ML063560358
| ML063560358 | |
| Person / Time | |
|---|---|
| Site: | Wolf Creek  |
| Issue date: | 11/30/2006 |
| From: | Kammerdeiner G Electric Power Research Institute |
| To: | Office of Nuclear Reactor Regulation |
| Mensah T | |
| References | |
| Download: ML063560358 (17) | |
Text
MRP-139 Analysis Basis MRP / NRC November 30, 2006 Greg Kammerdeiner, First Energy Chairman, Assessment ITG
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© 2006 Electric Power Research Institute, Inc. All rights reserved.
Purpose Provide overview of analysis documents that support required butt weld inspections in MRP-139 Summarize the purpose, method and main conclusions from each document Review supporting analysis documents in light of Wolf Creek inspection findings
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© 2006 Electric Power Research Institute, Inc. All rights reserved.
Roadmap to Butt Weld Analysis Documents
- Document preparation spanned five year period from June 2000 through August 2005
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© 2006 Electric Power Research Institute, Inc. All rights reserved.
MRP-21: Crack Growth of A182 in PWRs
Purpose:
To determine crack growth rates in A182 weld metal used to join A600 CRDM nozzles to vessel heads
- Methodology: Sample welds prepared using typical materials and procedures were then tested in autoclave
- Main
Conclusions:
- The crack growth rates in A182 welds were up to five times higher than for A600
- A preliminary crack growth model was developed Superseded by MRP-115
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© 2006 Electric Power Research Institute, Inc. All rights reserved.
MRP-44.1: Preliminary Butt Weld Safety Assessment
Purpose:
To prepare a preliminary safety assessment to address butt weld cracks such as occurred at Ringhals 3/4 and leak as occurred at VC Summer
- Methodology: Prepared a survey of A182 butt weld locations and assessed operating experience, crack orientation, flaw tolerance, leak detection, boric acid corrosion, and structural margin
- Main
Conclusions:
- Plants have adequate margin to continue operation
- Identified short-term inspection guidance Superseded by MRP-113
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© 2006 Electric Power Research Institute, Inc. All rights reserved.
MRP-57: A182 Experience in BWRs
Purpose:
To assess A182 butt weld cracks in BWR plants
- Methodology: GE reviewed public and proprietary crack data
- Main
Conclusions:
- There are more axial cracks than circumferential cracks
- Axial cracks can grow long if not arrested by resistant material
- Circumferential cracks were all less than 90º arc length (and some deep) except Duane Arnold
- 360º crack at Duane Arnold caused by crevice environment not present in PWRs. Nevertheless, showed leak before rupture
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© 2006 Electric Power Research Institute, Inc. All rights reserved.
MRP-33: RPV Nozzle Weld Stresses
Purpose:
To determine effect of weld geometry (single vs double V) and repairs on residual stresses to help understand Ringhals 3/4 and VC Summer cracks/leak
- Methodology: Prepared elastic-plastic finite element models of welds and repairs
- Main Findings:
- Hoop stresses exceed axial stresses at ID surface such that most cracks should be axial
- Through-wall hoop stresses are high suggesting potential for rapid crack growth
- Welds finished on the ID, or repaired on the ID, have high tensile stresses on inside surface
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© 2006 Electric Power Research Institute, Inc. All rights reserved.
MRP-106: Residual and Operating Stresses in A182 Butt Welds
Purpose:
To determine residual/operating stresses for full range of butt weld sizes (1 to 30) and ID repairs for larger (5 to 30) butt welds
- Methodology: Elastic-plastic finite element analyses for multi-pass welds and 30º, 60º, and 90º partial-arc ID repairs
- Main
Conclusions:
- For normal welds without repairs, tensile stresses at ID are low, consistent with small number of cracks/leaks
- Weld repairs to the ID create high tensile hoop and axial stresses of similar magnitude
- Many butt welds too small for ID repairs
- Adding weld deposit to OD can reduce ID tensile stress
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© 2006 Electric Power Research Institute, Inc. All rights reserved.
MRP-109: Safety Assessment for Westinghouse and CE Plant Butt Welds
Purpose:
To assess safety of A182 butt welds in Westinghouse and CE plants
- Methodology: Calculated:
- Size of critical axial and circumferential flaws,
- Time for cracks to grow through wall and circumferentially using generic residual stresses and MRP-21 crack growth model, and
- Potential to detect leaks before rupture
- Main
Conclusions:
- Axial flaws not a concern due to crack arrest at carbon and SS material
- Times for partial-arc cracks to grow through-wall can be <1 yr
- Time between leak detection and rupture >2 yr
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© 2006 Electric Power Research Institute, Inc. All rights reserved.
MRP-112: Safety Assessment for B&W Design Plants
Purpose:
To assess safety of A182 butt welds in B&W design plants
- Methodology: Essentially the same as MRP-109 since Westinghouse and Framatome collaborated on analysis methodology
- Main
Conclusions:
- Axial flaws not a concern
- Time for part-arc cracks to grow thru-wall can be <4 yr
- Some time between detectable leakage and rupture except for PZR relief nozzle
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© 2006 Electric Power Research Institute, Inc. All rights reserved.
MRP-114: Effect of Weld Repairs
Purpose:
To assess the effect of partial-arc weld repairs on circumferential cracking
- Methodology: Fracture mechanics crack growth calculations based on:
- As-welded and repaired residual stresses in MRP-106,
- PVP-365 K algorithm (superposition method) and
- Crack arrests for K < 0
- Main
Conclusions:
- Regions of high crack growth are limited to approximately the arc length of the weld repair region
- Even for cases of 360º ID repairs, uniform initiation is highly unlikely (i.e., Duane Arnold)
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© 2006 Electric Power Research Institute, Inc. All rights reserved.
MRP-116: Probabilistic Risk Assessment
Purpose:
To assess the probability and consequences of rupture of butt welds due to PWSCC
- Methodology: Probabilistic model based on standard Westinghouse approach considering crack initiation, crack growth, crack detection, leakage detection, rupture and core damage
- Main
Conclusions:
- Probability of leak due to axial crack is 2 orders of magnitude higher than for circ cracks
- Total risk (CDF or LREF) within Reg. Guide 1.174 guidelines for 40 year plant life
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© 2006 Electric Power Research Institute, Inc. All rights reserved.
MRP-115: A82/182 Crack Growth Rates
Purpose:
To develop A82/182 crack growth model considering worldwide data
- Methodology: All available data collected and screened by panel of international experts, including NRC and NRC contractors
- Main
Conclusions:
- Recommended deterministic curve at 75th percentile of data
- MRP-115 curve for A182 similar to MRP-21 but with no stress intensity threshold
- Curve for A82 between A600 base metal and A182 weld metal (closer to A600)
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© 2006 Electric Power Research Institute, Inc. All rights reserved.
MRP-113: Final Butt Weld Safety Assessment
Purpose:
To integrate all MRP safety assessment work into one overall summary document
- Methodology: Same as described for other reports
- Main
Conclusions:
- Most cracks expected to be axial and will arrest at carbon and SS material
- Deep partial-arc circ cracks should be limited to region of weld repairs with leak detection before rupture except at one location
- 360º part-depth circ cracks unlikely to occur
- BMVs should be performed within next two outages
- Inspection plan is being developed
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© 2006 Electric Power Research Institute, Inc. All rights reserved.
MRP-139: Butt Weld Inspection Requirements
Purpose:
To develop mandatory inspection intervals for A82/182 butt welds
- Main
Conclusions:
- Characterize all weld profiles by end of 2007
- Inspect PZR welds by end of 2007
- Inspect HL welds 4-14 NPS by end of 2008
- Inspect HL welds >14 NPS by end of 2009
- Inspect CL welds by end of 2010
- MRP Letter 2004-05 requires BMV of welds by 2007
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© 2006 Electric Power Research Institute, Inc. All rights reserved.
Review Based on Wolf Creek Inspections
- MRP-113/114 address partial-arc circ cracks caused by weld repairs
- Repairs known to be detrimental with potential for circumferential flaws
- Crack growth rate can be high
- Crack growth limited to approximate repaired region
- Unknown to what extent WC indications align with repairs
- Critical flaw size is large
- 144-166º for safety/relief from existing analyses
- Realistically much larger (remove SSE and thermal loads)
- Maximum Wolf Creek flaw is estimated at 26% thru-wall and 155º extent.
- Depth vs arc length is unknown
- Palisades and Duane Arnold confirm margin for large circumferential flaws
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© 2006 Electric Power Research Institute, Inc. All rights reserved.
Opportunities for Improvement
- Establish less conservative critical flaw size
- Better estimate of K vs crack depth with stress relaxation
- Probabilistic considerations