ML20155C168

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Safety Evaluation Accepting Proposed Alternative Plan for Exam of Reactor Pressure Vessel Shell Longitudinal Welds
ML20155C168
Person / Time
Site: Peach Bottom Constellation icon.png
Issue date: 10/22/1998
From:
NRC (Affiliation Not Assigned)
To:
Shared Package
ML20155C166 List:
References
NUDOCS 9811020100
Download: ML20155C168 (4)


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UNITED STATES

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NUCLEAR REGULATORY COMMISSION i

WASHINGTON, D.C. 2006H001

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SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION ALTERNATIVE TO THE AUGMENTED EXAMINATION OF THE REACTOR PRESSURE VESSEL LONGITUDINAL WELDS PEACH BOTTOM ATOMIC POWER STATION. UNIT 2 PECO ENERGY COMPANY DOCKET NO. 50-277

1.0 INTRODUCTION

Pursuant to 10 CFR 50.55a(g)(6)(ii)(A)(2), the licensees are required to perform augmented examinations of reactor pressure vessels. The licensees are required to implement an augmented examination of " essentially 100%" of the reactor pressure vessel (RPV) shell welds.

The shell welds are specified in the 1989 Edition of the American Society of Mechanical Engineers (ASME) Code,Section XI, Table IWB-25001, Examination Category B A, " Pressure l

Retaining Welds in Reactor Vessel," item B1.10. This ASME category includes item 81.12, i

longitudinal shell welds. The 10 CFR 50.55a(g)(6)(ii)(A)(2) defines " essentially 100%"

examination as "more than 90% of the examination volume of each weld." Under 10 CFR 50.55a(g)(6)(ii)(A)(5), the licensees unable to completely satisfy the requirements of the augmented reactor vessel examination may propose an attemative that provides an acceptable level of quality and safety. A licensee may use its proposed altemative when authorized by the l

Director of the Office of Nuclear Reactor Regulation, in a letter dated April 2,1998, the PECO Energy Company (the licensee) submitted a request to use an alternative to perform certain augmented examinations of the RPVlongitudinal shell welds at Peach Bottom Atomic Power Station (PBAPS), Unit 2. In a letter dated August 12, 1998, the licensee provided additional and clarifying information to support the original request.

The licensee examined the RPV intemal surface to the extent practical, even though 90%

coverage was not attained.

2.0 REQUEST FOR ALTERNATIVE EXAMINATION The licensee requests an altemative examination to performing additional examinations to achieve the full 90% coverage.

3.0 REQUIREMENT Under 10 CFR 50.55a(g)(6)(ii)(A)(2) requirements, all licensees shall augment their reactor l

vessel examination by implementing once, as part of the inservice inspection interval in effect on l

Septernber 8,1992, the examination requirements for reactor vessel shell welas specified in item B1.10 of Examination Category B A, " Pressure Retaining Welds in Reactor Vessel,"in Table IWB-2500-1 of subsection lWB of the 1989 Edition of section XI, Division 1, of the ASME Boiler and Pressure Vessel Code, subject to the conditions specified in 10 CFR 50.55a(g)(6)(ii)(A)(3) l and (4).

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4.0 BASIS FOR ALTERNATIVE EXAMINATION Pursuant to 10 CFR 50.55a(g)(6)(ii)(A)(5), the licensees that are unable to completely satisfy the augmented RPV shell weld examination requirement may submit information to the Commission to support such determination and propose attematives to the examination requirements that would provide an acceptable level of quality and safety.

5.0 PROPOSED ALTERNATIVE EXAMINATION The licensee is unable to meet the 90% volume coverage requirement for each longitudinal weld of the PBAPS, Unit 2, reactor pressure vessel as required by 10 CFR 50.55a(g)(6)(ii)(A)(2).

Therefore, it is proposing an attemative to 90% volume coverage requirement for each longitudinal weld, in accordance with 10 CFR 50.55a(s)(3)(i), pursuant to 10 CFR 50.55a(g)(6)(ii)(A)(5).

I The licensee stated that it intends to inspect the PBAPS, Unit 2, RPV during the upcoming PBAPS, Unit 2 refueling outage (2R12), scheduled to begin in early October 1998. The proposed alternative is to rform an examination of the RPV longitudinal shell welds to the maximum extent practical frora the Inner Diameter (ID), within the constraints of vessel intemal rostrictions.

This examination would be performed for longitudinal shell welds specified in item B1.10 of Examination Category B-A, " Pressure Retaining Welds in Reactor Vessel,"in Table IWB-2500-1 f

i of Subsection IWB of the 1989 Edition of Section XI, Division 1, of the ASME Boiler and Pressure Vessel Code. Further examination from the ID is not practical without disassembly of the vessel intemal components. Taba 1 provides the longitudinal welds, the estimated volumetric examination coverage from the ID, and the ID restrictions that will potentially obstruct scanning.

The volumetric examination co<erage and restrictions from the ID are based on a detailed weld coverage scan plan completed in February 1998. As shown in Table 1, eight of 15 wsids achieve greater than 90% volumetric coverage, crediting ID inspections only. There are no vessel intemals which pose a restrictica to the 10 examination which are easily removable. The only removable components (i.e., not weided to the vessel) which limit ccan coverage are the feedwater spargers. However, it is impractical to remove the feedwater spargers due to the potential for damage to the sparger seals and nonles. Therefore, there are no components which can be reasonably removed to increase coverage from the ID.

For those longitudinal welds where greater than 90% mjumetric examination may not be achieved from the ID, the estimated supplemental coverge and physical constraints on the vessel Outer Diameter (OD) are identified in Table 1. As neted in Table 1, further review has determined that two (2) welds (RPV-V1 A and RPV VSA) wouid exceed 90% volume coverage with a supplemental OD examination. In some locations, the editional weld volume that can be accessed from the OD is a subset of the ID examinations.

The restrictions which prohibit unrestrained access to 100% of the longltudinal weld volume from the OD are the vesselinsulation and the vessel noules.

The percentage of longitudinal weld volume coverage estimated from the ID Examination reflects a significant portion of the total reactor pressure vessel weld length but represerits greater than f

l-90% of the longitudinal welds within the beltline region. The beltline is the region in the RPV that is adjacent to the core, receives significant amounts of neutron radiation, and is susceptible to l

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3-embrittlement. The beltline welds are RPV VIA, VIB, VIC, V2A, V2B 2nd V2C. Attempting to perform supplemental OD examinations would result in a minimal increase to longitudinal weld volume coverage and would only increase the amount of inspection of beltline weld RPV VIA by l

a small amount (10%). Examination from the OD would only permit two additional welds to exceed 90% volumetric coverage requirement. Additional disassembly and reassembly of portions of the reactor vessel biological shield and insulation would result in further increases in personnel radiation exposure as well as an increase in the general area dose rates in the drywell for the entire population of workers. Additional doses to the entire population to perform the supplemental (OD) examinations contained in Table 1 are estimated to be 24 man-Rem.

Therefore, based on the incremental cost and radiation dose, in conjunction with the limited additional volumetric coverage, the licensee has concluded that performing OD examinations to increase coverage beyond that achieved from the ID would result in undue hardship without a l

compensating increase in safety.

5.0 EVALUATION The staff has evaluated the alternatives proposed by the licensee for the volumetric examination l

of the above mentioned reactor vessel shell welds in regard to the following factors:

. Physical constraints at each weld that limit the required examination coverage l

. Maximum extent of volumetric coverage obtained with the existing constraints l

. Supplementing inner diameter examination with examination from the outside

. Results of previous vessel examinations

. Detection of the presence of degradation mechanisms, if any, from the examination l

Based on its evaluation, the NRC staff agrees that supplementing the ID examination with an OD examination would result in only two additional welds to achieve an increased volumetric coverage to meet the Code requirement. However, it constitutes a hardship in that a significant amount of disassembly and reassembly of the reactor vessel biological shield and insulation is i

I required without a compensating increase in the level of quality and safety, and the licensee is unable to completley comply with the requirements.

l Curing the fabrication process of the PBAPS, Unit 2 RPV, all of the shell welds were thoroughly examined using several examination methods as required by the original construction code.

Additionally, all of the shell welds received volumetric examinations prior to initial plant l

operations, as prescribed by ASME Section XI preservice inspection requirements. Selected shell welds have received volumetric examinations during the first inservice inspection interval in accordance with ASME Section XIinservice inspection requirements. No rejectable indications were identified during any of these examinations. The preservice inspection and inservice inspection identified planar and/or linea indications are listed in Table 1. Allindications in Table 1 are Code acceptable.

The General Electric (GE) GERIS 2000 System will be used to perform the remote controlled, automated UT examinations of the RPV. This tool has been used previously at PBAPS, Unit 3 and other Boiling Water Reactors for the purpose of RPV examinations. GE demonstrated this

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4 system at the Performance Demonstration initiative (PDI), qualification Session No. 6102, in accordance with the 1992 Edition,1993 Addenda of ASME Boiler and Pressure Vessel Code,Section XI, Appendix Vill requirements. Appendix Vill was developed to ensure the effectiveness of UT examinations within the nuclear industry by means of a rigorous, item-specific, performance demonstration. The performance demonstration was conducted on an RPV mock-up containing flaws of various sizes and locations. The demonstration estabiished the capability of equipment, procedures, and personnel, which are similar or the same as those that will be used at PBAPS, Unit 2, to find flaws that could be detrimental to the integrity of the i

I RPV.

Greater than 90% of the portion of each longitudinal weld within the beltline region will be scanned. Hence the welds that are most susceptible to radiation embrittlement will receive greater than 90% examination.

The staff concludes from the results of previous volumetric examinations and the extent of the examination proposed by the licensee that there is reasonable assurance that any degradation mechanism that could exist in the welds would be detected during the proposed volumetric examination. Hence, the licensee's proposed altemative provides reasonable assurance of structuralintegrity, and provides an acceptable level of quality and safety.

6.0 CONCLUSION

Pursuant to 10 CFR 50.55a(g)(6)(ii)(A)(5), the staff evaluated the licensee's proposed attemative of examining reacior vessel shell welds specified in item B1.12 of Examination Category B-A

" Pressure Retaining Welds in Reactor Vessel,"in Table IWB-2500-1 of Subsection IWB of the 1989 Edition of the ASME Code,Section XI and the augmented examination requirements in 10 CFR 50.55a(g)(6)(ii)(A)(2). The staff has determined that the licensee is unable to completely comply with the requirements, and that the proposed altemative provides an acceptable level of quality and safety. The licensee's proposed attemative is authorized pursuant to 10 CFR 50.55a(g)(6)(ii)(A)(5).

Attachment:

Table Principal Contributor. H. Conrad Date: October 22, 1998

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