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.

Cycle-Wise Process-Zone Model for Prediction of Delayed Hydride Cracking Initiation Under Flaw-Tip Hydride Ratcheting Conditions

2018 ◽  
Author(s):  
Steven X. Xu ◽  
Jun Cui ◽  
Douglas A. Scarth ◽  
David Cho
Keyword(s):  
Structural Integrity ◽  
Process Zone ◽  
Operating Conditions ◽  
Zone Model ◽  
Evaluation Procedures ◽  
Fuel Bundle ◽  
Pressure Tubes ◽  
Delayed Hydride Cracking ◽  
Technical Basis ◽  
Hydride Cracking

Flaws found during in-service inspection of Zr-2.5Nb pressure tubes in CANDU(1) reactors 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. A key structural integrity concern with in-service blunt flaws is their susceptibility to delayed hydride cracking (DHC) initiation, particularly for debris fretting flaws under flaw-tip hydride ratcheting conditions. Hydride ratcheting conditions refer to situations when flaw-tip hydrides do not completely dissolve at normal operating temperature, and accumulation of flaw-tip hydrides occurs with each reactor heat-up/cool-down cycle. A significant number of in-service flaws are expected to be under hydride ratcheting conditions at late life of pressure tubes. DHC initiation evaluation procedures based on process-zone methodology for flaws under hydride ratcheting conditions are provided in CSA (Canadian Standards Association) N285.8-15. The process-zone model in CSA N285.8-15 predicts whether DHC initiation occurs or not for given flaw geometry and operating conditions, regardless of the number of reactor heat-up and cool-down cycles. There has been recent new development. Specifically, a cycle-wise process-zone model has been developed as an extension to the process-zone model in CSA N285.8-15. The cycle-wise process-zone model is able to predict whether DHC initiation occurs or not during a specific reactor heat-up and cool-down cycle under applied load. The development of the cycle-wise process-zone model was driven by the need to include flaw-tip stress relaxation due to creep in evaluation of DHC initiation. The technical basis for the development of the cycle-wise process-zone model for prediction of DHC initiation under flaw-tip hydride ratcheting conditions is 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.

Comments on the Process Zone Methodology When Applied to Delayed Hydride Cracking in CANDU Zr-Nb Pressure Tube Material

2008 ◽  
Author(s):  
E. Smith
Keyword(s):  
Process Zone ◽  
Pressure Tube ◽  
Zone Model ◽  
Tube Material ◽  
Delayed Hydride Cracking ◽  
The Way ◽  
Hydride Cracking

The paper discusses the application of the process zone model to the problem of hydrided region formation and Delayed Hydride Cracking (DHC) in CANDU Zr-Nb pressure tube material. The special characteristics of the process zone approach, as used for the DHC problem, are highlighted, while making comparisons with the way in which it is more generally applied in other engineering situations.

A Probabilistic Integrity Assessment of Flaw in Zirconium Alloy Pressure Tube Considering Delayed Hydride Cracking

2003 ◽  
Vol 17 (08n09) ◽  
pp. 1587-1593 ◽  
Author(s):  
Sang Log Kwak ◽  
Joon Seong Lee ◽  
Young Jin Kim ◽  
Youn Won Park
Keyword(s):  
Probabilistic Analysis ◽  
Nuclear Reactor ◽  
Failure Criteria ◽  
Pressure Tube ◽  
Initial Surface ◽  
Tube Material ◽  
Integrity Assessment ◽  
Pressure Tubes ◽  
Delayed Hydride Cracking ◽  
Hydride Cracking

In the CANDU nuclear reactor, pressure tubes of cold-worked Zr-2.5Nb material are used in the reactor core to contain the nuclear fuel bundles and heavy water coolant. Pressure tubes are major component of nuclear reactor, but only selected samples are periodically examined due to numerous numbers of tubes. Pressure tube material gradually pick up deuterium, as such are susceptible to a crack initiation and propagation process called delayed hydride cracking (DHC), which is the characteristic of pressure tube integrity evaluation. If cracks are not detected, such a cracking mechanism could lead to unstable rupture of the pressure tube. Up to this time, integrity evaluations are performed using conventional deterministic approaches. So it is expected that the results obtained are too conservative to perform a rational evaluation of lifetime. In this respect, a probabilistic safety assessment method is more appropriate for the assessment of overall pressure tube safety. This paper describes failure criteria for probabilistic analysis and fracture mechanics analyses of the pressure tubes in consideration of DHC. Major input parameters such as initial hydrogen concentration, the depth and aspect ratio of an initial surface crack, DHC velocity and fracture toughness are considered as probabilistic variables. Failure assessment diagram of pressure tube material is proposed and applied in the probabilistic analysis. In all the analyses, failure probabilities are calculated using the Monte Carlo simulation. As a result of analysis, conservatism of deterministic failure criteria is showed.

Multi-Variable Engineering Model for Axial Delayed Hydride Cracking Growth Rate in Pre-Irradiated Zr-2.5%Nb

2009 ◽  
Author(s):  
Leonid Gutkin ◽  
Douglas A. Scarth
Keyword(s):  
Growth Rate ◽  
Regression Model ◽  
Test Temperature ◽  
Pressure Tube ◽  
Tube Length ◽  
Variable Model ◽  
Pressure Tubes ◽  
Delayed Hydride Cracking ◽  
Absolute Test ◽  
Hydride Cracking

The growth rate of postulated delayed hydride cracks in CANDU Zr-2.5%Nb pressure tubes is an important material property required for flaw evaluations and leak-before-break assessments. It is monitored using surveillance pressure tubes according to the requirements of the Canadian Standards Association (CSA) Standard N285.4 [1]. Radial growth rate and axial growth rate are used to calculate the propagation of delayed hydride cracks in the through-wall direction and along the pressure tube length, respectively. The axial delayed hydride cracking growth rate had been previously found to increase exponentially with inverse absolute test temperature. This dependence had been described by an Arrhenius-type regression model with one explanatory variable. As more experimental results were obtained from surveillance pressure tubes, it has become possible to assess whether there may be statistically significant effects of other variables, which should be incorporated into the representative relation for the axial delayed hydride cracking growth rate. In this paper, multi-variable regression analysis has been used to develop an improved representative model for the axial delayed hydride cracking growth rate of irradiated Zr-2.5%Nb pressure tube material. The developed model explains approximately 93% of overall observed variation in the experimental data, and therefore has better predictive capabilities than the reference regression model with test temperature as a sole predictor. The developed multi-variable model is proposed to be incorporated into the scheduled revision (2010 edition) of the CSA Standard N285.8 as the representative predictive 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.

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.

The Effect of Plasticity on Process-Zone Predictions of DHC Initiation at a Flaw in CANDU Reactor Zr-Nb Pressure Tubes

2002 ◽  
Author(s):  
Douglas A. Scarth ◽  
Ted Smith
Keyword(s):  
Zirconium Alloy ◽  
Process Zone ◽  
Matrix Material ◽  
Zone Model ◽  
Hydrogen Atoms ◽  
Alloy Matrix ◽  
Model Calculations ◽  
Alloy Material ◽  
Crack Growth Mechanism ◽  
Service Inspection

Delayed Hydride Cracking (DHC) in Zr-2.5 Nb alloy material is of interest to the CANDU 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 tip. The hydrided region could then fracture to the extent that a crack forms, and is able to grow by the DHC crack growth mechanism. A procedure for evaluating DHC initiation at a blunt flaw that takes into account the effect of flaw geometry has been developed. 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. The authors have presented predictions of DHC initiation on the basis of elastic conditions in the surrounding zirconium alloy matrix material outside of the process zone in papers presented at the 1999, 2000 and 2001 ASME PVP Conferences. In the present paper, the effects of plasticity in the surrounding zirconium alloy matrix material have been incorporated into a simple theoretical process-zone model for DHC initiation, as well as into an engineering process-zone model that is suitable for evaluation of in-service flaws. The resulting process-zone model calculations and comparisons with DHC initiation experiments demonstrate that DHC initiation predictions can be overly-conservative if plasticity is not taken into account.

Statistical Modeling of Resistance to Crack Initiation due to Hydrided Region Overloads at Simulated Flaws in CANDU Pressure Tubes

2015 ◽  
Author(s):  
Leonid Gutkin ◽  
Douglas A. Scarth
Keyword(s):  
Crack Initiation ◽  
Stress Reduction ◽  
Process Zone ◽  
Reactor Core ◽  
Dual Process ◽  
Modeling Framework ◽  
Hydride Formation ◽  
Pressure Tubes ◽  
Delayed Hydride Cracking ◽  
Hydride Cracking

CANDU(1) Zr-2.5%Nb pressure tubes are susceptible to formation of hydrided regions at the locations of stress concentration, such as in-service flaws. Hydrided region overloads occur when the applied stress acting on a flaw with an existing hydrided region exceeds the stress at which the hydrided region has been formed. The overload events may potentially result in crack initiation and its subsequent growth by the mechanism of delayed hydride cracking. Therefore, evaluating the in-service flaws in the pressure tubes for crack initiation due to hydrided region overloads is required by the Canadian Nuclear Standards, and methodology is being developed to perform such evaluations. As part of this development, the resistance of pressure tube material to crack initiation due to hydrided region overloads was modeled statistically. In the proposed modeling framework, the overload resistance is expressed as a power-law function of the material resistance to initiation of delayed hydride cracking under constant loading. This approach fundamentally relies on the concept of a dual process zone introduced by E. Smith, as discussed in the paper. Both the overload crack initiation coefficient and the overload crack initiation exponent vary with the flaw geometry. The overload crack initiation coefficient also varies with the extent of stress reduction prior to hydride formation and with the number of non-ratcheting hydride formation thermal cycles. The developed model is suitable for use as a predictive model in probabilistic assessments of CANDU reactor core, and has been proposed for implementation into the scheduled revision (2015) of the Canadian Nuclear Standard CSA N285.8.

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