ML20086D687
| ML20086D687 | |
| Person / Time | |
|---|---|
| Site: | Seabrook  |
| Issue date: | 11/18/1991 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML20086D682 | List: |
| References | |
| NUDOCS 9111260222 | |
| Download: ML20086D687 (3) | |
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NUCLEAR REGULATORY COMMISSION r,
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SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION l
SUPPORTING AMENDMENT NO.8 TO FAtlLITY OPERATING LICENSE N0. HPF-86 PUBLIC SERVICE COMPANY OF NEW HAMPSHIRE SEABROOK STATION UNIT NO. 1 3
00CKE LNL 50-443
1.0 INTRODUCTION
By letter dated April 12, 1991, New Hampshire Yankee requested changes to the technical specifications (TS) of the Seabrook Station. Additional information was submitted by letter dated July 12, 1991. Tne proposed changes redefine the fully withdrawn position of all Rod Cluster Control Assedly (RCCA) banks to minimize localized RCCA wear. Curt ently, the fully withdrawn position for the control and shutdown RCCA t,anks is defined as ??8 steps above rod bottom. The proposed changes will allow the control and shutdown RCCA banks to be designated es fully withdrawn between steps 225 and 232, inclusive.
These changes are consistent with those approved for other plants. The RCCA repositioning will axially reposition the RCCAs to distribute wear to several locations on the RCCA rodlets in order to extend rod life.
The RCCAs in kestinghouse PWRs were originally estimated to last for at least 15 years before the absorber cladding, a thin tube, would snow excessive thinning as a result of sliding wear.
In 1983, after 13 years of operation, the RCCAs were inspected at Point Beach Nuclear Plent, Unit 2.
The results of this inspection showed that sliding wear was minor, but severe fretting wear had occurred on several tubes. Subsequer.t inspections at the Kewaunee and Haddam Neck plants which had been in operation for more than 12 years, also showed fretting wear. The marks of fretting wear were about 1 inch in length and were found adjacent to the guide blocks that position the rods when the RCCAs are in their withdrawn position.
The fretting resulted from flow-induced vibratory contact between the rods and the guide blocks during long periods of steady-state power operation.
Vibration is hydraelically induced by flow of the reactor coolant; therefore it is e continuous process when the reactor coolcnt pumps are in operation.
More recently plants have experienced nuticeable fretting wear after as little as one cycle of operation. As a result of this industry experience, Yankee Atomic Electric Company (Y/sEC) has advised New Hampshire Yankee to implenent the RCCA repositioning program as soon as possible.
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2 2.0 EVALUATION Currently the fully withdrawn position for all of the Seabrook Station RCCAs is 228 steps abovs rod bottom. To avoid the fretting wear at the same location, it is recommended that the fully withdrawn parked position be changed periodically.
In this way the wear will be spread over a greater surface area of the rodlet cladding.
The licensee proposed defining " fully withdrawn" to mean between 225 and 232 steps above reactor bottom for all RCCA banks. Between 228 and 232 steps, the RCCAs are withdrawn at least two steps above the active fuel. Thus with respect to core physics, the effects are equivalent. fiso at 232 steps, the RCCAF will remain inserted in the guide thimbles of the fuel assemblies and thus will allow for a smooth rod drop. The rod drop time assumed in the saf ety analysis will still be bounding. When the RCCAs are withdrawn to 225 steps, they will be actually inserted apreoximately 0.4 inches into the active core region, The RCCA repositioning program has been evaluated With respect to its effect on reactor physics, transient analysis, LOCA analysis, mechanical analysis and fuel management.
YAEC, using the 51MULATE-3 computer model, has determined that the repositioning to the 225th step will have en insignificant effect which is not quantifiable for both axial and radial power distributions, critical boron concentrations and temperature dependent shutdown nergins during Cycles 1 and 2.
The repositioning program will be taken into account in future core designs and safety anelyset.
The RCCA repositioning program allows the rods to be withdrawn to 232 steps, four steps higher than the current TS limit. Conservative calculations show that the rod crop time will be rell within the TS Rod Drop Time of 2.2 seconds.
Furthermore, the f 2 positioning will not be implemented until rod drop times have been ueasured at the new positions and shown to meet the TS values.
Because the proposed change involves small adjustments in tile fully withdrawn position of RCCAs relative to the active core region, we would expect essentially negligible effects of the proposed change as reported in the licensee's evaluation.
3.0 STATE CONSULTATION
In accordance with the Commission's regulations, the New Hampshire and Massachusetts State officials were notified of the proposed issuance of the amendment. The State officials had no comments.
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4.0 ENVIRONMENTAL CONSIDERATION
The amendment changes a requirement with respect to installation or use of a facility component-located within the restricted area as defined in 10 CFR Part 20. The NRC staff has determined that the amendment involves no i
significant increase in the amounts, and no significant change in the types, of any effluents that may be released offsite, and that there is no significant increase in individual or cumulative occupational radiation exposure. The Connission has previously issued a proposed finding that the amendrent involva.s no significant hazards consideration, and there has been no public comment on such' finding (56 FR 51928). Accordingly, the amendment meets the eligibility criteria for categorical exclusion set forth in 10 CFR 51.22(c)(9).
Pursuant to 10 CFR 51.22(b) no environmental'imoact statement or.
environmental assessment need be prepared in connection with the issuance of the amendment,
5.0 CONCLUSION
The Commission has concluded, based on the considerations discussed above, that:
(1) there is reasonable assurance that the health and safety of the publicwillnotbeendangeredbyoperationintheproposedmanner,(2)such activities will be conducted in compliance with the Commission's regulations, and (3) the issuance of the amendment will not be inimical to the common defense and security or to the health and safety of the public.
Principal Contributor: M. Chatterton Date:
November 18, 1991 p
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