ML20202J173

Safety Evaluation Concluding That NSP Proposed Alternative to Surface Exam Requirements of ASME BPV Code for CRD Mechanism Canopy Seal Welds Will Provide Acceptable Level of Quality & Safety

ML20202J173
Person / Time
Site:	Prairie Island
Issue date:	01/22/1999
From:	NRC (Affiliation Not Assigned)
To:
Shared Package
ML20202J147	List: ML20202J142 ML20202J173
References
NUDOCS 9902090057
Download: ML20202J173 (3)

Text

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SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION REVIEW OF PROPOSED ALTERNATIVE TO THE ASME CODE ON SURFACE EXAMINATION OF WELD REPAIRS AND OVERLAYS TO NON-STRUCTURAL CANOPY SEAL WELDS .

PRAIRIE ISLAND NUCLEAR GENERATING PLANT. UNITS 1 AND 2 NORTHERN STATES POWER COMPANY DOCKET NOS 50-282 AND 50-306

1.0 INTRODUCTION

Pursuant to 10 CFR 50.55a(a)(2), systems and components of boiling and pressurized water-cooled nuclear power reactors must meet the requirements of ASME Boiler and Pressure Vessel Code specified in paragraphs (b), (c), (d), (e), (f), and (g) of this section.10 CFR 50.55a(a)(3) proposed alternatives to the requirements of paragraphs (c) through (h) of this section or portions thereof may be used when authorized by the NRC. The applicant shall demonstrate that (i) the proposed alternatives would provide an acceptable leve: of quality and cafety, or (ii) compliance with the specified requirements of this section would result in hardship or unusual difficulty without a compensating increase in the level of quality and safety.

By letter dated November 30,1998, Northem States Power Company (the licensee) proposed an alternative to the surface examination requirements of paragraph N-518.4 of the 1968 American Cociety of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code (the code of record for Prairie Island) for control rod drive mechanism (CRDM) canopy seal welds. In place of the liquid penetrant (PT) surface examination required by the ASME Boiler and Pressure Vessel Code, the licensee-proposed altemative is to make weld repair / overlays using an automatic welding process, to perform visual examinations of the weld area with a remote 8x video camera, and to perform a post-outage system leakage test inspection. The proposed alternative would be used in the examination of one canopy seal weld repair and in the examinations of weld overlays applied on non-repaired canopy seal welds. The seal weld repair is at location E11 on the lower canopy seal weld. The weld overlays will be performed as a preemptive measure on the lower and intermediate canopy seal welds during the upcoming refueling outages.

The seal welds are used to ensure leak tightness of threaded joints holding the rod travel housing to the CRDM housing. Each seal weld is a small groove weld applied to a small protrusion (" canopy") over the end of the threads. Since the threads constitute the pressure boundary, the seal weld is non-structural. The presence of the canopy protrusion provides a 9902090057 990122 PDR ADOCK 05000282 P PDR

-a g 2 weld surface that avoids fusion of the ends of the threads and allows the seal weld to be more readily removed when necessary. The weld repair / overlays will increase the wall thickness of the protrusion at the seal weld. ,

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2.0 DESCRIPTION

2.1 Reauest for Relief The licensee requested relief from the PT testing requirements in N-518.4 of the 1968 Edition of Section 111 of the ASME Code for weld repair / overlays of CRDM canopy seal welds.

2.2 Basis for Relef Paragraph N-518.4 of the 1968 Edition of Section ill of the ASME Code requires that attachments welded to the pressure boundary be inspected by means of a PT, However, PT ,

weld examinations of the canopy seal welds are difficult. Surface preparation (grinding) of the  !

welds, PT examination, and subsequent cleanup would have to be performed around obstacles, would be time consuming, and would incur substantial personnel radiation exposure. Access between CRDMs is limited with a separation of approximately 7.2 inches, and canopy seal welds are in a high radiation field of approximately 400 mr/hr.

2.3 Proposed Alternative j The licensee proposed the following alternative to the liquid penetrant testing requirements for

. the weld repair / overlays described above:

The use of a controlled automatic welding process.

The observation of the weld puddle / deposit via a 8x camera during the welding process.

- A fina) visual examination of the weld surface using the same 8x camera.

The performance of a VT-2 inspection of the canopy seal weld area for leakage during the post-outage system leakage test inspection.

- The authorized nuclear inservice inspector approval of alternative testing and NIS-2 acceptance.

3.0 - EVALUATION

, The 1968 Edition of Section 111 of the ASME Code specifies that a surface examination be performed on weld repaired areas (para. N-514.2) or welded attachments (para. N 518.4).

! These paragraphs require PT examination be performed in accordance with N-627. In paragraph N-627, the most stringent acceptance criteria is the requirement for "no linear indications." For the proposed alternative, a no linear indication criteria is unrealistic. Instead, the licensee calculated the critical flaw (crack) size with fracture mechanics and limit load

7 3 analysis, then demonstrated a video camera system that had the capability of finding flaws smaller than the critical flaw size.

The licensee submitted a test report giving the results of a resolution test for the camera equipment that will be used by the welding contractor during the weld repair / overlays. In the test, a 0.0005-inch diameter by 0.4-inch long wire was used to simulate a crack. The wire was j taped to the surface next to a mock-up production weld. A review of the video recording made during the demonstration showed that the camera system was capable of recording the image of the test wire.

Since the camera demonstration was with a simulated crack, the licensee performed a bounding analysis using limit load (net section collapse) and linear elastic fracture mechanics (LEFM) analyses to determine critical crack size. Using the limit load method, the critical longitudinal and circumferential through-wall crack lengths were 4.3 inches and 8.1 inches, respectively.

Using LEFM, the critical longitudinal and circumferential through-wall crack lengths were 5 inches and 7.8 inches, respectively. The limit load analyses provide the most realistic calculation of the maximum tolerable crack length. Although known to be less accurate for the high toughness materials used, the LEFM results provide an independent verification of the limit load analyses.

Both sets of analyses give critical crack sizes 10 times larger then the length of wire detected in the weld head video camera performance demonstration. In the staff's opinion, the initiation and growth of a crack larger than the bounding critical length of 4.3 inches in and near a weld joining stainless steel-to-Inconel 600 material without being detected is unrealistic. Because of weld shrinkage, a crack, if present, would exhibit significant opening in the width dimension, thereby, enhancing detectability.

As part of the license's process control during welding, the video camera will be employed to monitor the weld puddle during performance of the production welds. The monitoring enables the welding operator to verify the welding process, take corrective actions during the course of welding, and to identify potential problem locations prior to weld completion. The licensee will also perform a VT-2 inspection of the canopy seal weld area for leakage during the post outage system leakage test inspection. With this additional process monitoring capability, the licensee can provide reasonable assurance that any crack formed in or near the canopy seal weld will be detected. This technique is now commonly employed in the industry with positive results.

4.0 CONCLUSION

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Based on the submittal and above discussion, the staff concludes that pursuant to 10 CFR  :

50.55a(a)(3)(i), the licensee's proposed attemative will provide an acceptable level of quality and i safety. . ,

Prinicpal Contributor: D. Naujock Date: January 22, 1999

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