ML13308B484
| ML13308B484 | |
| Person / Time | |
|---|---|
| Site: | San Onofre  |
| Issue date: | 11/16/1976 |
| From: | Arenal A Southern California Edison Co |
| To: | Engelken R NRC/IE, NRC/RGN-V |
| References | |
| NUDOCS 8103060509 | |
| Download: ML13308B484 (4) | |
Text
Southern California Edison Company P.O. BOX 800 2244 WALNUT GROVE AVENUE ROSEMEAD, CALIFORNIA 91770 November 16, 1976 9"
U. S. Nuclear Regu atory Commission Region V Office of Inspection and Enforcement Walnut Creek Plaza, Suite 202 1990 North California Boulevard Walnut Creek, California 94596 Docket No. 50-206 Attention: R. H. Engelken, Director
Dear Sir:
Reference:
Letter dated November 2, 1976, from SCE (H. L. Ottoson) to NRC Region V, Failed Thermal Shield Flexures This letter is a followup report concerning the failed thermal shield flexures cited previously in the letter referenced above. Cracks in four (4) of the six (6) flexure type supports of thethermal shield were found as a result of visual inspections performed on the reactor internals during the current refueling outage. These cracks have apparently rendered the affected flexure supports ineffective. There have been, however no apparent adverse effects resulting from operation with the flexures in this condition.
We have consulted the designer and manufacturer of the San Onofre Unit 1 reactor vessel internals (Westinghouse). Engineering analyses and information relative to the flexure installation have been reviewed. Data from similar operating plants, which have also experienced thermal shield flexure failure, has been studied. We have concluded from this review that satisfactory operation is possible with or without the flexure supports.
Extensive visual examinations of associated reactor vessel internals, conducted during the current refueling outage, have confirmed that no other part of the reactor vessel structure has been adversely affected by the flexure failures. This detailed inspection of the reactor internal structures and the thermal shield concentrated on uncovering evidence of any adverse effects caused by.the failures. The areas of major importance were the six bottom axial thermal shield supports, the four thermal shield upper radial alignment and limiter keys, the vessel irradiation specimen basket chutes and the six lower core radial 12003
- L
- 1.
r
-2 support keys. The visual inspection of these components revealed no indication on any portion of the reactor internals, or core components of abnormal vibration loading or wear. All structures, with the exception of the thermal shield flexure supports, were in good condition.
Specifically, the radiation specimen basket chutes, which are adjacent to the flexure supports and are Welded to the thermal shield and to supports above the thermal shield on the core barrel, were examined. The excellent condition of the welds on the chutes at the top of the thermal shield and at the first support above the thermal shield verifies that the thermal shield displacement was small after the flexures failed. Additionally, four radial alignment keys on the top end of the thermal shield at San Onofre, which fit in the core barrel keyways, act as radial movement limiters. Visual inspection of these keys indicated no excessive wear or damage. The visual inspection of the six bottom axial thermal shield supports and the six lower core radial support keys do not indicate the presence of high dynamic loading. Also, the locking devices on the bolts and the dowels in the supports are intact indicating*
no motion at the bottom support or transmittal of large forces. The mating fracture surfaces on each flexure were found in close contact. This observation indicates the flow induced oscillations of the thermal shield were not large enough to cause significant wear on the mating surfaces, and the total displace ment of the thermal shield, with respect to the core barrel, was not large enough to cause flexure failure.
This situation was reviewed by the On Site Review Committee on November 12, 1976 and by the Nuclear Audit and Review Committee on November 15, 1976. The Committees concluded that continued operation with the flexures in their present condition or with all six flexures cracked was satisfactory. This conclusion was based upon:
(1) Thermal shield structural support integrity is provided by the bottom support arrangements. Radial limiter keys at the top of the thermal shield provide additional margin in this design. The flexures were added as an additional precaution against thermal shield movement and are not required for structural support of the thermal shield under any operating conditions.
(2) The broken flexures are not potential loose parts due to the existing radial movement of the thermal shield being limited to 15 mils by the limiter keys. The existing break areas are relatively "clean"1 and even and their appearance indicates the absence of contact that would lead to metal fracture and subsequent loose parts.
(3) Connecticut Yankee has operated successfully without flexures since 1970 when all flexures were found to be cracked.
(4) The inspections noted above do not indicate any abnormal conditions.
Based on the foregoing, we intend to return the unit to service with the flexures in their present condition and take no further corrective action at this
-3 time. We anticipate completion of the in-service inspection of the reactor vessel on November 25, 1976. When it is complete, the core barrel will be returned to the vessel and fuel loading will be initiated. During the next refueling outage, we intend to visually inspect 50 percent of the flexures (those at 124, 205 and 244 degrees) in accordance with previously established procedures.
If you have any questions or require additional information, please let us know.
Sincerely, MIG!NL SIGNED (A. Arenal)
GRM/JCS:g20a
Attachment:
Licensee Event Report cc: Director, NRC Office of Inspection and Enforcement Director, NRC Office of Management Information and Program Control bcc: Each MARC Member Each OSRC.Member J. T. Head E. J. Bresnahan R. D. Britt J. A. Beoletto D. E. Nunn D. R. Pigott (Chickering & Gregory)
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