ML20217B532
| ML20217B532 | |
| Person / Time | |
|---|---|
| Site: | North Anna  |
| Issue date: | 04/20/1998 |
| From: | NRC (Affiliation Not Assigned) |
| To: | |
| Shared Package | |
| ML20217B505 | List: |
| References | |
| NUDOCS 9804230063 | |
| Download: ML20217B532 (2) | |
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,j NUCLEAR REGULATORY COMMISSION WASHINGTON, D.C. 2066H001 SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION REVIEW OF PROPOSED ALTERNATIVE TO THE ASME COQE FOR SURFACE EXAMINATION OF NONSTRUCTURAL SEAL WELDS VIRGINIA ELECTRIC AND POWER COMPANY NORTH ANNA POWER STATION (NAPS) UNITS 1 AND 2 i
DOCKET NOS. 50-338 AND 50-339
1.0 INTRODUCTION
By letter dated April 8,1998, Virginia Electric and Power Company (VEPCO, the licensee),
stated that NAPS Unit 2 is planning to replace the part-length control rod drive mechanism (CRDM) housings during the Spring 1998 refueling outage, which began on April 5,1996. The construction Code being used is the 1989 Edition of the American Society of Mechanical l
Engineers (ASME) Section Ill. The CRDM housing replacement will be performed in accordance with ASME Section XI 1986 Edition, which is the 2nd interval Code of reference.
Paragraph NB-5271 of the ASME Section lli requires examination of the final seal weld with a surface (magnetic particle or liquid penetrant) examination raethod. However, this Code examination requirement creates an unusual difficulty and hardship based on accessibility and personnel exposure. Therefore, pursuant to 10 CFR 50.55a(a)(3), the licensee requests the use of an attemate examination method to complete the part-length CRDM housing replacement activity. The proposed attemative would be used in conjunction with four j
nonstructural seal welds on threaded caps for the Unit 2 reactor vessel head penetrations for i
the part-length 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.
The examination of the welds required by Paragraph NB-5271 of the ASME Secticn til would be difficult. The seal welds will be in a high radiation area of approximately 2000 millirem / hour.
Additionally, access to the weld locations is difficult due to the limited clearance between adjacent CRDMs. The separation between the housings varies from approximately 4 to 8 inches. This is not adequate clearance to gain complete access to perform the liquid penetrant (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.
While the corrective action plan for the CRDM part-length housings for NAPS Unit 1 has not been finalized, the licensee has requested approval of the proposed alternative examination for ~
both Units 1 and 2. The proposed attemative examination requested and the basis are 9804230063 980420 PDR ADOCK 05000338 P
c.
identical, except that the Unit 1 Code of reference is the 1983 Edition with the Summer 1983 Addenda of ASME Section XI. The construction Code for Unit 1 would still be the 1989 Edition of ASME Section Ill.
2.0 EVALUATION
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Section 111 of the ASME Code,1989 Edition, specifies a surface examination of weld-repaired areas or welded attachments. For PT examinations, the acceptance criteria of paragraph NB-5320 must be met. The most stringent among the acceptance criteria is the requirement for "no linear indications." The Code defines 1/4 inch as the smallest length of a relevant linear indication. he prc;)osed altemative would include an 8-power visual examination (by means of i
a remote video camen) of the in-process automatic welding and a post-weld visual examination
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l with the same camera. Although the seal weld is nonstructural, the licensee had an analysis performed that treated tt e 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 i
examination with the camera. The results of the analysis demonstrated that the predicted i
critical flaw size in all cedes is of significant length and, therefore, the proposed 8-power visual l
examination will be ab:e to reliably detect much smaller flaws than the calculated critical flaw j
i size. A VT-2 visual'.sxamination of the welds in accordance with IWA-5241 would also be i
performed oud,9 a hydrostatic test of the reactor. This inspection assures that the function of I
the seal weld is being satisfied.
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 l
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.
1 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 alternative will provide an acceptable level of quality and safety.
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3.0 CONCLUSION
l 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 NAPS, Unit 1 and 2, reactor vessel head penetrations for the part-length CRDMs will provide an acceptable level of quality and safety. The proposed alternative inspection is authorized for the four stated repairs at NAPS, Unit 1 and 2. Authorization of the proposed attemative does not extend to future use without prior staff review.
Principal Contributor: N. Kalyanam Date: April 20, 1998 m