ML20217M976
| ML20217M976 | |
| Person / Time | |
|---|---|
| Site: | North Anna  |
| Issue date: | 04/01/1998 |
| From: | Kansler M VIRGINIA POWER (VIRGINIA ELECTRIC & POWER CO.) |
| To: | NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM) |
| References | |
| 98-185, NUDOCS 9804080278 | |
| Download: ML20217M976 (5) | |
Text
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WGINIA Ei.iccTunc ann Pownca COMPANY RictimoNo, VIMHNI A 2.U61 April 1, 1998 U. S. Nuclear Regulatory Commission Serial No.98-185 Attention: Document Control Desk NAPS /GSS/ETS R0 Washington, D.C. 20555 Docket Nos. 50-338 50-339 License Nos. NPF-4 NPF-7 Gentlemen:
VIRGINIA ELECTRIC AND POWER COMPANY NORTH ANNA POWER STATION UNITS 1 AND 2 PROPOSED TECHNICAL SPECIFICATION CHANGES TURBINE VALVE SURVEILLANCE FREQUENCY On November 18,1997, Virginia Electric and Power Company requested amendments, in the form of changes to the Technical Specifications and to Facility Operating License Numbers NPF-4 and NPF-7 for North Anna Power Station Units 1 and 2, respectively.
The proposed Technical Specification changes will permit quarterly testing of the turbine governor and throttle valves. The proposed changes were discussed with the NRC staff in a telephone conference call on March 25,1998 and additional clarification was requested by the NRC staff to complete the review of the proposed Technical Specification. This letter provides the information requested by the staff.
UFSAR Section 10.2 describes the turbine as "a conventional 1800-rpm, tandem-compound unit (Westinghouse Model BB-281), consisting of one double-flow, high pressure cylinder and two double-flow, low-pressure cylinders." The basis for the
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Technical Specification change is Westinghouse WCAP-14732, "Probabilistic Analysis jIl of Reduction in Turbine Valve Test Frequency for Nuclear Plants with BB-296 Turbines with Steam Chests." The low pressure turbines are Westinghouse Model BB-281, i
where as the high pressure turbine and steam chest is a Westinghouse Model BB-296.
g The UFSAR will be revised during a future revision to eliminate the confusion I
associated with the model numbers.
To support the proposed quarterly testing of the turbine throttle and governor valves, the WCAP-14732,"Probabilistic Analysis of Reduction in Turbine Valve Test Frequency for Nuclear Plants with BB-296 Turbines with Steam Chests," evaluated the risk associated with turbine throttle and governor valve testing at various intervals from monthly to annually.
However, in the introduction of the significant hazards deteimination we erroneously stated that the Westinghouse probabilistic analysis 9804080278 900401 PDR ADOCK 0500o338 P
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evaluated a test interval of up to eighteen months.
The basis of the Technical Specificatior)s and significant hazards consideration determination is WCAP-14732.
Thus, the actual significant hazards determination provided in our November 18,1997 letter remains valid. Attachment 1 contains a corrected introduction to the significant hazards consideration determination.
If you have any further questions, or require additional information, please contact us.
Very truly yours,
/
./
J; s
M.
. Kan er Vice Prdsident - Nuclear Operations Attachment 1.
Significant Hazards Consideration Determination Commitment made in this letter:
1.
The UFSAR will be changed during a future revision to reference both the high and low pressure turbine model numbers.
cc:
U. S. Nuclear Regulatory Commission Region ll Atlanta Federal Center 61 Forsyth St., SW, Suite 23T85 Atlanta, Georgia 30303 Mr. M. J. Morgan NRC Senior Resident inspector North Anna Power Station Commissioner Bureau of Radiological Health Room 104A 1500 East Main Street Richmond, VA 23219 l
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ATTACHMENT VIRGINIA ELECTRIC AND POWER COMPANY SIGNIFICANT HAZARDS CONSIDERATION DETERMINATION NORTH ANNA UNITS 1 AND 2 i
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. SIGNIFICANT HAZARDS CONSIDERATION The Technical Specifications surveillance requirements currently require testing and inspection of the Turbine Overspeed Protection System control valves to ensure their operability to prevent overspeeding of the turbine.
The proposed change would increase the surveillance test interval for the turbine governor and throttle valves from at least once every 31 days to at least once per 92 days.
The turbine control valves have been tested monthly with no failures attributable to
. valve or control system malfunctions. In addition, operational experience has shown that the valves have not failed to' close in response to turbine trip demands nor experienced valve stem sticking while the units were carrying load.
Westinghouse Electric Corporation has performed an evaluation of the probability of generating turbine missiles as a direct function of the reduced testing frequency of the turbine governor valves (GV) and throttle valves (TV) for nuclear. plants with Westinghouse BB-296 turbines with steam chests. Design and intermediate overspeed events are not major contributors to the turbine missile probability for BB-296 turbines.
Therefore, the evaluation focused on the one overspeed event (i.e., destructive overspeed) that is affected by the test intervals of the turbine throttle and governor valves. The North Anna turbines are also of the heavy hub design which reduces the probability of turbine generated missiles from disk failure resulting from a destructive overspeed.
. It was determined by the Westinghouse probabilistic evaluation that with a twelve month l test interval for the turbine governor and throttle valves, the total turbine missile ejection probability for North Anna Units 1 and 2 continues to meet the applicable turbine system reliability acceptance criteria of 1E-5. However, Virginia Electric and Power Company is only proposing to extend the test interval to 92 days.
Virginia Electric and Power Company has reviewed the proposed Technica!
Specification changes against the requirements of 10 CFR 50.92 and has determined that the proposed changes would ~ not pose a significant hazards consideration.
.Specifically, operation of the North Anna Power Station in accordance with.the proposed Technical Specifications changes will not:
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1.
Involve a significant increase in the probability or consequences of an accident previously evaluated.
No new or unique _ accident precursors are introduced by these changes in 1
I surveillance requirements.
The probability of turbine missile ejection with an extended test interval to 92 days for the turbine governor and throttle valves has been determined to remain within the applicable NRC acceptance criteria. The I
heavy hub ' design of the turbine rotors provides further assurance that the probability of ejection of turbine missiles due to destructive overspeed remains within the acceptance criteria.
Therefore, these changes do not involve a significant increase in the probability or consequences of an accident previously evaluated.
The demonstrated high reliability of the turbine governor and throttle valves and the verification of the operability of the other turbine control valves provide adequate assurance that the turbine overspeed protection system will operate as designed, if needed. Turbine govemor and throttle valve testing performed to date has demonstrated the reliability of these valves. In addition, the operability of the other turbine valves (i.e., reheat and intercept stop valves) will continue to be verified every 18 months as required by the Technical Specifications.
2.
Create the possibility of a new or different kind of accident from any accident previous evaluated.
Since the implementation of the proposed change to the surveillance requirements will not require hardware modifications (i.e., alterations to plant configuration),
operation of the facilities with these proposed Technicsl Specifications does not create the possibility for any new or different kind of accident which has not already been evaluated in the Updated Final Safety Analysis Report (UFSAR). In addition, the. results of the probabilistic evaluation indicate that no additional transients have been introduced.
The proposed revision to the Technical Specifications will not result in any physical alteration to any plant system, nor would there be a change in the method by which any safety related system performs its function. The design and operation of the turbine overspeed protection and turbine control systems are not being changed.
.The proposed Technical Specifications changes-do not affect the design, operation, or failure modes of the valves and other components of the turbine overspeed protection system. Therefore, the proposed change does not create the possibility of a new or different kind of accident from any accident previously evaluated.
3.
Involve a significant reduction in a margin of safety.
The proposed changes do not reduce the margin of safety as defined in the basis for any Technical Specifications. Furthermore, the total turbine missile ejection probability continues to be enveloped by the applicable acceptance criteria of 1E-5.
The design and operation of the turbine overspeed protection and turbine control systems are not being changed and the operability of the turbine governor and throttle valves will be demonstrated on a refueling outage basis. In addition, the results of the accident analyses, which are documented in the UFSAR, continue to bound operation with the proposed change in surveillance interval for the turbine throttle and governor valves, so that there is no safety margin reduction. Therefore, the proposed changes do not involve a significant reduction in a margin of safety.
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