NRC Generic Letter 98-05: Boiling Water Reactor Licensees Use of the BWRVIP-05 Re... Page 1 of 3

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UNITED STATES

NUCLEAR REGULATORY COMMISSION

OFFICE OF NUCLEAR REACTOR REGULATION

WASHINGTON, D.C. 20555-0001

November 10, 1998

NRC GENERIC LETTER 98-05: BOILING WATER REACTOR LICENSEES USE OF THE BWRVIP-05 REPORT TO

REQUEST RELIEF FROM AUGMENTED EXAMINATION REQUIREMENTS ON

REACTOR PRESSURE VESSEL CIRCUMFERENTIAL SHELL WELDS

Addressees

All holders of operating licenses (or construction permits) for boiling water reactors (BWRs), except those who

have permanently ceased operations and have certified that fuel has been permanently removed from the

reactor vessel.

Purpose

The U.S. Nuclear Regulatory Commission NRC is issuing this generic letter to inform addressees that the NRC

staff has completed its review of the "BWR Vessel and Internals Project [BWRVIP], BWR Reactor Pressure

Vessel Shell Weld Inspection Recommendations (BWRVIP-05)," and that licensees of BWRs may request

permanent (i.e., for the remaining term of operation under the existing, initial, license) relief from the

inservice inspection requirements of 10 CFR 50.55a(g) for the volumetric examination of circumferential

reactor pressure vessel (RPV) welds. No specific action or written response is required.

Background

By letter dated September 28, 1995, as supplemented by letters dated June 24 and October 29, 1996, May

16, June 4, June 13, and December 18, 1997, and January 13, 1998, the BWRVIP submitted the Electric

Power Research Institute (EPRI) proprietary report TR-105697, BWR Vessel and Internals Project [BWRVIP],

BWR Reactor Pressure Vessel Shell Weld Inspection Recommendations (BWRVIP-05)." The BWRVIP-05 report

evaluates the current inspection requirements for the reactor pressure vessel shell welds in BWRs, formulates

recommendations for alternative inspection requirements, and provides a technical basis for these

recommended requirements. It initially proposed to reduce the scope of inspection of the BWR reactor

pressure vessel (RPV) welds from essentially 100 percent of all RPV shell welds to 50 percent of the axial

welds and zero percent of the circumferential welds; however, as modified, it proposes to perform inservice

inspections (ISI) on essentially 100 percent of the RPV axial shell welds, and essentially zero percent of the

circumferential RPV shell welds, except for the intersections of the axial and circumferential welds.

Approximately 2 - 3 percent of the circumferential welds will be inspected under this proposal.

On August 7, 1997, the NRC issued Information Notice (IN) 97-63, "Status of NRC Staff's Review of BWRVIP05," regarding licensee requests for relief. IN 97-63 stated that the staff would "...consider technicallyjustified requests for reliefs from the augmented examination in accordance with 10 CFR 50.55a(a)(3)(i), 10

CFR 50.55a(a)(3)(ii), and 50.55a(g)(6)(ii)A(5) from BWR licensees who are scheduled to perform inspections

of the BWR RPV circumferential shell welds during the fall 1997 or spring 1998 outage seasons. The staff

issued schedular reliefs for inspections of the BWR RPV circumferential shell welds due during the fall 1997

outage season for four units who submitted technically-justified requests, and has issued schedular reliefs for

two units during the spring 1998 outage season.

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On May 7, 1998, the staff issued IN 97-63, Supplement 1, which informed BWR licensees that the staff was

extending the period in which it would "...consider technically justified requests for relief from the augmented

examination in accordance with 10 CFR 50.55a(a)(3)(i), 50.55a(a)(3)(ii), and 50.55a(g)(6)(ii)(A)(5) from

BWR licensees who are scheduled to perform inspections of the BWR RPV circumferential shell welds during

the fall 1998 or spring 1999 outage seasons. Acceptably justified relief would be considered for inspection

delays of up to two operating cycles for BWR RPV circumferential shell welds only. Licensees will still need to

perform their required inspections of 'essentially 100 percent' of all axial welds."

Discussion

The staff has completed its final review of the information submitted by the BWRVIP and the staff's safety

evaluation (SE) was transmitted to Carl Terry, Chairman of the BWRVIP, in a letter dated July 30, 1998.

The staff previously concluded that beyond design-basis events occurring during plant shutdown could lead to

cold over-pressure events that could challenge vessel integrity. The industry's response concluded that

condensate and control rod drive pumps could cause conditions that could lead to cold over-pressure events

that could challenge vessel integrity. The BWRVIP's estimate of the frequency of over-pressurization events

that could challenge the RPV is 9.5 x 10 4/yr for BWR-4 facilities and 9 x 10 4 /yr for other than BWR-4

facilities. After accounting for actual injections which were not included in the BWRVIP analysis, the staff

conservatively estimates that the total frequency could be as high as 1 x 10-3/yr (a point estimate).

The initial industry review determined that the failure frequency of circumferential welds was 2.2 x 10-4 1/yr.

This frequency was determined using importance sampling, generic weld variables and design basis events.

Subsequent analyses using "Monte Carlo" calculation methods, plant-specific weld variables and pressures and

temperatures associated with cold over-pressure events, determined that the limiting plant specific conditional

probability of vessel failure, P(FIE), for circumferential welds at 32 effective full power years (EFPY) were 1 x

10-6 from the BWRVIP's re-analysis and 8.2 x 10-5 from the staff's analysis. Combining the frequency of cold

over pressure events with the P(FIE), the BWRVIP failure frequency for the limiting circumferential welds was

9.0 x 10-10 /yr [(9 x 10-4 /yr event frequency for a BWR-3) x (1.0 x 10-6 conditional probability of failure)].

The limiting plant specific failure frequency forcircumferential welds at 32 EFPY was determined by the staff to

be 8.2 x 10-8 /yr [(1 x 10-3/yr event frequency) x (8.2 x 10-5 P(FIE))]. As depicted in NUREG 1560, Vol. I,

core damage frequencies (CDF) for BWR plants were reported to be approximately 10 7 /yr to 10 4/yr. In

addition, Regulatory Guide (RG) 1.154 indicates that PWR plants are acceptable for operation if the plantspecific analyses predict the mean frequency of through-wall crack penetration for pressurized thermal shock

events is less than 5 x 10 6/yr. The failure frequencies of circumferential welds in BWR vessels are

significantly below the criteria specified in RG 1.154.

RG 1.174 provides guidelines as to how defense-in-depth and safety margins are maintained, and states that

a risk assessment should be used to address the principle that proposed increases in risk, and their

cumulative effect, are small and do not cause the NRC Safety Goals to be exceeded. The estimated failure

frequency of the BWR RPV circumferential welds is well below the acceptable core damage frequency (CDF)

and large early release frequency (LERF) criteria discussed in RG 1.174. Although the frequency of RPV weld

failure can not be directly compared to the frequencies of core damage or large early release, the staff

believes that the estimated frequency of RPV circumferential weld failure bounds the corresponding CDF and

LERF that may result from a vessel weld failure. On the above bases, the staff has concluded that the

BWRVIP-05 proposal, as modified, to eliminate BWR vessel circumferential weld examinations, is acceptable.

Permitted Action

BWR licensees may request permanent (i.e., for the remaining term of operation under the existing, initial,

license) relief from the inservice inspection requirements of 10 CFR 50.55a(g) for the volumetric examination

of circumferential reactor pressure vessel welds (ASME Code Section XI, Table IWB-2500-1, Examination

Category B-A, Item 1.11, Circumferential Shell Welds) by demonstrating that: (1) at the expiration of their

license, the circumferential welds will continue to satisfy the limiting conditional failure probability for

circumferential welds in the staff's July 30, 1998, safety evaluation, and (2) licensees have implemented

operator training and established procedures that limit the frequency of cold over-pressure events to the

amount specified in the staff's July 30, 1998, safety evaluation. Licensees will still need to perform their

required inspections of "essentially 100 percent" of all axial welds.

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This generic letter requires no specific action or written response. Any action on the part of addressees to

request relief from the inservice inspection requirements of 10 CFR 50.55a(g) for the volumetric examination

of the circumferential reactor pressure vessel welds, in accordance with the guidance of this generic letter, is

strictly voluntary. If you have any questions about this matter, please contact one of the contacts listed

below.

/s/'d by

Jack W. Roe, Acting Director

Division of Reactor Program Management

Office of Nuclear Reactor Regulation

Technical Contacts:

Attachment:

B. J. Elliot, NRR

301-415-2709

E-mail: bje@nrc.gov

C. E. Carpenter, NRR

301-415-2169

E-mail: cec@nrc.gov

List of Recently Issued NRC Generic Letters

(NUDOCS Accession Number 9811030134)

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