Delayed Hydride Cracking Initiation Research Articles

Structural Reliability of pressure tube of a PHWR with volumetric flaws

The volumetric flaws can get generated in the pressure tubes (PT) of PHWR (made of Zr-2.5Nb material) during the operation of the reactor. The PT material has hydrogen present during the fabrication. Additionally, the zirconium material has a tendency to pick up hydrogen from the surrounding environment and can cause initiation of a crack through delayed hydride cracking (DHC) mechanism. This crack can grow due to the DHC and has the potential to cause leakage or break of the PT. The crack formation occurs only when a set of conditions are met. The model for the crack initiation ahead of the tip of a volumetric flaws is prescribed by the CSA Standard N285.8-15. The formation of the crack is dependent of the geometry of PT, geometry of the flaw (length, depth and root radius), applied loading (due to internal pressure, residual stresses), hydrogen concentration in the material and the material tensile and fracture properties. Most of these parameters are well defined. However, some of them like flaw geometry and material properties are not known with certainty. The structural reliability based methods quantify the uncertainty using probability distributions and present results in form of probability estimates. In this work, the probability of initiation of DHC ahead of a volumetric flaw and its subsequent growth is reported. Monte Carlo based simulation method along with variance reduction techniques is used to predict the probability. This study is useful to establish the Leak-Before-Break behaviour of the PT.

An assessment of delayed hydride cracking initiation in PHWR pressure tubes using in-house ZIPTAS code

The earlier fitness-for-service (FFS) evaluation of a service-induced volumetric flaw in the pressure tube of a Pressurized Heavy Water Reactor (PHWR) was based on the threshold peak stress that can lead to initiation of Delayed Hydride Cracking (DHC) as a function of number of reactor Heat-up/cool-down cycles. This design curve was a conservative lower bound to the limited test data available at that time. It, however, did not account for the effect of flaw geometry explicitly. To overcome this restriction, an improved flaw evaluation procedure that accounts for the stress relaxation due to hydride formation and decohesion process by a non-linear process zone lying at the tip of a blunt flaw was proposed by Scarth and Smith (2001). This model has been adopted by the Canadian Standards Association in their FFS code of practice. The process zone model is, however, tedious as it involves a set of non-linear equations that need to be solved simultaneously. In the present work, the process zone model is implemented in an in-house computational code (ZIPTAS) and a set of parametric studies are performed to assess the influence of flaw geometry, flaw size and service loads on the maximum nominal stress that will not lead to initiation of DHC from a volumetric flaw in a pressure tube. The developed ZIPTAS code is validated with several published results and is expected to be useful to both plant operators and regulators for quick and robust assessment of flaw size that can be safely permitted for continued operation of PHWRs.

Assessment of Delayed Hydride Cracking at volumetric flaws formed due to bearing pad fretting

Pressurized Heavy water reactor is a horizontal channel type reactor where fuel bundles are inside pressure tubes (PT) and supported at bearing pad locations. Bearing pad (BP) fretting results in formation of volumetric flaws at PT inner surface which may lead to delayed hydride cracking (DHC). In this paper, safety assessment of pressure tube was carried out against DHC initiation at volumetric flaws using CSA N285.8 procedures. Bounding envelop was developed for threshold bulk Heq and volumetric flaw depth for DHC initiation. Several flaw root radius (r) were assumed to arrive at bounding flaw sizes and threshold bulk Heq in the PT. Peak stress was calculated considering hydride ratcheting condition at flaw tip considering sustained hot condition. Four locations, rolled joint inlet/ outlet and PT main body inlet/outlet were selected to assess DHC initiation for volumetric flaws. Residual tensile stress was considered for the assessment at rolled joint location

An improved theoretical process-zone model for delayed hydride cracking initiation at a blunt V-notch

Delayed hydride cracking (DHC) is an important concern for pressure tubes used in nuclear reactors. In this paper, an improved analytical process-zone model is developed based on the deformation fracture criteria. A V-notch with rounded root, which is widely adopted in mechanical testing of DHC, is considered and the proposed model includes the effect of both notch angle and tip radius. Comparisons with experiments show that the proposed model has a prediction accuracy closer to the current engineering process-zone model but with slightly less conservatism. The model is extended to account for plasticity and constraint effects at the flaw tip by introducing an empirical factor that depends on key material and geometric parameters.

Integrated probabilistic assessment for DHC initiation, growth and leak-before-break of PHWR pressure tubes

A few hundred zirconium alloy pressure tubes in a pressurized heavy water reactor (PHWR) serve as the nuclear fuel channel, as well as the reactor coolant pressure boundary. The pressure tubes are inspected periodically and a fitness-for-service assessment (FFSA) must be conducted if any flaw is detected in the inspection. A Canadian standard provides FFSA procedures of PHWR pressure tubes, which include probabilistic assessment for flaws considering delayed hydride cracking (DHC) and leak-before-break (LBB). In the present study, an integrated approach with detailed stepwise calculation procedures and integration methodology for probabilistic assessment of pressure tube was developed. In the first step of this approach, a probability of the DHC initiation, growth and penetration for single initial flaw is calculated. In the next step, a probability of LBB failure, which means tube rupture, for single through-wall crack (TWC) is calculated. Finally, a rupture probability for all initial flaws in a reactor can be calculated using the penetration probability for single flaw and the LBB failure probability for single TWC, as well as the predicted total number of initial flaw in the reactor.

Assessment of Structural Integrity of Pressure Tubes during Cold Pressurization

Abstract Presently, double melted, cold worked and stress relieved Zr-2.5%Nb alloy pressure tubes are employed in some of Indian Pressurized Heavy Water Reactors (IPHWRs) where initial hydrogen content is limited to 25 wppm . Over a period of time, part of hydrogen/deuterium released by the corrosion reaction (metal-coolant reaction) between the Zr-2.5%Nb alloy and D 2 O may be picked up by the pressure tubes which further embrittles the pressure tube material at lower temperatures. Structural integrity of the irradiated pressure tubes of these reactors during cold pressurization is studied. Studies have been performed to ensure that there is no unstable crack growth and Delayed Hydride Cracking (DHC) during cold pressurization of pressure tubes. For these calculations, reported lower bound fracture toughness properties for the double melted Zr-2.5%Nb irradiated pressure tube material have been used. The calculations indicate that the applied stress intensity factor for the postulated part through wall flaw in the pressure tube is less than the threshold stress intensity factor for DHC initiation. Pressure tubes also meet the required factor of safety for the stability of a postulated leakage size crack in case of cold pressurization and the operator has sufficient time to take the necessary action before the crack reaches the critical size. The paper brings out the details of the safety assessment carried out for the pressure tube during cold pressurization.

Finite Element Simulation of Cyclic Ball Indentation on Zr 2.5 wt.% Nb Alloy and Assessment of the Residual Stress Produced

Abstract In-situ Property Measurement System based on ball indentation technique has been developed for estimation of mechanical properties of pressure tubes of Pressurised Heavy Water Reactors. Pressure tubes, made of Zr 2.5 wt% Nb material, operate under high pressure and high temperature and are susceptible to failure due to Delayed Hydride Cracking (DHC). Initiation of DHC from a location is possible under the presence of high tensile stress and hydrides. As the ball indentation process produces residual stress, it is necessary to quantify its magnitude and distribution to assess the fitness of the pressure tube for continued operation. The cyclic indentation process was simulated using a two dimensional axisymmetric finite element model and inelastic analysis was carried out to assess the effect of various parameters on the stress distribution. Th s paper describes the finite element simulation of the indentation process and results obtained from the parametric studies.

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.

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

A risk-informed approach to assessment of DHC initiation in pressure tubes of CANDU reactors

The delayed hydride cracking (DHC) of flaws and cracks in pressure tubes is a serious form of degradation in the reactor core. CSA standard N285.8 (2005) recommends deterministic and probabilistic procedures for assessing the potential of DHC initiation from flaws that are generated by fretting or any other mechanism. Although the deterministic method is simple, it lacks a quantitative risk-informed basis for the assessment. On the other hand, a full probabilistic method based on simulation is tedious to implement. This paper presents an efficient, reliability-based approach that bridges the gap between a simple deterministic analysis and full Monte Carlo simulations. In the proposed method, a deterministic DHC initiation criterion is calibrated to specified target probability levels. The main advantage of the proposed approach is that it provides a practical, risk-informed basis for DHC initiation assessment while retaining the simplicity of the deterministic method.

Delayed Hydride Cracking Initiation at Notches in Zr-2.5Nb Alloys

Delayed hydride cracking (DHC) is an important crack initiation and growth mechanism in Zr-2.5Nb alloy pressure tubes of CANDU nuclear reactors. DHC is a repetitive process that involves hydrogen diffusion, hydride precipitation, growth, and fracture of a hydrided region at a flaw tip. In-service flaw evaluation requires analyses to demonstrate that DHC will not initiate from the flaw. The work presented in this paper examines DHC initiation behavior from V-notches with root radii of 15 μm, 30 μm, and 100 μm, which simulate service-induced debris fretting flaws. Groups of notched cantilever beam specimens were prepared from two unirradiated pressure tubes hydrided to a nominal hydrogen concentration of 57 wt. ppm. The specimens were loaded to different stress levels that straddled the threshold value predicted by an engineering process-zone (EPZ) model, and subjected to multiple thermal cycles representative of reactor operating conditions to form hydrides at the notch tip. Threshold conditions for DHC initiation were established for the notch geometries and thermal cycling conditions used in this program. Test results indicate that the resistance to DHC initiation is dependent on notch root radius, which is shown by optical metallography and scanning electron microscopy to have a significant effect on the distribution and morphology of the notch-tip reoriented hydrides. In addition, it is observed that one tube is less resistant to DHC initiation than the other tube, which may be attributed to the differences in their microstructure and texture. There is a reasonable agreement between the test results and the predictions from the EPZ model.

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.

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.

Overload Fracture of Flaw Tip Hydrides in Zr-2.5Nb Pressure Tubes

Abstract CANDU Zr-2.5Nb pressure tubes are susceptible to a crack initiation mechanism known as Delayed Hydride Cracking (DHC), which is a process that involves hydrogen diffusion, hydride precipitation, hydride region formation, and fracture at a flaw tip. An overload occurs when the hydrided region is loaded to a stress higher than that at which this region is formed. Service-induced flaws are present in some pressure tubes, which can act as crack initiation sites. Most experimental data to assess DHC initiation are obtained under constant loading conditions in which hydride formation and fracture occur at the same load, and therefore they are not suitable to assess crack initiation under overload condition. A series of step-wise increasing load experiments was performed on unirradiated Zr-2.5Nb pressure tube samples to determine the fracture stress of hydrides formed at notches with 15 μm root radius under different hydride formation stresses and thermal histories. Crack initiation in the overload tests was detected by the acoustic emission technique. The notch tip hydride morphologies were examined by optical and scanning electron microscopy. Test results indicated that the resistance to overload fracture was dependent on the hydride formation stress and thermal histories, which affected the notch tip hydride size, density, and distribution. Overload tests were performed at different temperatures, and a transition temperature to high resistance to overload fracture was observed.

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.

Hydride-induced embrittlement and fracture in metals—effect of stress and temperature distribution

A mathematical model for the hydrogen embrittlement of hydride forming metals has been developed. The model takes into account the coupling of the operating physical processes, namely: (i) hydrogen diffusion, (ii) hydride precipitation, (iii) non-mechanical energy flow and (iv) hydride/solid-solution deformation. Material damage and crack growth are also simulated by using de-cohesion model, which takes into account the time variation of energy of de-cohesion, due to the time-dependent process of hydride precipitation. The bulk of the material, outside the de-cohesion layer, is assumed to behave elastically. The hydrogen embrittlement model has been implemented numerically into a finite element framework and tested successfully against experimental data and analytical solutions on hydrogen thermal transport (in: Wunderlich, W. (Ed.), Proceedings of the European Conference on Computational Mechanics, Munich, Germany, 1999, J. Nucl. Mater. (2000a) 279 (2–3) 273). The model has been used for the simulation of Zircaloy-2 hydrogen embrittlement and delayed hydride cracking initiation in (i) a boundary layer problem of a semi-infinite crack, under mode I loading and constant temperature, and (ii) a cracked plate, under tensile stress and temperature gradient. The initial and boundary conditions in case (ii) are those encountered in the fuel cladding of light water reactors, during operation. The effects of near-tip stress intensification as well as of temperature gradient on hydride precipitation and material damage have been studied. The numerical simulation predicts hydride precipitation at a small distance from the crack-tip. When the remote loading is sufficient, the near-tip hydrides fracture. Thus a microcrack is generated, which is separated from the main crack by a ductile ligament, in agreement with experimental observations.

The use of failure assessment diagrams to describe delayed hydride cracking initiation at a blunt flaw

Delayed hydride cracking (DHC) in Zr–2.5Nb alloy material is of interest to the CANadian Deuterium Uranium (CANDU) (1) industry in the context of the potential to initiate DHC at a blunt flaw in a CANDU reactor pressure tube. The existing CANDU blunt flaw DHC evaluation procedure is based on a threshold peak flaw-tip stress for DHC initiation that is independent of flaw geometry. Work is underway to improve the existing blunt flaw DHC evaluation procedure by developing a methodology that takes into account the effect of flaw geometry parameters. The methodology is based on representing the stress relaxation due to hydride formation and crack initiation by a process zone. A process-zone model was used to develop failure assessment diagrams in the conventional format of R6 as described in this paper, although the coordinates of the diagram have a different physical meaning. The failure assessment diagrams were shown to be independent of the scale of the configuration as well as specific values of material properties. The failure assessment curves as derived in the conventional R6 format exhibit a strong dependence on the geometry of the flaw due to the stress concentration and associated stress gradient. By redefining the ordinate of the failure assessment diagram in terms of peak flaw-tip stress the geometry dependence of the failure assessment curves was reduced significantly. This minimal geometry dependence of the failure assessment curves is valuable with regards to practical engineering flaw evaluations. Agreement between the failure assessment diagram predictions and experimental results is reasonable.

Developments in Flaw Evaluation for CANDU Reactor Zr-Nb Pressure Tubes

Delayed hydride cracking (DHC) in Zr-2.5Nb alloy material is of interest to the CANDU (CANada Deuterium Uranium) industry in the context of the potential to initiate DHC at a blunt flaw in a CANDU reactor pressure tube. The material is susceptible to DHC when there is diffusion of hydrogen atoms to the flaw, precipitation of hydride platelets, and development of a hydrided region at the flaw root. The hydrided region can then fracture to the extent that a crack forms, and is able to grow by the DHC crack growth mechanism. The existing CANDU blunt flaw DHC evaluation procedure is based on a threshold peak flaw-tip stress for DHC initiation that is independent of flaw geometry. Work is underway to improve the existing blunt flaw DHC evaluation procedure by developing a methodology that takes into account the effect of flaw geometry parameters. This methodology can, in principle, be applied to any flaw that is found during in-service inspection. The methodology is based on representing the stress relaxation due to hydride formation, and crack initiation, by a process zone. This approach has been used successfully in other engineering applications. A description of how the process-zone methodology can be used to quantify the effect of flaw geometry on the threshold peak stress for DHC initiation is provided. A model that is suitable for actual engineering applications is described, and the engineering model illustrates the dependency of the threshold peak stress for DHC initiation on flaw geometry. Agreement between engineering model predictions and experimental results is reasonable. Various aspects of the DHC initiation problem that are being addressed, in addition to flaw geometry, are described.

A theoretical estimation of fracture toughness of zirconium hydride

The presence of hydrogen in zirconium alloys causes the hydrides to precipitate when hydrogen solubility limits are exceeded. Hydrides in zirconium alloys can lead to a significant embrittlment effect under certain circumstances, and mechanical properties, especially crack growth resistance, may be affected. As a result, there is a phenomenon of delayed hydride cracking (DHC) of zirconium alloys which is characterized by crack propagation at low values of the stress intensity factor. One of important mechanical properties of zirconium alloys is the threshold stress intensity factor, KIH, below which cracks do not grow. Therefore, the problem of hydrogen embrittlment of zirconium alloys is a very important, particularly for structural components in the nuclear industry. For example, the gradual build-up of deuterium in the Zr-2.5Nb pressure tubes of a pressurized heavy water reactor as a result of corrosion, combined with the possibility of the generation of flaws of sufficient size, makes the tubes susceptible to the initiation of DHC [1]. In this case, the service life of a pressure tube is determined by its capability to resist the crack initiation and propagation. Because of the technological importance of this phenomenon, the crack initiation threshold due to the hydride formation has been studied. A theoretical prediction of the KIH value of zirconium alloys can be based on modelling of the process zone ahead of the crack tip (e.g. [2–4]). The process zone is treated as a zone of fracture process. On the other hand, hydrided zirconium alloy can be interpreted as zirconium-zirconium hydride mixture. In this case, to predict the value of KIH of hydrided zirconium alloy it is necessary to estimate the fracture toughness of the hydride. However, measurements of fracture toughness of δ-zirconium hydrides showed large scatter due to specimen problems because of their extreme brittleness [5]. Therefore, a theoretical estimation of the value of K H IC for bydride is very important for an analysis and optimisation of crack growth resistance in zirconium alloys. This paper presents a theoretical estimation of fracture toughness of zirconium hydrides. The theory is based on a local approach to fracture of zirconium alloys. Recently, a model for the threshold stress intensity factor, KIH, was suggested leading to an analysis of crack growth initiation in zirconium alloys during DHC [4]. Local criterion of brittle fracture and modelling of the fracture process zone were used to describe fracture initiation at the hydride platelet in the process zone ahead of the crack tip. The hydride platelet was assumed to cover the process zone and the crack growth proceeds through fracturing of the hydride. The critical condition for fracture initiation in the hydride is reached when the maximum of the local stress approaches the fracture strength of the hydride at some characteristic (critical) distance, rc, ahead of the crack tip (Fig. 1). It was shown that the theoretical KIH-estimation applied to the case of mixed plane condition within the process zone is qualitatively consistent with experimental data for unirradiated Zr-2.5Nb alloy. In the framework of the proposed model, the theoretical value of KIH for a single hydride platelet at the crack tip characterizes the ultimate lower bound value of crack resistance in hydrided zirconium alloy during DHC, whereas the minimum of the threshold stress intensity factor, (KIH)min, should be considered as the minimal capability of the hydride to resist fracture within the hydride, i.e. the fracture toughness, K H IC, of hydride. The minimum fracture toughness of structural materials can be described by a model called the Kμ-model [6]. The following equation should be valid

Assessment of aging of Zr2.5Nb pressure tubes in CANDU ™ reactors

In modern CANDU ™ nuclear generating stations, pressure tubes of cold-worked Zr2.5Nb material are used in the reactor core to contain the fuel bundles and the heavy water (D 2O) coolant. The pressure tubes operate at an internal pressure of about 10 MPa and temperatures ranging from about 250°C at the inlet to about 310°C at the outlet. Over the expected 30 year lifetime of these tubes they will be subjected to a total fluence of approximately 3 × 10 26 n m −2. In addition, these tubes gradually pick up deuterium as a result of a slow corrosion process. When the hydrogen plus deuterium concentration in the tubes exceeds the hydrogen-deuterium solvus, the tubes are susceptible to a crack initiation and propagation process called delayed hydride cracking (DHC). If undetected, such a cracking mechanism could lead to unstable rupture of the pressure tube. A fitness-for-service methodology has been developed which assures that this will not happen. A key element in this methodology is the acquisition of data and understanding—from surveillance and accelerated aging testing—to assess and predict changes in the DHC initiation threshold, the DHC velocity and the fracture toughness (critical crack length) as a function of service time. The most recent results of the DHC and fracture toughness properties of CANDU pressure tubes as a function of time in service are presented and used to suggest procedures for mitigation and life extension of the pressure tubes.