ML20248L633

SER of EPRI Topical Rept TR-105873 BWR Vessel & Internals Project,Evaluation of Crack Growth in BWR Stainless Steel Internals,(BWRVIP-14)

ML20248L633
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Issue date:	06/08/1998
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U.S. NUCLEAR REGULATORY COMMISSION 1 OFFICE OF NUCLEAR REACTOR REGULATION DIVISION OF ENGINEERING SAFETY EVALUATION OF EPRI TOPICAL REPORT TR 1.QERZ2

. gER VESSEL AMD INTERNALS PROJECT. EVALUATION OF CRACK GROWTH J.N BWR STAINLESS STEEL INTERNALS. (BWRVIP-14)"

1.0 INTRODUCTION

i By letter dated March 18,1996, as supplemented by letter dated July 28,1997, the Boiling Water Reactor Vessel and Intemals Project (BWRVIP) submitted the Electric Power Research Institute (EPRI) Proprietary Report TR 105873, "BWR Vessel and intemals Project, Evaluation i of Crack Growth in SWR Stainless Steel Intemals (BWRVIP-14)," dated March 1996, for staff review and approval. The staff requested additional inforraation (RAI) in a letter dated December 9,1996, and the BWRVIP provided its response, Structural integrity Associates Report No. SIR-97 025, Revision 3, " Responses to NRC Request for Additional Information on BWRViP Report BWRVIP-14 on Evaluation of Crack Growth in RPVIntemais," dated June 1997, by letter dated July 28,1997.

The BWRVIP-14 report provides a methodology for assessment o! crack growth in BWR stainless steel shrouds and other stainless steel intemals components. The assessment was limited to the circumferential welds of the shroud whete most of the reported intergranular stress corrosion cracking (IGSCC) has occurred to date. The methodology was developed specifically for crack growth in the radial (through thickness direction). Residual and applied stresses and stress intensity factors (K) have been developed for crack propagation in this direction.

1.1 Background

The cracking of stainless steel BWR core shrouds was identified as a significant issue beginning

. In 1993 and 1994. To investigate the issues of core shroud integrity and other BWR intemals, the BWR utilities formed the Boiling Water Reactor Vessei and Intemals Project (BWRVIP) to address service-relEsd degradation of BWR vessels and intemais. One of the major issues involved is the reinspection intervals. The current methodology is to determine the inspection interval based on characterizing all observed cracks as being through the wall and propagating the cracks around the circumference of the core shroud (assuming a crack growth rate of 4

5 x 10 in/hr). This crack growth rate (CGR) is based on a stainless steel crack growth rate correlation provided in the NRC Technical Report, NUREG 0313, Revision 2, " Technical Report on Material Selection and Processing Guidelines for BWR Coolant Pressure Boundary Pipir.g,"

issued February 1984.

The NRC staff, with assistance from the Argonne National Laboratory (ANL) has assessed the BWRVIP's submittals in this safety evaluation (SE). The Argonne Technical Evaluation Report, ENCLOSURE n

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- Terhnical Evaluation Report on EPRI TR-105873, BWR Vessels and intemals Project, Evt.luation of Crack Growth in BWR Stalniess Steel RPV Intemals,"is enclosed.

1i2 SER Organization

' 'This SER contains a brief summary of the general contents of the BWRVIP-14 report and a

' summary of the ANL Technical Evaluation Report, followed by the staff's evaluation end conclusions.

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SUMMARY

OF BWRVIP-14 TOPICAL REPORT The BWRVIP methodology involved the development of an empirical model to account for the variability of important IGSCC parameters in providing an assessment of the crack growth rate in BWR stainl=,ss steel components. The BWRVIP-14 report includes a data base of unirradiated .

stainless steel crack growth rates, which is limited to data that had defined environmental conditions and well characterized important crack growth parameters. This database was used I to derive an empirical crack growth rate law to account for stress intensity (K), and environmental conditions such as conductivity, electrochemical potential (ECP), and temperature.

~ The BWRVIP-14 report also discusses the weld residual stresses developed during fabrication of the weld, in which both experimental measurements and analytical techniques were used, in general, the results on residual stress dis.ributions were similar to that recommended in NU'AEG-0313, Rev. 2, for large diameter stainless steel pipe. The BWRVIP-14 report wams that variability - due to fit-up stresses, weld joint geometry, and welding parameters such as heat input, weld secuence, weld starts and stops and repairs - can affect local residual stress distribution.

Further, the report estimates the through wall stress intensity factor distributions for the assumed

' ideal residual stress profiles using fracture mechanics models, and it outlines three parallel altemate approaches to an evaluation methodology for determining crack growth rates (CGRs) in BWR shrouds. In the first approach, a K-independent crack growth rate of 2.2 x 10~51n/hr is recommended. This CGR represents the highest CGR - with a : tress intensity factor of 25 ksi

- /(in) and the 95th percentile curve of the model with conductivity of 0.15 pS/cm and ECF of 200 mV(SHE) -- which are typical for BWR plants operating under moderate hydrogen water chemistry conditions. The second approach involves the use of the 95th percentile curve of the model with conductivity of 0.15 pS/cm and ECP OF 200 mV(SHE) but with K-dependence. The third approach involves using all the variables in the crack growth model to establish the 96th percentile crack growth curve. An example problem representing actual BWR shroud conditions is_also presented. A9pendices to the BWRVIP-14 report present operating plant ultrasonic data that indicate that the present NRC (NUREG-05t3) CGR is conservative. The report proposes that the BWRVIP's crack growth methodology be used for crack evaluation.

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SUMMARY

OF ARGONNE TECHNICAL EVALUATION REPORT lt should be noted that the attached Argonne National Labcratory (ANL) techaical evaluation report (TER) does not necessarily represent approved staff positions, but is one of the sou,ces that the staff has consulted in the formulation of its conclusions in this SER.

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ANL, in their TER, found that the BWRVIP's first calculational approach (e.g., using a CGR of 2.2 x 10~5 in/hr) can be used irt welds with well characterized residual stress profiles. ANL's TER points out that the BWRVIP CGR model does not oenend on stress intensity explicitly; rather, it was developed from the BWR 95th percentile curve whose stress intensity and chemistry values are given in the above summary. Therefore, it is applicable only to weldment geometries for which the constant value of K represents a conservative estimate of the actual variable value.

The staff concludes this approach can be used if the value of K is explicitly determined to be less than 25 ksi/in. for weldment geometr'es to which that rate would be applied. Further, the licensee would need to certify that the components are operated in accordance with the EPRI BWR WaW Chemistry Guidelines.

One Umitation of the BWRVir s data base correlation is that it consists solely of unirradiated base matarials. Wcidments and their associated fusion heat affected zones could have somewhat higher impurity !evels due to flux / copper contamination or impurity diffusion than the wrought base metals that are solely considered .in the data base correlat.'on. It is well known that changes in irradiated materials properties are a strong function of certain impurities. In addition, tnere are few, if any, valid crack growth measurements for irradiated metals. Benchmark comparisons with ultrasonic measurements made on actual hradiated core shroud cracks may not be valid because the uncertainties in the ultrasonic measurements could mask any variations in the crack growth rate correlation.

Therefore, in crder to use a K-based crack growth model such as the BWRVIP's correlation, careful estimates of the residual stresses, as well as the applied loads, are required. The prediction of the CGR is a strong function of the assumed residual stress distribution.

Unfortunately, when it comes to the welds in actual fabricated core support structures, only in the case of Double-V cylinder-to-cylinder welds (such as the H-4 welds) have the residual stresses been adequately characterized by measurements and by calculations. Other weld geometries may have deleterious residual strest, configurations. For example, one calculation for Nine Mile Point Nuclear Plant on the H-8 weld geometry that joins the conical shroud support to the shroud cylinder revealed a very unfavorable stress distribution in that case, rather than decreasing as the crack grows, the K values associated witn this weld increased in this example, having an environment with a conductivity of 0.15 mS/cm and an ECP of 200 MeV, after the crack has grown about a third through-well, the predicted crack growth rate is in excess of the current NRC accepted bounding crack growtn rate.

4.0 ._ STAFF EVALUATION The staff accepts the recommendation in the ANL TER that any use of the correlation or of any data derived from unirradiated specimens should be limited to materials with fluences in the range less than 5 x 102' n/cm" (E>1MEV), unless further technical justification or data are provided for a case by case review. The BWRVIP-14 report should be revised to more fully address the applicability of its model to irradiated material.

In those cases where it can be demonstrated that the electrochemical potential (ECP) and condut,tivity are reduced (e.g., such as by direct ECP measurements or by methods demonstrated to achieve lower ECP such a hydrogen injection), the staff may consider a reduction in CGR on a case by case basis.

The discussion in the BWRVIP-14 report on the effects of weld repairs arid other residual stress

. distribution uncertainties (e.g., fit-up stre.sses, weld sequence, weld starts and stops) stated that the BWRVlP can give little guidance as to how to address the individual parameters for each

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- individual BWR shroud. The ANL TER recommends that any calculations should be performed using a more conservative estimate of the stress profile (i.e., by adding a uniform 10 ksi stress to the profile). Based on the ANL TER, the staff concludes that specific calculations of the stress

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intensity profile for each weldment geometry is necessary. This needs to be done also for the CGR whenever K is noi explicitly addri ssed. Weld repairs have a significant effect on residual stress distributions. Therefore, the staif expects repairs to be documented, their effect on the residual stess distribution evaluated, and be subject to staff approval on an application-specific basis.

5.0 CONCLUSION

S The three approaches for the evaluation of crack growth given in the BWRVIP-14 report have been found acceptable for use subject to staff review and the following conditions:

1. the first approacn (constant K and a CGR of 2.2 x 104in/hr) may be used provided that repairs, etc., are considered in evaluating the residual stresses; ,

2. that the component is operated in accordance with the EPRI BWR Water Chemistry Guidelines; and,

3. the stress intensity factor K is explicitly determined to be less than 25 ksi/in.

However, for each of the approaches, in addition to the consideration of repairs and water chemistry considerations, it will be necessary for the staff to review the licensee's crack growth evaluation, including an evaluation of the applied and residual stresses to determine the acceptability of the assumed crack growth rate. Residual stress determinations must include

. repairs and any other relevant factors.

Cracking in weldments that have been irradiated by fluences in the range greater than 5 x 10 2o 8

n/cm (E > 1 MEV)is outside of the scope of this SER and would require review on a case basis

- that is supported with relevant data.

Enclosure:

Technical Evaluation Report

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