutron fluences below 5 × 10 25 n/m 2 ( E > 1 MeV) and leveled off to a constant value at higher neutron fluences over it. This neutron fluence dependence of the DHC velocity was found to be similar to that of the Nb concentration in the β-Zr, not that of its tensile stress, which is hard to understand in view of the old DHC models. Normalization of the DHC velocity by hydrogen diffusivity or D H is found to lessen the apparent yield stress effect on the DHC velocity of Zr–2.5Nb tubes. Consequently, it is demonstrated that the DHC velocity of the Zr–2.5Nb tubes is governed not by the crack tip stresses but by hydrogen diffusion, corroborating the validity of the new DHC model.

Anisotropic delayed hydride cracking velocity of CANDU Zr–2.5Nb pressure tubes

Delayed hydride cracking (DHC) tests were conducted on CANDU 1 CANadian Deuterium Uranium. 1 Zr–2.5Nb tubes containing hydrogen with a microstructure of elongated α-Zr and semi-continuous β-Zr at temperatures ranging from 100 to 300 °C. DHC velocity (DHCV) was found to be around two times higher in the axial direction than that in the radial direction even on the same cracking plane. According to Kim’s DHC model that DHCV at temperatures lower than 300 °C is governed mainly by hydrogen diffusion and to a lesser extent by the hydrogen concentration gradient at the crack tip, we suggest that the enhanced DHCV in the axial direction arises from a faster hydrogen diffusion in the axial direction with a semi-continuous distribution of β-Zr. Evidence to this suggestion is provided by Levi’s experiment that Zr–2.5Nb tubes or plates with a fully discontinuous β-Zr due to annealing at 400 °C for 1000 h or 650 °C for 9 h has no anisotropic DHCV with the orientation. Hence, we conclude that Kim’s DHC model is plausible. An anisotropic DHCV of irradiated CANDU Zr–2.5Nb tubes is discussed based on their microstructural evolution with the neutron fluence and operating temperatures.

Temperature dependency of delayed hydride cracking velocity in Zr–2.5Nb tubes

Delayed hydride cracking (DHC) tests were conducted on Zr–2.5Nb tubes with different distributions of the β-Zr at temperatures ranging from 398 to 573 K. Compact tension specimens charged to 27–100 ppm hydrogen were used to determine the temperature dependencies of their DHC velocity (DHCV) and their striation spacing. The CANDU Zr–2.5Nb tube with a higher yield strength and a semi-continuous β-Zr had a higher DHCV and a smaller striation spacing than the RBMK Zr–2.5Nb tube with a fully discontinuous β-Zr and a lower yield strength. It is found that the activation energy for the DHCV is the sum of the activation energies for hydrogen diffusion and the striation spacing representing the hydrogen concentration gradient at the crack tip. Quantitative contribution of hydrogen diffusion and the hydrogen concentration gradient to the DHCV is discussed. This study provides supportive evidence for the plausibility of Kim's DHC model.

Stage I and II behaviors of delayed hydride cracking velocity in zirconium alloys

Delayed hydride cracking (DHC) tests were conducted on Zr–2.5Nb compact tension (CT) specimens with 60–80ppm H at temperatures ranging from 160 to 280°C in the load increasing mode where the applied stress intensity factor, KI was increased step-wise by 0.5MPam−1/2 from 4.5MPam−1/2 until a crack grew. Critical hydride lengths, lc, corresponding to the spacing of the striation lines, were determined with the applied stress intensity factor, KI from the fracture surfaces of the Zr–2.5Nb CT specimens. The lc was the largest at as low a KI as just above KIH in Stage I and leveled off to 30μm at a larger KI of above 9MPam−1/2 especially at 250°C, corresponding to Stage II. This finding was also observed at all the investigated temperatures. Since DHCV is inversely governed by lc according to Kim's DHC model, it is concluded that the KI dependency of DHCV is caused by the KI dependency of lc. The increased lc in Stage I is suggested to arise from the creep effect on the increased KIH that is experimentally supported by Sagat's results. The rationale for this suggestion is provided using Kim's hypothesis that cracking of hydrides occurs by their interaction with twins.

Delayed Hydride Crack Velocity of Zirconium Alloys with the Direction of an Approach to Temperature

The delayed hydride cracking (DHC) tests were conducted on Zr-2.5Nb compact tension specimens with the test temperatures reached by a heating and a cooling. The Zr-2.5Nb specimens were either furnace-cooled or water-quenched after a hydrogen charging treatment to contain 10 to 100 ppm H. On an approach to the test temperatures by a cooling, both the Zr-2.5Nb specimens showed the DHC velocity increasing with an increasing temperature over a temperature range of 100-300°C, irrespective of the cooling rate. However, on an approach to the test temperatures by heating, the furnace-cooled Zr-2.5Nb showed a DHC arrest at temperatures over 180°C and no DHC at 250°C, and the water-quenched ones did have a DHC growth, even at 250°C. Using Kim's DHC model we elucidate the DHC arrest in the furnace-cooled Zr-2.5Nb at temperatures over 180°C and the DHC growth in the water-quenched specimen, even at 250°C, upon an approach by a heating.

Effect of Irradiation on the Fracture Properties of Zr-2.5Nb Pressure Tubes at the End of Design Life

Abstract To determine the fracture properties of Zr-2.5Nb pressure tubes irradiated until the end of design life, cantilever beam, curved compact toughness, and transverse tensile samples were prepared from a typical pressure tube and irradiated in the high flux reactor OSIRIS at CEA, Saclay, France. Experiments were conducted on two batches of samples mounted in two irradiation inserts. Each insert held sixteen samples of each type of specimen. The first insert was irradiated to a fluence corresponding to approximately half of the design life in a CANDU3 reactor. The experimental results were reported in [1]. Samples in the second insert were irradiated for 10.5 years in OSIRIS and received a maximum neutron fluence of 2.61 × 1026 n/m2 (E > 1 MeV), being equivalent to 2.98 × 1026 n/m2 (E > 1 MeV) in a CANDU reactor, i.e., corresponding to ∼30 years operation in CANDU reactors at 80 % capacity factor. The present report describes the results of tensile, fracture toughness, and Delayed Hydride Cracking (DHC) tests and XRD microstructure analysis from the second batch of specimens. A continuous and gradual evolution in tensile, fracture, DHC properties, and dislocation densities is demonstrated without any evidence of a sudden change following the initial transitient at very low fluence. In the whole high fluence range, there is a very slow rate of increase in c-component dislocation density, strength, and DHC velocity and a slow reduction in elongation and Nb concentration in the β-phase. The a-type dislocation density and fracture toughness remain approximately constant. The results from the second insert of specimens confirm that, following the initial transient at very low fluence, there is little further change in the fracture properties of Zr-2.5Nb pressure tube material. Therefore, material properties behave in a stable and predicable manner to the end of a 30 years design life for CANDU reactor pressure tubes.

Delayed Hydride Cracking Velocity of Irradiated Zr-2.5Nb Tubes after a 30-Year Operation in the Wolsong Unit 1

The aim of this study is to investigate a change in delayed hydride cracking (DHC) velocity of Zr-2.5Nb tubes with fast neutron fluence (E>1MeV) and predict the DHC velocity of the irradiated Wolsong 1 Zr-2.5Nb tubes at a neutron fluence corresponding to the 30 year design lifetime. To this end, the DHC velocity were determined at temperatures ranging from 100 to 280 oC on unirradiated Zr-2.5Nb tubes and the irradiated Zr-2.5Nb tubes in the Wolsong Unit-1 to the neutron fluence of 8.9x1025 n/m2 (E>1MeV). DHC tests were conducted on the compact tension specimens charged with 34 to 100 ppm hydrogen in accordance with the KAERI DHC procedures that have been validated through a round robin test on DHC velocity of Zr-2.5Nb tubes as an IAEA coordinated research project. Irradiated Zr-2.5Nb tubes had 3 to 5 times higher DHC velocity than that of unirradiated Zr-2.5Nb tubes while the inlet region of the irradiated Zr-2.5Nb tube with the highest yield strength had a slightly higher DHC velocity compared to that of the outlet region with the lowest yield strength. From a normalized correlation of yield strength and DHC velocity of the Zr-2.5Nb tubes, the yield strength was found to govern the DHC velocity of the Zr-2.5Nb tubes irrespective of the neutron fluence and operating temperatures. The DHC velocity of the irradiated Zr-2.5Nb tubes is predicted after a 30 year operation in the Wolsong Unit 1 on the basis of an increase in the yield strength with neutron fluence and a DHC velocity dependence on the yield strength of Zr-2.5Nb tubes.