ML20082M195
| ML20082M195 | |
| Person / Time | |
|---|---|
| Site: | River Bend  |
| Issue date: | 08/20/1991 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML20082M193 | List: |
| References | |
| NUDOCS 9109050081 | |
| Download: ML20082M195 (3) | |
Text
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NUCLEAR REGULATORY COMMISSION WAsHINoToN, D.C. 2o56s yw y
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SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION RELATED TO AMENDMENT NO. 59 TO FACILITY OPERATING LICENSE NO. NPF-47 GULF STATES UTILITIES COMPANY RIVER BEND STATION, UNIT 1 DOCKET NO. 50-458
1.0 INTRODUCTION
By letter dated April 16, 1991, Gulf States Utilities Company (GSU) (the licensee) requested an amendment to Facility Operating License No NPF-47 for the River Bend Station, Unit 1.
The proposed amendment would modify Technical Specificatiun (TS) Surveillance Requirement 4.3.8.2.a " Turbine Overspeed Protection System" by reducing the testing frequency of the high pressure turbine control valves from once every 7 days to once every 31 days.
The turbine overspeed protection system protects the turbine from excessive overspeed by closing the turbine steam arimission valves when a high turbine speed is sensed.
Although steam turbines and their auxiliaries are not safety-related systems as defined by NRC regulations, excessive overspeed of the turbine can produce large, high-energy missiles.
If such a missile were to strike and damage plant safety-related structures, systems, or components, they could render them unavailable to perform their safety function.
The current surveillance frequency is based on the original General Electric (GE) recommendations of March 1973, which were based on the probability of overspeed and wheel burst of a turbine.
Operating experience accumulated during the 24 years prior to 1984 has shown considerably lower valve failure rates than those values upon which the original GE recommendations were based.
GE documented these findings in Service Infermation Letter (SIL) No.
413 dated October 4, 1984, which recommended monthly testing of high pressure turbine control valves.
This has been reviewed and approved for inclusion in the Technical Specifications of all recently licensed BWRs and GSU has already incorporated recommendations for reduced testing frequencies for the other three sets of turbine valves.
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2.0 EVALUATION The current Technical Specification requires that the four high pressure turbine control valves be cycled through at least one complete cycle from the running position at least once per 7 days.
Reactor power must be reduced to 180% in order to perform this surveillance requirement.
The change to a 31-day testing interval would reduce the number of reactor power reductions necessary to accommodate turbine control valve testing.
Fewer deviations from steady state design conditions would reduce the probability of a reactor / turbine trip and the attendant challenge to safety systems.
The testing frequency for turbine control valves is based on the theoretical probability of a missile being generated in the turbine due to overspeed.
General Electric presented to the NRC their methodology for calculating the probability of failure in their report " Probability of Missile Generation in General Electric Nuclear Turbines" (Ref.1).
GE determined that, based on past in-service experience with nuclear turbine steam valves, turbine steam inlet valve reliability is no longer the major contributing f actor in deter-mining hypothetical missile generation.
Instead, the dominant failure mode is considered to result from propagation of turbine rotor cracks, and failure is most likely to occur at normal running speed not an overspeed condition potentially attributable to control valve malfunctions.
The methodology was i
ieviewed and found acceptable by the NRC for use in establishing inspection and testing intervals for turbine systems.
In a study of the River Bend turbine system, GE calculated a probability of 5 x 10-6 (Ref. 2) for a missile being generated in the turbine based on a six year rotor inspection interval and weekly testing of the turbine control valves.
This is well within the limit of I x 10" recommended by Regulatory Guide (RG) 1.115.
The probability of damage to safety-related systems, calcu-lated as 5 x 10-8 (Ref. 3), is also within the limit of 1 x 10" specified in RG 1.115 and the Standard Review Flan (SRP) Section 3.5.1.3.
GE Technical Information Letter (TIL) No. 969 documents GE's findings that the probability of missile generation increases only a negligible amount by increasing valve testing intervals from weekly to monthly.
This assures that the probability of failure will still be within the limits identified above when the revised surveillance frequency is used.
Based on the staff's review of the licensee's submittal, RG 1.115, SRP Section 3.5.1.3, GE SIL 413, and GE TIL 969, the change to a 31-day testing interval for the turbine control valves is acceptable.
3.0 STATE CONSULTATION
In accordance with the Commission's regulations, the Louisiana State official was notified of the proposed issuance of the amenument.
The State official had no comments.
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l 4.0 ENVIRONMENTAL CONSIDFRATION I
The amendment changes surveillance requirements.
The NRC staff has determined that the amendment involves no significant increase in the amounts, and no significant change in the types, of any effluents that may be released offsite,
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7 and that there is no significant increase in individual or cumulative occupa-tional radiation exposure.
The Commission has previously issued a proposed finding that the amendment involves no significant hazards consideration, and there has been no public comment on such finding (56 FR 24212).
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 impact statement or environmental assessment need be prepared in connection l
with the issuance of the amendment.
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5.0 CONCLUSION
r The Commission has concluded, based on the considerations discussed above, that:
(1) there is reasonable assurance that the health and safety of the j
public will not be endangered by operation in the proposed manner, (2) such i
activities will be conducted in compliance with the Commission's regulations,
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and (3) the issuance of the amendment will not be inimical to the common i
defense and security or to the-health and safety of the public, i
t REFERENCES 1.
Hope Creek Safety Evaluation Report, Supplement 6, Appendix U, dated July 1986.
I 2.
Letter from J. E. Booker of GSU to hRC, dated May 13, 1987.
l Approved by. letter from W. Paulson of NRC to GSU dated August 26, 1987.
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!j 3.
River Bend Updated Safety Analysis Report, Section 3.5.1.3.4.1.
t Principal Contributor:
Donna Skay I
Date:
August 20, 1991 I
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