ML20207B593
| ML20207B593 | |
| Person / Time | |
|---|---|
| Site: | Prairie Island  |
| Issue date: | 05/26/1999 |
| From: | NRC (Affiliation Not Assigned) |
| To: | |
| Shared Package | |
| ML20207B589 | List: |
| References | |
| NUDOCS 9906020019 | |
| Download: ML20207B593 (3) | |
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SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION REVIEW OF PROPOSED ALTERNATIVE TO THE ASME CODE FOR SURFACE EXAMINATION OF NONSTRUCTURAL SEAL WELDS NORTHERN STATES POWER COMPANY PRAIRIE ISLAND NUCLEAR GENERATING PLANT. UNIT 1 DOCKET NO. 50-282
1.0 INTRODUCTION
By letter dated April 12,1999, 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 attemative would be used in conjunction with four nonstructural seal welds on threaded caps for the Unit 1 reactor vessel head penetrations for the part-length control rod drive mechanisms (CRDMs) after the CRDMs are removed.
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 constitute the pressure boundary, the seal weld is nonstructural.
Paragraph N-518.4 of the 1968 ASME Code, Section lil, requires that attachments (welds) to the pressure boundary be inspected by means of a liquid penetrant examination (PT). However, a PT examination of the welds would be difficult. The seal welds will be in a high radiation area of approximately 1000 millirem / hour. Additionally, access to the weld locations is difficult due to the limited clearance between adjacent CRDMs. The separation between the housings is approximately 7.2 inches. This is not adequate clearance to geh ur,ylak a6cem 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.
A similar repair for the Unit 2 reactor vessel head penetrations for the part-length CRDMs was completed in February 1998, with an attemative to liquid penetrant examination requirement as approved by the staff in a safety evaluation dated February 20,1998.
9906020019 990526 PDR ADOCK 05000282 P
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l 2.0 EVALUATION Section til of the ASME Code,1968 Edition (Code of record for the reactor coolant pressure boundary systems and components for both Unit 1 and Unit 2), 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 altemative 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. Although the seal weld is nonstructural, the licensee had an analysis performed (for Unit 2 weld repair) that treated the weld as a pressure boundary in order to calculate a critical flaw size. Based on similarity of design, geometry, and materials, the analysis also applies to Unit 1 cap welding. 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 8-power visual examination will be able to rebably 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.
The licensee submitted a test report giving the results of a resolution test of the camera equ!pment used by the welding contractor during the weld repairs (for Unit 2), 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 i
lighting for illumination. Review of the tape demonstrated the visibility of the test wires with the I
camera system. Since the same camera will be used for the Unit 1 cap welding, these camera test results are applicable to the Unit 1 cap welding.
As a further process control during welding, the same video camera will be 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.
l 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.
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3.0 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 1 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 1.
Principal Contributor: T. J. Kim Date: May 26, 1999
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. Ql.STRIBUTION FOR PRAIRIE ISLAND RELIEF REQUEST l
DATED: May 26,1999 Docket File (50-282, 50-306)
PUBLIC PDlli-1 Reading-J. Zwolinski/S. Black T. Harris T. Kim l
G. Dick -
. C.-Thomas l
OGC l
G. Hill (4)
ACRS T. Hiltz, EDO R. Lanksbury, Rlli RCN i
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