ML20236T727

Informs That During 454th Meeting of ACRS on 980708-10,NRC Reviewed Proposed Final SE of BWR Vessel & Internals Project Rept Re Industry Recommendations for Reducing Scope of ISI of BWR Reactor Vessel Welds

ML20236T727
Person / Time
Issue date:	07/21/1998
From:	Seale R Advisory Committee on Reactor Safeguards
To:	Callan L NRC OFFICE OF THE EXECUTIVE DIRECTOR FOR OPERATIONS (EDO)
References
ACRS-R-1769, NUDOCS 9807280355
Download: ML20236T727 (2)

Text

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ACRSR-1769

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UNITED STATES PDR 8'

NUCLEAR REGULATORY COMMISSION p

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ADVISORY COMMITTEE ON REACTOR SAFEGUARDS d

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g WASHINGTON, D. C. 20555 July 21,1998 l

Mr. L. Joseph Callan Executive Director for Operations U.S. Nuclear Regulatory Commission Washington, D.C. 20555-0001

SUBJECT:

PROPOSED FINAL SAFETY EVALUATION OF THE BWR VESSEL AND INTERNALS PROJECT, BWR PRESSURE VESSEL SHELL WELD INSPECTION l

RECOMMENDATIONS (BWRVIP-05) REPORT l

During the 454th meeting of the Advisory Committee on Reactor Safeguards, July 8-10,1998, we reviewed the proposed final safety evaluation of the BWR Vessel and Internals Project (BWRVIP-

05) report concerning industry recommendationsfor reducing the scope of inservice inspection of k

BWR reactor vessel welds.

During our review, we had the benefit of discussions with representatives of the NRC staff and of the documents referenced.

Conclusions We endorse the staffs recommendation that licensees be granted permanent relief from inservice inspection requirements for volumetric examination of BWR circumferential reactor pressure vessel welds if the licensee can demonstrate that the generic evaluation performed by the staff is applicable to its vessel.

We concur with the staffs request that the BWRVIP provide a plan for followup analyses to determine more realisticestimates of the frequency of axialweld failures caused by cold-overpressure events and propose appropriate tecnnical approaches to address this issue.

Discussion in our September 10,1997 letter, we recommended that the staff review the BWRVIP-05 report using the risk-informed process in Regulatory Guide 1.174, "An Approach for Using Probabilistic l

Risk Assessmentin Risk-Informed Decisions on Plant-Specific Changes to the Current Licensing

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Basis." We also recommended that additionalefforts be taken to address uncertainties associated I

with the industry and staff analyses,in particularthose associated with flaw size distributions and l

l the sequences that could lead to vessel challenges. In addition, we recommended that the staff considerthe value of partialinservice inspection of welds. In response, the staff and the industry

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I estimated the frequency of vessel challenges through studies of potential precursor events, used recent research results from examination of actual pressure vessel welds to update flaw size distributions, and performed additional probabilistic fracture mechanics analyses.

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i The staff and industry studies confirmed that the failure frequency of axial welds during cold over-pressure events is much greater than the failure frequency for circumferential welds. The

'circumferential weld failure frequency is below the criteria specified in Regulatory Guide 1.154,

" Format and Content of Plant-Specific Pressurized Thermal Shock Safety Analysis Reports for Pressurized Water Reactors," for pressure vessel integrity. This finding supports the conclusion that inservice inspection of circumferential welds is not necessary during the current license term for operating BWRs.

The computed failure frequency of the axial welds does not meet the criteria of Regulatory Guide 1.154. This assessments based on end-of-life neutron fluence levels that will not occur for many years and includes a number of additional conservatism, and hence, failure of the axial welds is not a near-term safety concem. The staff requested additional analyses from the BWRVIP to obtain more realistic estimates of axial weld failure frequency.

The studies performed by the staff and the industry also demonstrate that inservice inspection of the axial welds is ineffective in reducing the likelihood of vessel failure due to fabrication flaws. An inspection program for these welds consistent with the intent of ASME Section XI, however, does -

provide assurance that service-induced degradation mechanisms will be detected and is an important element of defense in depth.

6 Dr. William J. Shack did not participate in the Committee's deliberation regarding this matter.

l Sincerely, R. L. Seale Chairman i,

References 1.

Letter dated September 10,1997, from R. L. Seale, Chairman, ACRS, to L. Joseph Callan, I

Executive Director for Operations, NRC,

Subject:

Boiling Water Reactor Pressure Vessel Shell Weld inspection Recommendations (BWRVIP-05).

2.

Memorandum dated June 3,1998, from Frank J. Miraglia, Office of Nuclear Reactor Regulation, NRC, to Robert L. Seale, Chairman, ACRS,

Subject:

Transmittalof NRC Staffs Draft Safety Evaluation of the "BWR Vessel and Intemals Project BWR Reactor Pressure Vessel Shell Weld Inspection Recommendations (BWRVIP-05)* Report i

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