ML17157B077
| ML17157B077 | |
| Person / Time | |
|---|---|
| Site: | Susquehanna  |
| Issue date: | 03/03/1992 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML17157B076 | List: |
| References | |
| NUDOCS 9203110366 | |
| Download: ML17157B077 (4) | |
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UNITED STATES NUCLEAR REGULATORY COMMISSION WASHINGTON, D. C. 20555 ENCLOSURE SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION OF INSERVICE INSPECTION RELIEF RE UESTS PENNSYLVANIA POWER
& LIGHT SUS UEHANNA STEAM ELECTRIC STATION UNITS 1
AND 2 DOCKET NO. 50-387
- 1. 0 INTRODUCTION CRD housing bolting are visually inspected (VT-1) during scheduled maintenance as required by ASNE Section XI, Code Category B-G-2, Item B7.80 from the 1980 Edition, Winter Addenda of the ASHE Code.
IWB-2430 states that if relevant indications exceeding the requirements of IWB-3000 are identified, additional components in the same examination category shall be examined.
In addition, the visual indications must be inspected using either magnetic particle or liquid penetrant examinations.
If additional relevant indications are identified during the expanded examination, then all similar components shall be examined.
Pursuant to 10 CFR 50.55a(g)(5), if the licensee determines that conformance with an examination requirement of Section XI of the ASME Code is not practical for his facility, information shall be submitted to the Commission in support of that determination and a request made for relief from the ASHE Code requirement.
After evaluation of the determination, pursuant to 10 CFR 50.55a(g)(6)(i),
the Commission may grant relief and may impose alternative requirements that are determined to be authorized by law, will not endanger life, property, or the common defense and security, and are otherwise in the public interest, giving due consideration to the burden upon the licensee that could result if the requirements were imposed.
In a letter dated October 31, 1991, the licensee, Pennsylvania Power
& Light (PP&L), submitted Relief Request Nos.
1RR-22 and 2RR-19 for the Susquehanna Steam Electric Station (SSES),
Units 1 and 2, Inservice Inspection (ISI)
Program.
1RR-22 and 2RR-19 request relief from the sample expansion
'equirements for Control Rod Drive (CRD) housing bolting as required by Section XI of the ASME Code.
The licensee found crack like indications on CRD housing bolting (cap screws) on Unit 2 during the 1988 refueling outage.
The CRD housing bolting for Unit 1 is similar to the housing bolting for Unit 2 and similar crack-like indications are anticipated for Unit 1 housing bolting.
- 2. 0 EVALUATION OF RELIEF RE VEST The information provided by the licensee in support of the request for granting relief from impractical requirements has been evaluated, and the basis for granting relief is documented below.
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Re uest for Relief Nos.
1RR-22 and 2RR-19 Examination Cate or B-G-2 Item B7.80 Code Re uirement:
Section XI, Examination Category B-G-2, Item B7.80, requires that when examinations performed during an inspection reveal indications exceeding the allowable standards of IWB-3000, an additional number of components (or areas) within the same examination category shall be examined to the extent specified in Table IWB-2500-1.
Licensee's Code Relief Re uest:
Relief is requested from performing additional examinations of CRD bolting when cracks are detected in the head to shank fillet area.
Licensee's Pro osed Alternative:
None.
Plant maintenance practices require all visually rejected bolting to be documented on a plant nonconformance report.
Visually accepted bolts are magnetic particle (HT) inspected and any bolts with linear indications greater than I/16" are rejected, segregated and documented on a plant nonconformance report.
No rejected/cracked bolts are returned to service.
Only bolts which are accepted both by visual and magnetic particle examination are returned to service.
A sampling of bolts with cracks in the head to shank fillet area undergo subsequent engineering and metallurgical analysis.
If this analysis discloses new conditions which have not been evaluated then additional examinations
( to the extent practicable) will be performed.
Licensee's Basis for Re uestin Relief:
The licensee states that the CRD bolting is located in an adverse radiological environment, and that removal of CRD bolting involves plant and personnel safety hazards, thus making the ASME Code requirements impractical for its facility.
The licensee states that engineering and metallurgical analysis performed on the subject bolting show that the cracking does not compromise plant safety or CRD bolting integrity.
Detailed metallurgical analysis of the defects has determined the cracking mechanism to be stress corrosion blunted by general corrosion.
Crack growth calculations conclude that the 'cracks grow at a decreasing rate and that it would take 22 years for the worst case crack in a single bolt to grow to the minimum required average bolt diameter.
The longest period of time a bolt will be in service without receiving the code required visual examination is 12 years (inspection frequency coincides with CRD mechanism refurbishment exchange frequency).
Therefore, cracked bolting will be detected prior to the worst case single bolt failure.
In addition, analysis of the CRD bolted joint indicates that only 3 bolts out of 8 are needed to meet ASNE Code margins.
- Thus, adequate safety margin exists even when a bolted joint contains potentially cracked bolts.
Staff Evaluation:
The staff finds that visual examination of additional CRD housing bolting is impractical to perform to the extent required by the Code because of the adverse radiological environment and plant and personnel safety hazards experienced during removal of the CRD bolting.
The additional
examinations would only augment the total sample of cracked bolts with no change to the results of the engineering and metallurgical analyses.
- Thus, additional examinations will not increase plant safety or improve bolting integrity.
Additional examinations are performed to determine if unanticipated severe degradation of bolting integrity is occurring due to erosion, corrosion, or cracking.
However, analysis done by GE has shown that the crack growth rates seen at SSES are comparable to published crack growth rates for similar materials in similar environments and that the structural integrity of the housing would not be affected by the presence of the cracks.
Five of the eight bolts could be fully cracked without affecting the structural integrity of the housing.
Code required VT-I visual examinations during CRD mechanism refurbishment should provide an acceptable level of quality and safety.
3.0 CONCLUSION
Paragraph 10 CFR 50.55a(g)(4) requires that components (including supports) which are classified. as ASHE Code Class 1, 2, and 3 meet the requirements, except design and access provisions and preservice requirements, set forth in applicable editions of the ASME Code,Section XI, to the extent practical within the limitations of design,
- geometry, and materials of construction of the components.
Pursuant to 10 CFR 50.55a(g)(5)(iii), the licensee determined that conformance with certain Code requirements is impractical for its facility and submitted supporting information.
The staff has reviewed the licensee's submittal and concludes that relief can be granted as requested.
Pursuant to 10 CFR 50.55a(g)(6)(i),
the staff concludes that the requirements of the Code are impractical and relief may be granted for Relief Request Nos.
Such relief is authorized by law and will not endanger life, property, or the common defense and security, and is otherwise in the public interest.
This relief has been granted giving due consideration to the burden upon the licensee that could result if the requirements were imposed on the facility.
Principal Contributors: J.
Davis J. Raleigh
"'ate:
Harch 3, 1992