Correlation Between Irradiated and Unirradiated Fracture Toughness of Zr-2.5Nb Pressure Tubes

2009 ◽  
pp. 135-135-33 ◽  
Author(s):  
PH Davies ◽  
RR Hosbons ◽  
M Griffiths ◽  
CK Chow
Keyword(s):  
Fracture Toughness ◽  
Pressure Tubes

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.

Tensile properties and fracture toughness of Zr–2.5Nb alloy pressure tubes of IPHWR220

2015 ◽  
Vol 293 ◽  
pp. 138-149 ◽  
Author(s):  
H.K. Khandelwal ◽  
R.N. Singh ◽  
A.K. Bind ◽  
S. Sunil ◽  
J.K. Chakravartty ◽  
...  
Keyword(s):  
Fracture Toughness ◽  
Tensile Properties ◽  
Pressure Tubes

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.

Feasibility of Developing a Risk-Informed Procedure to Define Pressure-Temperature Limits for Fracture Protection of Zr-Nb Pressure Tubes Under Heat-Up and Cool-Down Conditions

2015 ◽  
Author(s):  
Steven X. Xu ◽  
Douglas A. Scarth ◽  
Preeti Doddihal ◽  
Paula Mosbrucker
Keyword(s):  
Fracture Toughness ◽  
Nuclear Industry ◽  
Calculation Procedure ◽  
Operational Flexibility ◽  
Probabilistic Evaluation ◽  
Pressure Tubes ◽  
History Of ◽  
Current Calculation ◽  
Operational Constraints ◽  
The U.S

A fracture mechanics based calculation procedure is provided in the CSA Standard N285.8 to define pressure-temperature limits for fracture protection of CANDU Zr-Nb pressure tubes. The calculated pressure-temperature limits are used to construct a plant-specific operating envelope for pressure tubes under reactor heat-up and cool-down conditions. The current calculation procedure to define pressure-temperature limits for fracture protection is deterministic, and makes use of conservative inputs, including an axial through-wall crack of 20 mm in length, lower-bound fracture toughness, and a safety factor of 1.3 on the calculated critical internal pressure. The deterministic procedure is straightforward to use, and has a long history of successful applications of protecting pressure tubes from rupture. However, the deterministic procedure will potentially impose challenging operational constraints on pressure tubes at late life conditions, due to the predicted low fracture toughness of pressure tubes with high levels of hydrogen equivalent concentration. As an alternative, the CSA Standard N285.8 also permits probabilistic evaluation of fracture protection, which implies the acceptability of using risk-informed pressure-temperature limits for pressure tubes under reactor heat-up and cool-down conditions. The feasibility of developing a risk-informed procedure to define pressure-temperature limits for fracture protection of pressure tubes under heat-up and cool-down conditions is described in this paper. The intent is to use the risk-informed methodology to develop alternate pressure-temperature limits that allow more operational flexibility and still satisfy safety goals. The proposed risk-informed approach is consistent with risk-informed approaches that have been used in the U.S. nuclear industry.

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.

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