A Framework for Estimating Burst Test Fracture Toughness for Zr-2.5Nb Pressure Tubes Using Data From Small Specimen Tests

2019 ◽  
Author(s):  
Steven X. Xu ◽  
Kim Wallin ◽  
David Cho
Keyword(s):  
Fracture Toughness ◽  
Low Cost ◽  
Pressure Tube ◽  
Current Status ◽  
Operating Pressure ◽  
Small Specimen ◽  
Burst Test ◽  
Pressure Tubes ◽  
Using Data ◽  
Technical Basis

Abstract Zr-2.5Nb pressure tubes are primary pressure boundaries in a CANDU2 reactor. Design of pressure tube dimensions allows testing of a pressure tube section at its full size in the laboratory. Burst tests, i.e., internally pressuring pressure tube sections containing axial through-wall cracks till burst, have been used to provide test data of fracture toughness for pressure tubes with axial flaws. The advantage of measuring fracture toughness from burst tests is that measured toughness values are directly applicable to operating pressure tubes. Burst tests, however, are costly and consume considerable amount of material. Only a small number of burst tests can be performed in practice. There is strong motivation to estimate burst test fracture toughness using data from small specimen tests. The estimated burst test fracture toughness can fill the gap in the measured burst test toughness data, as well as provide information on material variability and data scatter. The technical challenge for estimating burst test toughness is that the estimated burst test toughness using data from low cost, small specimen tests must be reliable and representative of burst test specimen behavior with high confidence. A framework for accurately estimating burst test toughness using data from curved compact tests has been under development and is described in this paper. Aspects of technical basis and current status of developing analytical procedures for systematically estimating burst test toughness are presented.

Acceptance Criteria for Probabilistic Fracture Protection Evaluations of CANDU Zr-Nb Pressure Tubes

2018 ◽  
Author(s):  
Douglas Scarth ◽  
Leonid Gutkin
Keyword(s):  
Fracture Toughness ◽  
Conditional Probability ◽  
Nuclear Reactor ◽  
Reactor Core ◽  
Service Level ◽  
Pressure Tube ◽  
Stability Evaluation ◽  
Acceptance Criteria ◽  
Pressure Tubes ◽  
Technical Basis

Requirements for pressure-temperature limits to protect against rupture of CANDU nuclear reactor Zr-Nb pressure tubes are provided in the Canadian Standards Association (CSA) Standard N285.8. The requirements are based on a stability evaluation of a postulated axial through-wall flaw for all ASME Service Level A, B, C and D loadings. The flaw stability evaluation is strongly dependent on the fracture toughness of the Zr-Nb pressure tube material. The fracture toughness of Zr-Nb pressure tubes is decreasing with operating hours. The decrease in fracture toughness as well as compounding conservatisms based on using bounding values make deterministic evaluations more challenging. The CSA Standard N285.8 permits probabilistic evaluations of fracture protection, but does not provide acceptance criteria. Proposed acceptance criteria that meet the intent of the design basis for Zr-Nb pressure tubes have been developed. The proposed acceptance criteria consist of a proposed maximum allowable conditional probability of pressure tube rupture for the entire reactor core, as well as a proposed maximum allowable conditional probability of rupture of a single pressure tube. The paper provides a description of the technical basis for the proposed acceptance criteria for probabilistic evaluations of fracture protection.

Quantitative Characterization of Hydride Spacing for Cohesive-Zone Modelling of Fracture Toughness in Zr-2.5Nb Pressure Tubes

2016 ◽  
Author(s):  
Cheng Liu ◽  
Leonid Gutkin ◽  
Douglas Scarth
Keyword(s):  
Fracture Toughness ◽  
Cohesive Zone ◽  
Cohesive Zone Model ◽  
Model Development ◽  
Threshold Value ◽  
Pressure Tube ◽  
Zone Model ◽  
Empirical Distributions ◽  
Hydride Formation ◽  
Pressure Tubes

Zr-2.5Nb pressure tubes in CANDU 1 reactors are susceptible to hydride formation when the solubility of hydrogen in the pressure tube material is exceeded. As temperature decreases, the propensity to hydride formation increases due to the decreasing solubility of hydrogen in the Zr-2.5Nb matrix. Experiments have shown that the presence of hydrides is associated with reduction in the fracture toughness of Zr-2.5Nb pressure tubes below normal operating temperatures. Cohesive-zone approach has recently been used to address this effect. Using this approach, the reduction in fracture toughness due to hydrides was modeled by a decrease in the cohesive-zone restraining stress caused by the hydride fracture and subsequent failure of matrix ligaments between the fractured hydrides. As part of the cohesive-zone model development, the ligament thickness, as represented by the radial spacing between adjacent fractured circumferential hydrides, was characterized quantitatively. Optical micrographs were prepared from post-tested fracture toughness specimens, and quantitative metallography was performed to characterize the hydride morphology in the radial-circumferential plane of the pressure tube. In the material with a relatively low fraction of radial hydrides, further analysis was performed to characterize the radial spacing between adjacent fractured circumferential hydrides. The discrete empirical distributions were established and parameterized using continuous probability density functions. The resultant parametric distributions of radial hydride spacing were then used to infer the proportion of matrix ligaments, whose thickness would not exceed the threshold value for low-energy failure. This paper describes the methodology used in this assessment and discusses its results.

Development and Application of Oxide Wedging Crack Growth Model for CANDU Zr-2.5Nb Pressure Tubes

2016 ◽  
Author(s):  
Douglas A. Scarth ◽  
Preeti Doddihal ◽  
Monique Ip
Keyword(s):  
Crack Growth ◽  
Growth Model ◽  
Growth Mechanism ◽  
Pressure Tube ◽  
Operating Pressure ◽  
Metallographic Examination ◽  
Reactor Coolant ◽  
Pressure Tubes ◽  
Crack Growth Mechanism ◽  
Crack Growth Model

Surface breaking and subsurface manufacturing flaws have been detected by in-service ultrasonic examination in a number of CANDU reactor Zr-2.5Nb pressure tubes. The manufacturing flaws are oriented in the circumferential direction in the Zr-2.5Nb pressure tube, and are axially aligned along the pressure tube. Metallographic examination of these flaws in an ex-service Zr-2.5Nb pressure tube revealed a series of parallel circumferentially oriented discontinuous features oriented at an angle of nominally 40 degrees relative to the axial direction in the pressure tube. Since the surface breaking flaws are exposed to the reactor coolant, it was considered prudent to evaluate potential growth of the flaws by an oxide wedging crack growth mechanism. Oxide wedging crack growth is a slow crack growth mechanism that can occur when zirconium oxide forms on the crack faces due to a corrosion reaction with the reactor coolant. An oxide wedging crack growth model was developed to predict crack growth rates and future flaw sizes as a part of the fitness-for-service evaluation of a Zr-2.5Nb pressure tube containing this type of manufacturing flaw. The model was then applied to predict crack growth from manufacturing flaws that were detected in an operating pressure tube, and the evaluation results were used as part of the justification for continued operation.

Effects of Hydride Morphology and Test Temperature on Fracture Toughness of Zr-2.5Nb Pressure Tube Material

2009 ◽  
Author(s):  
Jun Cui ◽  
Gordon K. Shek
Keyword(s):  
Fracture Toughness ◽  
Test Temperature ◽  
Fracture Behavior ◽  
Room Temperature ◽  
Fracture Initiation ◽  
Compact Tension ◽  
Pressure Tube ◽  
Primary Coolant ◽  
Tube Section ◽  
Pressure Tubes

CANDU® reactor uses Zr-2.5Nb alloy pressure tubes as the primary coolant containment. Fracture toughness properties of the pressure tubes are required for evaluation of fracture initiation and leak-before-break. This paper presents an experimental study on the effects of hydride morphology and test temperature on axial fracture toughness of a cold-worked, unirradiated Zr-2.5Nb pressure tube. Compact tension specimens were prepared from one tube section which contained as-received hydrogen concentration and another section which was electrolytically hydrided to 70 ppm hydrogen. Reoriented hydrides were formed in the hydrided tube section in ten thermal cycles under an applied tensile hoop stress of 160 MPa. The hydride morphologies were characterized by a parameter referred to as the hydride continuity coefficient (HCC), which provided a measure of the extent to which the hydrides were reoriented with respect to the applied stress direction. Partially reoriented hydrides with HCC between 0.3–0.4 were formed under the stress and temperature cycles used to precipitate the hydrides. J-R curves were generated to characterize the fracture behavior of the specimens tested at five different temperatures: 25°C (room temperature), 100°C, 150°C, 200°C and 250°C. Test results indicate that, for the as-received specimens, the fracture toughness is relatively high at room temperature and not significantly affected by the test temperature between room temperature and 250°C. For the 70 ppm hydrided specimens containing partially reoriented hydrides, the fracture toughness is significantly lower than that of the as-received specimens at room temperature. At 100°C, the fracture toughness is higher than that at room temperature but the average value is still lower than that of the as-received specimens. The specimens exhibit either brittle or ductile fracture behavior with a sharp transition to an upper-shelf toughness value. At 150°C, the specimens achieve an upper-shelf toughness level. Between 150°C and 250°C, the fracture toughness is similar to that of the as-received specimens and not affected by the reoriented hydrides.

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.

Specimen Curvature and Size Effects on Crack Growth Resistance From Compact Tension Specimens of CANDU Pressure Tubes

2019 ◽  
Author(s):  
Bruce W. Williams ◽  
William R. Tyson ◽  
C. Hari M. Simha ◽  
Bogdan Wasiluk
Keyword(s):  
Fracture Toughness ◽  
Crack Growth ◽  
Crack Extension ◽  
Compact Tension ◽  
Crack Growth Resistance ◽  
Pressure Tube ◽  
Small Scale ◽  
J Integral ◽  
Tube Material ◽  
Pressure Tubes

Abstract CSA Standard N285.8 requires leak-before-break and fracture protection for Zr-2.5Nb pressure tubes in operating CANDU reactors. In-service deuterium uptake causes the formation of hydrides, which can result in additional variability and reduction of fracture toughness. Pressure tube fracture toughness is assessed mainly through rising pressure tube section burst tests. Given the length of the ex-service pressure tubes required for burst testing and the requirement to increase the hydrogen content of irradiated ex-service pressure tubes, only a limited number of burst tests can be performed. Using small-scale compact tension, C(T), specimens are advantageous for obtaining a statistically significant number of fracture toughness measurements while using less ex-service pressure tube material. This work focuses on the study of C(T) geometry designs in order to obtain crack growth resistance and fracture toughness closer to those deduced from burst tests. Because C(T) specimens must be machined from pressure tubes of about 100 mm in diameter and 4 mm in wall thickness, they are out-of-plane curved. As well, they undergo significant tunnelling during crack extension. These two factors can result in a violation of the ASTM standard for fracture toughness testing. The current work examined the influence of specimen curvature and tunnelled crack front on the crack growth resistance curve, or J-R curve. Finite element (FE) models using stationary and growing cracks were used in a detailed numerical investigation. To capture crack tunnelling in the FE models, a damage mechanics approach was adopted, with the critical strain to accumulate damage being a function of crack front stress triaxiality. The J-integral numerically estimated from the domain integral approach was compared to the J-integral calculated from the analytical equations in the ASTM E-1820 standard. In most cases, the difference between the numerical and the standard estimations was less than 10%, which was considered acceptable. It was found that at higher load levels of load-line-displacement, specimen curvature influenced the J-integral results. Crack tunnelling was shown to have a small influence on the estimated J-integrals, in comparison with the straight crack fronts. A modest number of experiments were carried out on unirradiated Zr-2.5Nb pressure tube material using three designs of curved C(T) specimens. It was found that the specimens of both designs that featured a width of 34 mm had more than twice the crack extension of the specimens of the 17-mm width design. The 17-mm width specimens are used mainly to assess the small-scale fracture toughness of pressure tube material. Additionally, the applied J-integral at the maximum load was about 1.4 times higher for the larger-width C(T) specimens. These C(T) specimens also produced J-R curves with greater crack extensions, which were closer to those obtained from the pressure tube section burst tests. Artificially hydrided pressure tube material was not considered in the current work, to avoid any potential source of experimental variability; however, it should be considered in future work.

Exploratory Analysis to Estimate Axial Fracture Toughness for Zr-2.5Nb Pressure Tubes Using Test Data From Small Curved Compact Specimens

2017 ◽  
Author(s):  
Steven X. Xu ◽  
Kim Wallin
Keyword(s):  
Fracture Toughness ◽  
Prediction Model ◽  
Test Data ◽  
Late Life ◽  
Exploratory Analysis ◽  
Pressure Tube ◽  
Life Conditions ◽  
Pressure Tubes ◽  
Axial Fracture ◽  
Leak Before Break

Zr-2.5Nb pressure tubes are in-core, primary coolant containment of CANDU(1) nuclear reactors. Technical requirements for in-service evaluation of pressure tubes are provided in the Canadian Standards Associate (CSA) N285.8. These requirements include the evaluation of service conditions for protection against fracture of operating pressure tubes and demonstration of leak-before-break. Axial fracture toughness for pressure tubes is a key input in the evaluation of fracture protection and leak-before-break. The 2015 Edition of CSA N285.8 provides a pressure tube axial fracture toughness prediction model that is applicable to pressure tubes late life conditions. The fracture toughness prediction model in CSA N285.8-15 is based on rising pressure burst tests performed on pressure tube sections with axial cracks under simulated pressure tube late life conditions. Due to the associated high cost of testing and high consumption of pressure tube material, it is not practical to perform a large number of fracture toughness burst tests. On the other hand, more fracture toughness data is required to improve the existing pressure tube axial fracture toughness prediction model. There is strong motivation to estimate pressure tube axial fracture toughness using test data from small specimens. The estimated pressure tube fracture toughness using test data from small specimens can fill the gaps in the burst test toughness data, as well as provide information on material variability and data scatter. Against this background, an exploratory analysis of estimating pressure tube axial fracture toughness using test data from small curved compact specimens has been performed and is described in this paper. The estimated values of pressure tube axial fracture toughness using the test data from small curved compact specimens are compared with the measured toughness from burst tests of pressure tube sections with axial cracks to check the feasibility of this approach.

Fracture Protection of CANDU Zr-2.5Nb Pressure Tubes With High Hydrogen Equivalent Concentration

2015 ◽  
Author(s):  
Preeti Doddihal ◽  
Douglas Scarth ◽  
Paula Mosbrucker ◽  
Steven Xu
Keyword(s):  
Fracture Toughness ◽  
Heavy Water ◽  
Pressure Tube ◽  
Normal Operation ◽  
Tube Material ◽  
High Hydrogen ◽  
Equivalent Concentration ◽  
Corrosion Reaction ◽  
Pressure Tubes ◽  
Heavy Water Reactor

The core of a CANDU®1 (CANada Deuterium Uranium) pressurized heavy water reactor includes horizontal Zr-2.5Nb alloy pressure tubes that contain the fuel. Pressure-temperature limits are used in CANDU® reactors for normal operation heat-up and cool-down conditions to maintain margins against fracture. The pressure-temperature limits are determined by postulating a 20 mm long axial through-wall crack in the pressure tube and using a fracture toughness-based calculation procedure. Due to a corrosion reaction with the heavy water coolant, pressure tubes absorb deuterium isotope in service, resulting in an increase in hydrogen equivalent concentration. Experiments have shown that high hydrogen equivalent concentration reduces the fracture toughness of pressure tube material at low temperatures during reactor heat-up and cool-down from normal operating temperatures. New fracture toughness curves that are applicable to material with high hydrogen equivalent concentration have been developed to address this issue. These curves are being used to develop new pressure-temperature limits for fracture protection of CANDU® pressure tubes. The methodology for deriving the pressure-temperature limits for a CANDU® Zr-2.5Nb pressure tube using the new fracture toughness curves is presented in this paper. Preliminary results of pressure-temperature limits for a CANDU® reactor are also provided.

A Numerical Study on Fracture Toughness of CANDU Pressure Tube

2017 ◽  
Author(s):  
Elisabeth Keim ◽  
Tomas Nicak ◽  
Bogdan Wasiluk
Keyword(s):  
Fracture Toughness ◽  
Fracture Behavior ◽  
Numerical Study ◽  
Compact Tension ◽  
Operating Conditions ◽  
Pressure Tube ◽  
Crack Tip Opening Displacement ◽  
Opening Displacement ◽  
Two Phase ◽  
Pressure Tubes

CANDU pressure tubes carry nuclear fuel and belong to the primary heat transport system. They are fabricated from cold-worked Zr-2.5Nb alloy prone to deuterium ingress under normal operating conditions. Increased hydrogen equivalent concentration and reactor pressure-temperature transients result in development of a brittle phase, hydride, changing mechanical behavior. The hydride downgrades fracture toughness properties in the transition region and reduces material ductility. Canadian Nuclear Safety Commission founded a two-phase project to improve understanding of the parameters governing fracture toughness properties and load carrying capacity of Zr-2.5Nb pressure tubes with elevated hydrogen equivalent concentrations. This paper presents preliminary results obtained in the first phase. The fracture behavior of a curved compact tension specimen (CCTS) and a pressure tube burst specimen (PT) with axial through-wall crack used in destructive burst test were studied in details. The intention was to identify any differences between fracture behavior of the CCTS and the PT potentially affecting fracture toughness estimates. The stress and deformation states ahead of the crack front, calculated fracture toughness parameters including J-integral and crack tip opening displacement (CTOD), as well as fracture constraint by means of elastic T-stress have been discussed.

AISI 1030

Alloy Digest ◽  
1983 ◽  
Vol 32 (5) ◽  
Keyword(s):  
Fracture Toughness ◽  
Corrosion Resistance ◽  
Low Cost ◽  
Heat Treating ◽  
Plain Carbon Steel ◽  
General Purpose ◽  
Fine Grain ◽  
Steel Mills ◽  
Medium Strength ◽  
Hot Rolled

Abstract AISI 1030 is a plain carbon steel containing nominally 0.30% carbon. It is used in the hot-rolled, normalized, oil-quenched-and-tempered or water-quenched-and-tempered conditions for general-purpose engineering and construction. It provides medium strength and toughness at low cost. Among its many uses are axles, bolts, gears and building sections. All data are on a single heat of fine-grain steel. This datasheet provides information on composition, physical properties, hardness, elasticity, and tensile properties as well as fracture toughness. It also includes information on corrosion resistance as well as forming, heat treating, machining, joining, and surface treatment. Filing Code: CS-94. Producer or source: Carbon and alloy steel mills.

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