Effect of Flaw Orientation on Delayed Hydride Crack Initiation in Zr-2.5Nb Pressure Tubes

2013 ◽  
Author(s):  
Gordon K. Shek ◽  
Jun Cui ◽  
Douglas A. Scarth ◽  
Steven Xu
Keyword(s):  
Stress Concentrator ◽  
Hydrogen Diffusion ◽  
Process Zone ◽  
Axial Direction ◽  
Pressure Tube ◽  
Experimental Program ◽  
Evaluation Procedure ◽  
Oblique Angle ◽  
Repetitive Process ◽  
Pressure Tubes

The Zr-2.5Nb pressure tubes of CANDU reactor are susceptible to a cracking mechanism known as Delayed Hydride Cracking (DHC), which is a repetitive process that involves hydrogen diffusion, hydride precipitation and fracture at a stress concentrator such as a flaw or a crack. Service-induced flaws are present in some pressure tubes and they need to be assessed for susceptibility to DHC initiation. An engineering procedure based on the process-zone methodology has been developed and incorporated into the Canadian standard to determine the susceptibility of flaws in pressure tubes to DHC initiation. The engineering procedure was validated against experiments on flaws which were oriented in the axial direction of the pressure tube. However, many of the service-induced flaws are oriented at some oblique angle with respect to the axial direction of the tube and they may have higher threshold stresses for DHC initiation than those of the axial flaws. It would be advantageous to apply the process-zone evaluation procedure to such angled blunt flaws. For this purpose, an experimental study was carried out to measure the threshold stresses for DHC initiation from machined V-notches with different orientations (0°, 15°, 30° and 45°) with respect to the axial direction of an unirradiated pressure tube. The experimental results were used to support the development of the evaluation procedure for angled blunt flaws. The experimental program and the validation of the engineering procedure for angled blunt flaws are described in this paper.

Delayed Hydride Cracking Initiation at Simulated Secondary Flaws in Zr-2.5 Nb Pressure Tube Material

2004 ◽  
Author(s):  
Jun Cui ◽  
Gordon K. Shek ◽  
Douglas A. Scarth ◽  
William K. Lee
Keyword(s):  
Hydrogen Diffusion ◽  
Process Zone ◽  
Pressure Tube ◽  
Experimental Program ◽  
Small Surface ◽  
Integrity Assessment ◽  
Pressure Tubes ◽  
Delayed Hydride Cracking ◽  
The Impact ◽  
Hydride Cracking

Flaws in Zr-2.5 Nb alloy pressure tubes of CANDU nuclear reactors are susceptible to a crack initiation and growth mechanism called Delayed Hydride Cracking (DHC), which is a repetitive process that involves hydrogen diffusion, hydride precipitation, growth of the hydrided region and fracture of the hydrided region at the flaw-tip. The presence of small surface irregularities, or secondary flaws, at the bottom of service-induced fretting flaws in pressure tubes requires an integrity assessment in terms of DHC initiation. Experimental data and analytical modeling are required to predict whether DHC initiation can occur from the secondary flaws. In the present work, an experimental program was carried out to examine the impact of small secondary flaws with sharp radii on DHC initiation from simulated fretting flaws. Groups of cantilever beam specimens containing blunt notches with and without secondary flaws were prepared from unirradiated pressure tube materials hydrided to a nominal concentration of 50 wt ppm hydrogen. The specimens were subjected to multiple thermal cycles to form hydrides at the flaw-tip at different applied stress levels, which straddled the threshold value for DHC initiation. The threshold conditions for DHC initiation were established for different simulated fretting and secondary flaws. The experimental results are compared with predictions from the engineering process-zone DHC initiation model.

Effect of Hydrogen Isotope and Concentration on Delayed Hydride Crack Growth Rates in Zr-2.5Nb Pressure Tubes

2010 ◽  
Author(s):  
Gordon K. Shek ◽  
Harry Seahra
Keyword(s):  
Test Temperature ◽  
Growth Rates ◽  
Hydrogen Isotope ◽  
Hydrogen Diffusion ◽  
Critical Length ◽  
Potential Drop ◽  
Axial Direction ◽  
Peak Temperature ◽  
Pressure Tube ◽  
Pressure Tubes

CANDU Zr-2.5 Nb pressure tubes are susceptible to a cracking mechanism known as Delayed Hydride Cracking (DHC), which is a repetitive process that involves hydrogen diffusion, hydride precipitation and fracture at a crack tip. As defense-in-depth, when DHC is postulated to have initiated from a flaw, it is required to demonstrate that the crack can be detected by the leak monitoring system and the reactor safely shut down before the crack reaches the critical length for pressure tube rupture. DHC growth rates (DHCR) in the axial direction of the tube are required for such leak-before-break assessment. In this test program, the effect of hydrogen isotope and its concentration on DHCR in an unirradiated Zr-2.5 Nb pressure tube is studied. Pressure tube sections were hydrided or deuterided to different concentrations (nominal concentrations of 60, 100 and 190 ppm by weight). For the deuterided tube sections, they contained about 10 ppm of hydrogen from the manufacturing process. The DHC growth rate tests were performed on fatigue pre-cracked curved compact tension specimens, machined from the hydrided or deuterided tube sections, in several stepper-motor controlled load frames with cracking being monitored by direct current potential drop and acoustic emission techniques. DHCR at three test temperatures (270°C, 240°C and 200°C) were obtained from each specimen with the test temperatures approached from a peak temperature of 330°C. Some specimens were tested with a peak temperature of either 370°C or 300°C. The two main conclusions from the study are: (1) DHCR are affected by the hydrogen in solution at the test temperature and not by the amount of bulk hydrides present. The hydrogen in solution at a given test temperature depends on the hydrogen concentration of the specimen, as well as the thermal history (peak temperature in the initial thermal cycle and the test temperature) as a result of the hysteresis of Terminal Solid Solubility between hydride dissolution during heating and precipitation during cooling. (2) The DHC growth rates of the hydrided material are higher than those of the deuterided material because of the higher diffusion rate of hydrogen than deuterium. The isotope effect of hydrogen on DHC growth rates depends on the test temperature, with no apparent effect at 200°C and about 37% difference at 270°C which is slightly below the factor of √2 expected from the mass law of diffusion. The observed temperature dependence could be due to the presence of about 10 ppm hydrogen in the deuterided specimens, which dominates the DHC process at 200°C but insufficient to have a large effect at 270°C. The implication of the observed isotope and concentration effect of hydrogen on DHC growth rates on leak-fore-break assessment of flaws in pressure tubes is discussed.

Overload Fracture of Hydrided Region at Simulated Blunt Flaws in Zr-2.5Nb Pressure Tube Material

2009 ◽  
Vol 131 (4) ◽  
Author(s):  
Jun Cui ◽  
Gordon K. Shek ◽  
Zhirui Wang
Keyword(s):  
Crack Initiation ◽  
Hydrogen Diffusion ◽  
Pressure Tube ◽  
Test Results ◽  
Hydride Formation ◽  
Repetitive Process ◽  
Pressure Tubes ◽  
Reactor Operating ◽  
Canada Limited ◽  
Precipitation Formation

A crack initiation and growth mechanism known as delayed hydride cracking (DHC) is a concern for Zr-2.5Nb alloy pressure tubes of CANada Deuterium Uranium or CANDU (CANDU is a trademark of the Atomic Energy of Canada Limited, Ontario, Canada) nuclear reactors. DHC is a repetitive process that involves hydrogen diffusion, hydride precipitation, formation, and fracture of a hydrided region at a flaw tip. An overload occurs when the flaw-tip hydrided region is loaded to a stress, higher than that at which this region is formed. For the fitness-for-service assessment of the pressure tubes, it is required to demonstrate that the overload from the normal reactor operating and transient loading conditions will not fracture the hydrided region, and will not initiate DHC. In this work, several series of systematically designed, monotonically increasing load experiments are performed on specimens, prepared from an unirradiated pressure tube with hydrided region, formed at flaws with a root radius of 0.1 mm or 0.3 mm, under different hydride formation stresses and thermal histories. Crack initiation in the overload tests is detected by the acoustic emission technique. Test results indicate that the resistance to overload fracture is dependent on a variety of parameters including hydride formation stress, thermal history, hydrogen concentration, and flaw geometry.

Overload Fracture of Hydrided Region at Simulated Blunt Flaws in Zr-2.5Nb Pressure Tube Material

2005 ◽  
Author(s):  
Jun Cui ◽  
Gordon K. Shek
Keyword(s):  
Crack Initiation ◽  
Hydrogen Diffusion ◽  
Preliminary Test ◽  
Pressure Tube ◽  
Test Results ◽  
Hydride Formation ◽  
Repetitive Process ◽  
Pressure Tubes ◽  
Reactor Operating ◽  
Increasing Load

Flaws in Zr-2.5Nb alloy pressure tubes in CANDU nuclear reactors are susceptible to a crack initiation and growth mechanism known as Delayed Hydride Cracking (DHC), which is a repetitive process that involves hydrogen diffusion, hydride precipitation, growth of the hydrided region and fracture of the hydrided region at the flaw-tip. An overload occurs when the hydrided region at a flaw is loaded to a stress higher than that at which this region is formed. Flaw disposition requires justification that the hydrided region overload from normal reactor operating and transient loading conditions will not fracture the hydrided region, and will not initiate DHC. Some preliminary test results on the effect of hydrided region overload on DHC initiation were presented in Reference [1]. In the present work, several series of more systematically designed monotonically increasing load experiments were performed on specimens prepared from an unirradiated pressure tube with hydrided region formed at flaws with a root radius of 0.1 or 0.3 mm under different hydride formation stresses and thermal histories. Crack initiation in the overload tests was detected by the acoustic emission technique. Test results indicate that the resistance to overload fracture is dependent on a variety of parameters including hydride formation stress, thermal history, flaw geometry and hydrogen concentration.

Fracture Behavior of Finite Length Part Through Wall Flaws in Zirconium-Niobium Pressure Tubes

2006 ◽  
Author(s):  
Don R. Metzger ◽  
Gordon K. Shek ◽  
Ed T. C. Ho
Keyword(s):  
Finite Length ◽  
Process Zone ◽  
Three Dimensional ◽  
Analytical Techniques ◽  
Fracture Initiation ◽  
Zirconium Hydride ◽  
Experimental Program ◽  
Two Dimensional ◽  
Finite Element Program ◽  
Pressure Tubes

Flaws encountered in nuclear pressure tubes must be evaluated to ensure that a delayed hydride cracking (DHC) mechanism is not initiated where the stress concentration at a flaw tip causes diffusion of hydrogen and precipitation of zirconium hydride at the flaw tip. A fracture initiation model for DHC involves a process zone description for the interaction of hydride precipitation with the flaw tip stress distribution. Analytical techniques for this model are practical and accurate for two-dimensional geometry, but cannot be easily applied to the three-dimensional features of finite length surface flaws. Recently, a numerical rendition of the model has been incorporated into a finite element program so that arbitrary geometry and material properties can be managed. The three-dimensional finite length model is applied to specific flaw geometries used in an experimental program. Comparison with corresponding two-dimensional tests demonstrates that the finite length flaw has a significantly higher threshold load than that predicted on the basis of a two-dimensional model.

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

2009 ◽  
Vol 131 (4) ◽  
Author(s):  
Jun Cui ◽  
Gordon K. Shek ◽  
D. A. Scarth ◽  
Zhirui Wang
Keyword(s):  
Hydrogen Diffusion ◽  
Process Zone ◽  
Notch Root ◽  
Threshold Value ◽  
Operating Conditions ◽  
Test Results ◽  
Pressure Tubes ◽  
Delayed Hydride Cracking ◽  
Notch Tip ◽  
Hydride Cracking

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.

Statistical Assessment of Crack Initiation at Simulated Flaws due to Hydrided Region Overload in CANDU Zr-2.5%Nb Pressure Tube Material

2011 ◽  
Author(s):  
Leonid Gutkin ◽  
Douglas A. Scarth
Keyword(s):  
Stress Concentration ◽  
Crack Initiation ◽  
Applied Stress ◽  
Pressure Tube ◽  
Experimental Program ◽  
Tube Material ◽  
Statistical Assessment ◽  
Hydride Formation ◽  
Pressure Tubes ◽  
Threshold Stress Intensity Factor

CANDU Zr-2.5%Nb pressure tubes are susceptible to formation of hydrided regions at the locations of stress concentration, such as in-service flaws. When the applied stress acting on a flaw with an existing hydrided region exceeds the stress at which the hydrided region has been formed, hydrided region overload may occur. Probabilistic methodology is being developed to evaluate in-service flaws in the pressure tubes for crack initiation due to hydrided region overload. Statistical assessment of relevant experimental data on the overload resistance of Zr-2.5%Nb has been performed as part of this development. The results of this assessment indicate that the critical nominal stress for crack initiation due to hydrided region overload increases with increasing the nominal applied stress during hydrided region formation, decreasing the stress concentration factor and increasing the threshold stress intensity factor for initiation of delayed hydride cracking. These findings are consistent with our fundamental understanding of hydrided region overload, as well as with the previous modeling work by E. Smith, as referenced in the paper. The overload resistance also appears to increase with the number of thermal cycles in the course of hydride formation. The results of this assessment have been used to develop a preliminary probabilistic model to predict the critical stress for crack initiation due to hydrided region overload under ratcheting hydride formation conditions, as well as a comprehensive experimental program to further investigate the overload behavior of CANDU pressure tube material.

Validation of Weight Function Based Process-Zone Model for Evaluation of Delayed Hydride Cracking Initiation in Zr-2.5Nb Pressure Tubes

2016 ◽  
Author(s):  
Steven X. Xu ◽  
Dennis Kawa ◽  
Jun Cui ◽  
Heather Chaput
Keyword(s):  
Weight Function ◽  
Process Zone ◽  
Matrix Material ◽  
Pressure Tube ◽  
Zone Model ◽  
Hydrogen Atoms ◽  
Fuel Bundle ◽  
Pressure Tubes ◽  
Delayed Hydride Cracking ◽  
Hydride Cracking

In-service flaws in cold-worked Zr-2.5 Nb pressure tubes in CANDU(1) reactors are susceptible to a phenomenon known as delayed hydride cracking (DHC). The material is susceptible to DHC when there is diffusion of hydrogen atoms to a service-induced flaw, precipitation of hydrides on appropriately oriented crystallographic planes in the zirconium alloy matrix material, and development of a hydrided region at the flaw tip. The hydrided region could then fracture to the extent that a crack forms and DHC is said to have initiated. Examples of in-service flaws are fuel bundle scratches, crevice corrosion marks, fuel bundle bearing pad fretting flaws, and debris fretting flaws. These flaws are volumetric in nature. Evaluation of DHC initiation from the flaw is a requirement of Canadian Standards Association (CSA) Standard N285.8. This paper describes the validation of the weight function based process-zone model for evaluation of pressure tube flaws for DHC initiation. Validation was performed by comparing the predicted threshold load levels for DHC initiation with the results from DHC initiation experiments on small notched specimens. The notches in the specimens simulate axial in-service flaws in the pressure tube. The validation was performed for both un-irradiated and pre-irradiated pressure tube material.

Engineering Process-Zone Model for Evaluation of Structural Strength of Fuel Channel Annulus Spacers in CANDU Nuclear Reactors

2017 ◽  
Author(s):  
Douglas Scarth ◽  
Steven Xu ◽  
Cheng Liu
Keyword(s):  
Structural Strength ◽  
Process Zone ◽  
Maximum Load ◽  
Pressure Tube ◽  
Irradiation Damage ◽  
Zone Model ◽  
Engineering Process ◽  
Fuel Channel ◽  
Load Carrying ◽  
Pressure Tubes

The core of a CANDU(1) (CANada Deuterium Uranium) pressurized heavy water reactor consists of a lattice of either 390 or 480 horizontal Zr-Nb pressure tubes, depending on the reactor design. These pressure tubes contain the fuel bundles. Each pressure tube is surrounded by a Zircaloy calandria tube that operates at a significantly lower temperature. Fuel channel annulus spacers maintain the annular gap between the pressure tube and calandria tube throughout the operating life. To meet this design requirement, annulus spacers must have adequate structural strength to carry the interaction loads imposed between the pressure tube and calandria tube. Crush tests that have been performed on specimens from as-received and ex-service Inconel X-750 alloy spacers have demonstrated that the structural strength of Inconel X-750 spacers has degraded with operating time due to irradiation damage. There was a need for an engineering model to predict the future maximum load carrying capacity of the spacer coils for use in Fitness-for-Service evaluations of spacer structural integrity. An engineering process-zone model has been developed and used to analyze the spacer crush test results, and provide predictions of the Inconel X-750 spacer coil future maximum load carrying capacities. The engineering process-zone model is described in this paper. The process-zone model is based on the strip-yield approach of a process zone with a uniform restraining stress representing the fracture region that is surrounded by elastic material.

Development of Evaluation Procedures for Crack Initiation From In-Service Flaws in CANDU Zr-2.5Nb Pressure Tubes With Hydrogen

2016 ◽  
Author(s):  
David Cho ◽  
Steven X. Xu ◽  
Douglas A. Scarth ◽  
Gordon K. Shek
Keyword(s):  
Crack Initiation ◽  
Hydrogen Isotope ◽  
Operating Conditions ◽  
Pressure Tube ◽  
Evaluation Procedure ◽  
Operating Pressure ◽  
Evaluation Procedures ◽  
Technical Requirements ◽  
Fuel Bundle ◽  
Pressure Tubes

Flaws found during in-service inspection of CANDU(1) Zr-2.5Nb pressure tubes include fuel bundle scratches, debris fretting flaws, fuel bundle bearing pad fretting flaws and crevice corrosion flaws. These flaws are volumetric and blunt in nature. Crack initiation from in-service flaws can be caused by the presence of hydrogen in operating pressure tubes and resultant formation of hydrided regions at the flaw tips during reactor heat-up and cool-down cycles. Zr-2.5Nb pressure tubes in the as-manufactured condition contain hydrogen as an impurity element. During operation, the pressure tube absorbs deuterium, which is a hydrogen isotope, from the corrosion reaction of the zirconium with the heavy water coolant. In addition, deuterium ingresses into the pressure tube in the rolled joint region. The level of hydrogen isotope in pressure tubes increases with operating time. Over the years, Canadian CANDU industry has carried out extensive experimental and analytical programs to develop evaluation procedures for crack initiation from in-service flaws in Zr-2.5Nb pressure tubes. Crack initiation experiments were performed on pressure tube specimens with machined notches to quantify resistance to crack initiation under various simulated flaw geometries and operating conditions such as operating load and hydrogen concentration. Predictive engineering models for crack initiation have been developed based on understandings of crack initiation and experimental data. A set of technical requirements, including engineering procedures and acceptance criteria, for evaluation of crack initiation from in-service flaws in operating pressure tubes has been developed and implemented in the CSA Standard N285.8. A high level review of the development of these flaw evaluation procedures is described in this paper. Operating experience with the application of the developed flaw evaluation procedure is also provided.

Sign in / Sign up

Export Citation Format

Share Document