ML20203H833

SE Accepting Proposed Alternative to ASME Code for Surface Exam of Nonstructural Seal Welds for Prairie Island Nuclear Generating Plant,Unit 2

ML20203H833
Person / Time
Site:	Prairie Island
Issue date:	02/20/1998
From:	NRC (Affiliation Not Assigned)
To:
Shared Package
ML20203H828	List: ML20203H823 ML20203H833
References
NUDOCS 9803030339
Download: ML20203H833 (2)

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SAFETY EVALUATION SY THE OFFICE OF NUCI WAR REACTOR REGULATION  !

REVIEW OF PROPOSED ALTERNATIVE TO THE ASIWE CODE l FOR SURFACE EXAMINATION OF NONSTRUCTURAL SEAL WELDS NORTHERN STATES POWER COMPANY L

j PRAIRIE ISLAND NUCLEAR GENERATING PLANT. UNIT 2 l DOCKET NO. 50 306 i.

1.0 INTRODUCTION

i By letter dated February 13,1998, Northem States Power (NSP) Company proposed an

[ attemative 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 attomative would be used in conjunction with four nonstructural seal welds on threaded caps for

the Unit 2 reactor vessel head penetrations for the *part length control rod drive mechanisms -

(CRDMs) after the CRDMs are removed, The penetration locations are E7, G5, Og, and 17. -

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The welds will be used to ensure leak tightness of the thread joint holding the new cap to the

! CRDM penetrations after the CRDM is removed. Because the threads constitu's the pressure  ;

i . boundary, the seal weld is nonstructural.  !

i Paragraph N 518.4 of the 1968 ASME Code, Section lil, requires that attachments (welds) to -

L the pressure boundary be inspected by means of a liquid penetrant examination (PT).

l However, a PT examination of the welds would be difficult. The seal welds will be in a high .

t radiation area of approximately 1000 millirem / hour. Additior, ally, access to the weld locations is -;

L 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

j. subsequent cleanup _would be difficult to execute property, would be time consuming, and would '

incur substantial personnel radiation exposure.

L 2.0 EVALUATION I Section ill ef the ASME Code,1968 Edition, specifies a surface examination of weld-repaired 4 i areas (paragraph N 514.2) or welded attachments (paragraph N 518,4). For PT examinations, 4 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, 1

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2-The proposed attemative would include an 8 power visual examination (by means of a remote  !

video camera) 3f the in process automs .,c welding and a post weld visual examination with the same camera. Although the seal weld is nonstructural, the licensee had an anatyals perfoMed that treated the weld as a pressure boundary in order to calculate a critical flaw size. The results were used as a reference for the evaluation of the effectiveness of the visual examination with the camera. The results of the analysis demonstrated that the predicted critical flaw size in all cases is of significant length and therefore the proposed d power 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. This inspection assures that the function of the seal weld is being satisfied.

i The licensee submPted 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 and another 0.001-inch diameter, both 0.4 inch long, were 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 wires with the camera system.

As a further process control during welding, the same video camera will be employed to monitor the weld puddle during performance of the production we!ds. This technique is now commonly emoloyed in the industry with positive results, it e3bles 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 undetecte* % defects is substantially diminished.

Therefore, based on the use of a controlled automatic welding process, observation of the weld puddle / deposit during the welding process, a final visual examination of the weld surface using the camera, and a VT 2 examination of the weld and the cap for leakage during the post-outage hydrostatic test, the proposed attemative will provide an acceptable level of quality and safety.

30 CONCLUSION Under the provisions of 10 CFR 50.55a(a)(3)(i), the staff has determined that the licensee's proposed attemative to the ASME Code for surface examination (PT) of the seal welds on threaded caps for the Unit 2 reactor vessel head penetrations for the part-length CRDMs will provide an acceptable level of quality and safety. The proposed attemative inspection is authorized for the four stated repairs at Prairie Island Unit 2. Authorization of the proposed alternative does not extend to future use without prior staff review, if simi!ar repairs are necessary for the Unit 1 penetrations in the future, a separate relief request submittal will be required.

Principal Contributor: A. Kugler Date: February 20, 1998 u -