ML20138J996

Safety Evaluation Accepting Proposed Alternative to ASME Code for Surface Exam of CRD Mechanism Canopy Seal Welds

ML20138J996
Person / Time
Site:	Prairie Island
Issue date:	05/06/1997
From:	NRC (Affiliation Not Assigned)
To:
Shared Package
ML20138J990	List: ML20138J986 ML20138J996
References
NUDOCS 9705090228
Download: ML20138J996 (3)

Text

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_ put%q l g 3 UNITED STATES {

. , s* E NUCLEAR REGULATORY COMMISSION '

f WASHINGTON, D.C. 206tH001

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SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION l

, REVIEW OF PROPOSED ALTERNATIVE TO THE ASME CODE FOR SURFACE EXAMINATION OF WELD REPAIRS TO NONSTRUCTURAL SEAL WEL M f

NORTHERN STATES POWER COMPANY PRAIRIE ISLAND NUCLEAR GENERATING PLANT DOCKET NOS. 50-282 AND 50-306

1.0 INTRODUCTION

By letter dated January 19, 1996, Northern States Power Company (NSP), the i licensee, proposed an alternative to the surface examination requirements of paragraph N-518.4 of the 1968 American Society of Mechanical Engineers (ASME)

Boiler and Pressure Vessel Code. The proposed alternative would be used in

. conjunction with weld repairs to four nonstructural seal welds. The seal welds are on the canopy seal of four control rod drive mechanisms (CRDMs) at penetration ?ocations H8, F6, F8, H6 and G7 on Unit I and F6 and G7 on Unit 2.

By letter dated March 6,1996, the Nuclear Regulatory Commission (NRC)

authorized alternative inspections to the ASME Code paragraph N-518.4 for surface exa

nination of the control rod drive mechanism canopy seal welds pursuant to 10 CFR 50.55a(a)(3)(i). The NRC safety evaluation of March 6, j 1996, listed weld locations H6 and G7 on Unit 1 and F6 and G7 on Unit 2. In your letter dated February 14, 1997, you requested clarification that our acceptance of the alternative applied to all seven locations that you requested in your January 19, 1996, submittal.

Based on our review, we have determined that weld locations H8, F6, and F8 on Unit I also should have been evaluated and listed as locations that were acceptable for performance of the alternative inspections. j The welds are used to ensure leak tightness of the thread joint holding the rod travel housing to the CRDM housing. The seal weld is a small groove weld applied to a small protrusion (" canopy") over the end of the threads. Because the threads constitute the pressure boundary, the seal weld is nonstructural.

The presence of the canopy protrusion provides a weld surface that avoids fusion of the ends of the threads and allows the seal weld to be more readily i removed when necessary, 10 CFR 50.55a(g) requires that nuclear power facility piping and components to meet the applicable requirements of Section XI of the ASME Code (hereafter referred to as the Code), including components inspected under Section III of the Code. Alternatives to Code requirement may be used by nuclear plant licensees when authorized by~the Commission if the proposed alternatives are shown to provide an acceptable level of quality and safety (10 CFR 50.55a(a)(3)(i) or if compliance with the Code requirements would result in ,

9705090228 970506 PDR ADOCK 05000282 G PDR

i' i hardship or unusual difficulty without a compensating increase in the level of

quality and safety. [10 CFR 50.55a(a)(3)(ii)].

t

, Paragraph N-518.4 of the 1968 ASME Code,Section III, requires that

attachments (welds) to the pressure boundary be inspected by means of a liquid

! penetrant-(PT) examination. However, a PT examination of the welds would be difficult. The canopy seals are in a high radiation area of approximately 250 to 450 mR/ hour. Additionally, access to the canopy seals is difficult due to the limited clearance between adjacent CRDMs. The separation between the housings is approximately 7 inches. This is not adequate clearance to gain

- complete access to perform the PT examination. Surface preparation (grinding)

of the welds, PT examination, and subsequent cleanup would be difficult to

execute properly, would be time consuming, and would incur substantial

personnel radiation exposure.

l 2.0 EVALUATION Section III of the ASME Code, 1968 Edition, specifies a surface examination of weld-repaired areas (paragraph N-514.2) or welded attachments (paragraph N-518.4). For PT examinations, the acceptance criteria of paragraph N-627 must be met. The most stringent among the acceptance criteria is the requirement for "no linear indications." Later editions of the Code define 1/16 inch as the smallest length of a relevant linear indication.

The proposed alternative would _ include an 8-power visual examination (by means of a remote video camera) of the in-process automatic welding and a post-weld visual examination with the same camera. Additionally, a fracture mechanics and limit load analysis of the pressure boundary in the region adjacent to the seal welds has been performed by the licensee. The results of the fracture mechanics analysis demonstrated that the predicted critical flaw size in all cases is of significant length and, therefore, the proposed 8x visual examination will be able to reliably detect much smaller flaws than the calculated critical flaw size. A VT-2 visual examination of the welds would also be performed during a hydrostatic test of the reactor.

The licensee submitted a test report giving the results of a resolution test of the camera equipment used by the welding contractor during the weld repairs. In the test, a wire 0.0005-inch diameter by 0.4-inch long was taped to the surface adjacent to a mockup of the production welds. The wire was filmed using the weld head lighting for illumination. Review of the tape demonstrated the visibility of the test wire with the camera system.

As a further process control during welding, the same video camera was employed to monitor the weld puddle during performance of the production welds. This technique is now commonly employed in the industry with positive

~ results. It enables the welding operator to further verify the welding process, take corrective actions during the course of welding, and to identify potential problem locations prior to weld completion and performance of weld acceptance examinations. With the additional process. monitoring this method provides, the probability of undetected weld defects is substantially diminished. Therefore, the proposed alternative is sufficient to ensure weld integrity.

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3.0 CONCLUSION

Under the provisions of 10 CFR 50.55a(a)(3)(1), the staff has determined that the licensee's proposed alternative to the ASME Code for surface examination 1

(PT) of the CRDM canopy seal welds will provide an acceptable level of quality

and safety. The proposed alternative inspection is authorized for the three  !

penetration locations at Prairie Island Units I during the winter 1996 outage.

i Staff authorization of the proposed alternative does not extend to future use i without prior staff review. 1 Principal Contributor: G. Georgiev i

Date: May 6, 1997 l

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