ML18267A085
| ML18267A085 | |
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|---|---|
| Issue date: | 09/24/2018 |
| From: | Robert Tregoning NRC/RES/DE |
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| Download: ML18267A085 (15) | |
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Fatigue Crack Growth Rate Curves for Low Alloy Steels in the BWR Environment Sam Ranganath XGEN engineering Bob Carter EPRI NRC Public Meeting on EAF Research and Related ASME Activities, Rockville, MD September 25, 2018
© 2018 Electric Power Research Institute, Inc. All rights reserved.
Background
Fatigue crack growth in low alloy steel (LAS) in the BWR environment is an important concern in BWRs
- Feedwater nozzle cracking in the late 1970s is an example LAS fatigue crack growth in the environment exhibits some unusual characteristics
- Data scatter of up to three orders of magnitude
- The same material may exhibit Environmentally Assisted Cracking (EAC) effect under some conditions and no EAC effect at all under other conditions The first attempt to understand the reasons for the data scatter and develop a predictive model was in the EPRI-sponsored program by Eason and published in August 1993 as TR-102793
- One limitation of this work was that it was mostly based on data for PWR environment. BWR data was very limited ASME Code Case N-643-2 incorporated the findings from the EPRI work; it is intended only for the PWR environment The objective of this project is to develop a similar Ref. Analysis of Fatigue Crack Growth Rate Data for A508 and A533B Steels in methodology for the BWR environment and develop a LWR Environments, EPRI Report TR-102793, August 1993.
Code Case for BWR crack growth rates 2
© 2018 Electric Power Research Institute, Inc. All rights reserved.
Onset of Accelerated Crack Growth due to EAC Onset of EAC Ref. Analysis of Fatigue Crack Growth Data for A508 and A533B in LWR Environments, EPRI Report TR-102793, August 1993 3
© 2018 Electric Power Research Institute, Inc. All rights reserved.
Time Domain Modeling for EAC Crack Growth Prediction The traditional da/dN vs. K plots show large data scatter because of the many variables, e.g., rise time, R ratio, environment, K. The time domain plot of da/dtenv vs. da/dtair reduces the scatter.
Shoji and Takahashi1 suggested the time domain approach based the hypotheses that the crack tip strain rate controls EAC growth rate and is directly proportional to the fatigue crack growth rate in inert environment (air)
Ref. Analysis of Fatigue Crack Growth Data for A508 and A533B in where a is in mm, tR is the rise time in seconds, K LWR Environments, EPRI Report TR-102793, August 1993 is the stress intensity in MPam and R is Kmin/Kmax 1Ref. T.Shoji, H.Takahashi, M.Suzuki and T.Kondo, A new Parameter for characterizing Corrosion Fatigue Crack Growth, Journal of Engineering Materials and Technology, Transactions ASME, Vol.
103, October 1981, pp. 298-304 4
© 2018 Electric Power Research Institute, Inc. All rights reserved.
Recommendations from Easons Report (EPRI TR-102793)
Air Model
- da/dN = 7.78 x 10-8 [K / (2.88-R)]3.07 Low S Model (<0.005 wt.%)
- da/dN = 1.4 x 10-7 [K / (2.88-R)]3.07 Upper Slope
- da/dN = 2.36 x 10-5 TR0.652 [K / (2.88-R)]3.07 Second Slope
- da/dN = 3.16 x 10-6 [K / (2.88-R)]3.07 EAC Threshold
- Ka = 0.599 TR0.125 (2.88-R)2.99 , 0 < R < 0.9 4.61 TR0.125 exp [(0.9-R / (1.0-R)] , 0.9 < R < 1.0
- Kb =2.73 TR0.326 (2.88-R)
- Kc =17.32 TR0.326 (2.88-R) a is in mm, da/dN is in mm/s, K is in Mpa-m Based on statistical analysis Most of the data is from PWR environment 5
© 2018 Electric Power Research Institute, Inc. All rights reserved.
Predictive Model for BWRs Based on Seifert and Ritter modification of the Ford-Andresen model for the BWR environment Predictive model qualified by extensive test data for the BWR environment
- Almost seven hundred data points with CT specimens; a majority with crack length monitored by reversing DC potential method
- 15 heats of low alloy steels mostly with high sulfur
- Explicit consideration of BWR water chemistry, specifically the ECP associated with BWR oxygenated water in BWR normal water chemistry (NWC) and hydrogen water chemistry (HWC)
The objective is to develop a Code case applicable to BWRs (similar to N-643-2 for PWRs) 6
© 2018 Electric Power Research Institute, Inc. All rights reserved.
Seiferts Modification of the Ford-Andresen Model Seifert and Ritter used time domain analysis of data in conjunction with the Ford-Andresen slip dissolution model The time-based EAF crack growth rate (CGR) in high-temperature water, da/dtCF is just a linear superposition of the time-based CGR in air da/dtAir (by pure mechanical fatigue) and of the corrosion-assisted CGR, da/dtENV
- The source of the first term, da/dNAir is from a purely cyclic-controlled process and independent of loading frequency or the environment
- The second contribution only occurs during the rising load part of the fatigue cycle and is strongly dependent on crack-tip (CT) strain rate High-sulfur behavior: da/dtENV proportional to (d/dtCT)n Low-sulfur behavior: da/dtENV proportional to (d/dtCT)
Uses the GE high and low sulfur lines in the Ford-Andresen model 7
© 2018 Electric Power Research Institute, Inc. All rights reserved.
Seifert Superposition Model for EAF Crack Growth 8
© 2018 Electric Power Research Institute, Inc. All rights reserved.
Typical Operating BWR Conditions Typical NWC BWR operating conditions are 0.2-0.4 ppm oxygenated water (0 to +50 mVSHE)
For HWC conditions, the ECP is less than -230 mVSHE For air saturated water (the ECP is around +200 mVSHE)
The PSI model has predictions for three ECP values: +200 mVSHE,-100 mVSHE and -500 mVSHE Prediction for 200 mVSHE used conservatively to represent NWC conditions in the proposed model Prediction for -100 mVSHE 9
© 2018 Electric Power Research Institute, Inc. All rights reserved.
Predictions and Test Data for NWC Environment 10
© 2018 Electric Power Research Institute, Inc. All rights reserved.
Predictions and Test Data for HWC Environment 11
© 2018 Electric Power Research Institute, Inc. All rights reserved.
Features of the EAF CGR Seifert Superposition Model Unlike the Eason Model which is based on statistical treatment of data, the Seifert model is based on the mechanistic application of the Ford-Andresen model to the Shoji time domain analysis Unlike the Eason Model and Code Case N-643-2 which do not consider ECP explicitly, the Seifert BWR model accounts for different operating conditions (NWC, HWC and 8 ppm oxygenated water)
- For planned ASME Code Case two environments are considered: NWC (conservatively combines both 0.2 and 8 ppm oxygenated water) and HWC (including moderate HWC and Noble Metal Chemical Addition).
Since almost all the data is for high sulfur steels and the NWC environment is oxidizing, EAC susceptibility is assumed. There is no explicit check for whether the material is susceptible to EAC as in PWR Code Case N-643-2 12
© 2018 Electric Power Research Institute, Inc. All rights reserved.
Percentile Fit to NWC Data The Seifert Model appears to be a good fit to the available NWC data (~ 600 data points)
The fit appears to be even closer when chemistry transients (chloride and sulfate transients) are excluded The next step is to develop a statistical fit to the data (e.g. 75th percentile bound) using the 0.6 power model for the middle region
- The approach is to use methodology similar to that published in MRP-55 for Ni-based material where a power law relationship is assumed for data in different heats.
- For this example, the data for different oxygen levels are used to determine the fit assuming a power law relationship between da/dtair and da/dtenv of 0.6
- Preliminary results are shown here for NWC conditions 13
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Future Plans Prepare a technical basis document for review by the ASME Section XI WG- Flaw Evaluation Reference Curves (FERC)
Develop a draft Code Case (CC)
Develop example problems for the WG-FERC members to assure consistency of results for different users Make revisions to the draft CC based on the lessons from the example problems and member input Work with the ASME for Code Case approval 14
© 2018 Electric Power Research Institute, Inc. All rights reserved.
TogetherShaping the Future of Electricity 15
© 2018 Electric Power Research Institute, Inc. All rights reserved.