ML20082E267
| ML20082E267 | |
| Person / Time | |
|---|---|
| Site: | Seabrook  |
| Issue date: | 07/24/1991 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML20082E264 | List: |
| References | |
| NUDOCS 9107310311 | |
| Download: ML20082E267 (3) | |
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. UNITED STATES
~S-NUCLEAR REGULATORY COMMISSION k
WASHINGTON D C. 20555
<y w.....f SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR P.EGULATION RELATED TO AMENDMENT N0. 4 TO FACILITY OPERATING LICENSE NO. NPF-86 PUBLIC SERVICE COMPANY OF Nai HAMPSHIRE SEABROOK STATION, UNIT N0. 1 DOCKET NO. 50-443
1.0 INTRODUCTION
By letter dated November 13, 1990, New Hampshire Yankee (the licensee) requested an amendment to the Seabrook Station Operating License incorpo ating a change to the Seabrook Station Technical SpvMications (TSs). The proposed TS changes would delete the existing requirement to perform a weekly stroke test of the high pressure turbine control vhlves.
The surveillance requirements for the high pressure turbine stop valves and the combined intermediate valves are unchanged, as is the monthly stroke test requirement for the high pressure turbine control valves. Additionally, ec'itorial changes with respect to valve nomenclature have been submitted to provide consistency throughout the TS.
The turbine gener; tor is controlled and protected by an electro-hydraulic control (EHC) system that combines solid state electronic and high pressure hydraulic components to. control the steam flow through the turbine. The four high pressure turbine control valves are automatically positioned by the EHC system to control turbine speed and/or load. The high pressure turbine I
control valves combined with the four high pressure turbine stop valves provide a redundant means of isolating the high pressure turbine from the main steam supply in the event of an overspeed condition. The six combined intermediate valves provide a means of isolating the low pressure turbines o
l from their steam supply, the moisture separator-reheater.
A single failure of any one component would not lead to destructive overspeed.
2.0 EVALUATION The test frequency of the high pressure turbine control valves was originally based on the recommendation of the turbine manufacturer (General Electric).
These test frequencies were derived from the test frequencies established for l
fossil-fuel turbines.
Fossil-fuel turbines typically operate at higher L
temperatures and pressures than nuclear plant turbines.
As a result, fossil-fuel turbines are more susceptible to oxidation and sticking of control valve stems. Operating experience at nuclear power plants has indicated significantly lower failure rates than those from which the recommended test intervals were derived. Based upon these findings, General Electric Company issued Technical Information Letter (TIL) No. 969, dated May 22, 1984. The test frequency specified in this TIL is monthly testing of the control valves, and weekly testing for all other turbine steam valves.
9107310311 910724 PD9 ADOCK 05000443 P
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-2 The probability of multiple failures involving undetected electronic faults and/or stuck valves at the instant of load loss is extremely low due to the demonstrated high reliability of the control system components, and periodic in-service testing and inspection. General Electric Company states in TIL No. 969 that, ba:ed on past in-service experience with nuclear turbine steam valves, turbine steam inlet valve rel'r 'lity and testing intervals are no longer the major contributing factors in determining hypothetical turbine missiles.
The current Seabrook Station TSs require cycling of the four high pressure turbine stop valves, four high pressure turbine control valves anc six low pressure combined intermediate valves at least once every seven days. The Seabrook Station Probabilistic Safety Assessment (SSPSA), dated January 30, 1984, provided estimates of the mean annual frequency of turbine missile generation based on thg weexly test intervals.
This analysis yielded estimates of 2.0 x 10- forthemeanannualfrequgncyofturbinemissile generation due_to turbine ovurspeed and 8.3 x 10 for the total mean annual frequency of turbine missile generation. The staff considers these estimates to be conservative.
In the request for license amendment, the licensee addressed the significance of turbine missilt generaticn with regard to public risk. The SSPSA estimates the likelihood of ger.erating turbine missiles and analyzes the most probable consequences. The results indicate that the contribution from turbine nissile generation to public risk is negligible. The staff accepts this analysis in the context that small changes in the probability of turbine missile generation are unlikely to involve a significant increase in the probability of an j
accident or a significant reduction in the margin of safety.
l-l The extended recomriended testing interval for the control valves is also based on a reduction in the number of plant transients caused by the test, in order J:
to perform the control valve test, turbine output must be reduced below 93 percent of full power.
Peducing the frequency of this power reduction decreases the probability of an inadvertent turbine trip (and reactor trip) caused by testing.
The General Electric Company provided comparative values for increased turbine overspeed probabD ities due to increased test intervals in Nuclear Wheel Information Letter No. 2 dated November 8,1982. Applying these values to tie situation where the control valve test frequency is changed from weekly to monthly increases the turbine overspeed probability by a factor of 3.3.
The staff utilized this information to quantitatively evaluate the effect of the proposed change on the probability of turbine missile generation. This evaluation yielded an overspeed and total mean annual frequency of turbine missile generation of 6.6 x 10-g and 1.3 x 10-4, respectively.
1 The staff does not consider the change in total mean annual frequency of turbine missile grneration resulting from the proposed change in high pressure l
turbinc control valve testing interval to be significant based on:
the l
conservative approach of the licensee, the uncertainties inherent in estimates of the probability of turbine missile generation, and the turbine manufacturer's
.~-
c position:that. turbine steam inlet valve reliability and testing are no longer-major contributing factors in determining the probability of generating
-turbine missiles. Additionally, the decrease in the frequency of power transient; which will result from the proposed change in testing interval will provide an improved level of safety. The proposed editorial changes improve the consistency of valve nomenclature.
Based on the above, the staff finds the proposed TS amendment acceptable.
. Based on information available from the licensee and the turbine manufacturer, an extension of-the control valve testing _ interval from weekly to monthly will not significantly increase the probability of turbine missile generation, the probability of accidents initiated by such an event, or the consequences of thest postulated accidents.
Moreover, the extended test interval reduces the
. probability of-inadvertent plant trips which challenge safety systems.
Therefore, the staff finds the proposed TS amendment acceptable.
3.0 STATE CONSULTATION
In accordance with the Commission's regulations, the New Hampshire State official was notified of the proposed issuance of the amendment.
The State official had no comments.
4.0. ENVIP;0g ENTAL CONSIDERATION The amendment changes an inspection or surveillance requirement with respect to installation or use of a facility component located within the restricted area as defined in 10 CFR Part 20 and changes surveillance requirements. The NRC staff has vetermined 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,-and that there is no significant increase in individual or cumulative occupational 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
-24216). 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 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 public will not be endangered by operation in the proposed manner, (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: S. Jones, SPLB Date:
July 24, 1991
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