ML20199H400

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Proposed Tech Specs Permitting Quarterly Testing of Turbine Governor & Throttle Valves
ML20199H400
Person / Time
Site: North Anna  Dominion icon.png
Issue date: 11/18/1997
From:
VIRGINIA POWER (VIRGINIA ELECTRIC & POWER CO.)
To:
Shared Package
ML17339A053 List:
References
NUDOCS 9711260104
Download: ML20199H400 (18)
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PLANT SYSTEMS TURBINE OVERSPEED LIMITING CONDITION FOR OPERATION 3.7.1.7 At least one turbine overspeed ptotection system shall be OPERABLE.

APPLICABILITY; MODE 1,2 and 3 ACTIOt With the above required turbine overspeed protection system inoperable, within 6 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br /> either restore the system to OPERABLE status or isolate the turbine from the steam supply, i

SURVEILLANCE REQUIREMENTS

4.7.1.7.1 The provisions of Specification 4.0.4 are not applicable.

4.7.1.7.2 The above required turbine overspeed protection system shall be demonstrated l

OPERABLE:

l l a. By cycling each of the following valves through one complete cycle of full travel and verifying movement of each of the valves through one complete cycle by direct observation:

1. Four Turbiac Throttle valves at least once per 92 days,
2. Four Turbine Governor valves at le:.st once per 92 days, *
3. Four Turbine Reheat Stop valves at least once per 18 months, and
4. Four Turbine Reheat Intercept valves at least once per 18 months.
b. At least once per 18 months, by performance of CliANNEL CALIBRATION on the turbine overspeed protection instruments.
c. At least once per 40 months **, by disassembly of at least one of each of the above j valves and performing a visual and surface inspection of all valve seats, disks and

! stems and verifying no unacceptable flaws or corrosion. If unacceptable flaws or excessive corrosion are found, all other valves of that type shall be inspected unless the nature of the problem can be attributed to a service condition specific to that i

valve.

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  • Testing of the turbine governor valves may be suspended during end-of-cycle power coastdown operation between 835 MWe and 386 MWe,

" For reheat stop and reheat intercept valves, the inspection cycle may be increased to a maximum of once per 60 months provided there is no indication of operational distress.

NORTil ANNA - UNIT 1 v4 7-15 Amendment No '6,56,136,495

'9711260104 971118 r PDR ADOCK 05000338 l- (p PDR _

. PLANT SYSTEMS TURBINE OVERSPEED

' LIMITING CONDITION FOR OPERATION

- 3.7.1.7 - At least one turbine overspeed system shall be OPERABLE.

APPLICABIl ITY: MODE 1,2 and 3 ACTION:

With the above required turbine overspeed protection system inoperald.c within 6 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br /> either restore the system to OPERABLE status or isolate the turbine from the steam supply.

SURVEILLANCE REQUIREMENT 4.7.1.7.1 The provisions of Specification 4.0.4 are not applicable.

4.7.1.7.2 The above required turbine overspeed protection system shall be demonstrated OPERABLE:

a. By cycling each of the following valves through one complete cycle of full travel and verifying movement of each of the valves through one complete cycle by direct obsenation:
1. Four Turbine Throttle valves at least once per 92 days,
2. Four Turbine Governor valves at least once per 92 days, *
3. Four Turbine Reheat Stop valves at least once per 18 months, and
4. Four Turbine Reheat Intercept valves at least once per 18 months.
b. At least once per 18 months, by performance of CHANNEL CALIBRATION on the turbine overspeed protection instruments.
c. At least once per 40 months **. by disassembly of at least one of each of the above valves and performing a visual and surface inspection of all valve seats, disks and stems and verifying no unacceptable flaws or corrosion if unacceptable flaws or excessive corrosion are found, all other valves of that type shall be inspected unless the nature of the problem can be attributed to a service condition specific to that valve.
  • Testing of the turbine governor valves may be suspended during end-of-cycle power coastdown operation between 835 MWe and 386 MWe.
    • For reheat stop and reheat intercept valves, the inspection cycle may be increased to a maximum of once per 60 months provided there is no indication of operational distress.

- NORTH ANNA - UNIT 2 3/4712 Amendment No. 38, ? !, ! 19.-4-74

i ATTACHMENT 3 f

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I VIRGINIA ELECTRIC AND POWER COMPANY SIGNIFICANT HAZARDS CONSIDERATION

!. NORTH ANNA UNITS 1 AND 2

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 overspoeding of the turbine. The proposed change would increase the surveillance test interval for the turbine govemor 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 govemor valves (GV) and throttle valves (TV) for nuclear plants with Westinghouse B8-296 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 fo'used on the one overspeed event (i.e., destructive overspeed) that is affected by the test intervals of the turbine throttle and governor valves. The Nonh 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.

i it was determined by the Westinghouse probabilistic evaluation that with an eighteen month 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 Technical Specification changes agains' 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 surveillance requirements. The probability of turbine missile ejection with an extended test interval to 92 days for the turbine govemor and throttle valves has been determined to remain within the applicable NRC acceptance criteria. The 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 govemor and throttle valves and the verification of the operability of the other turbine control valves provide adequate assurance that the turbine overspced 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 previcus 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 Technical 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

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which any safety-related system performs its function. The design and operation Page 2 of 3

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 orveloped 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 govemor 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 govemor valves, so that there is no safety margin reduction. Therefore, the proposed change does not involve a significant reduction in a margin of safety.

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1 ATTACHMENT 5 WESTINGHOUSE ELECTRIC COMPANY -

IDENTIFICATION OF PROPRIETARY !NFORMATION PER NUREG-0390 l

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l VIRGINIA ELECTRIC AND POWER COMPANY NORTH ANNA UNITS 1 AND 2 PROPOSED TECHNICAL SPECIFICATION CHANGES I

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Discussion of Changs lDitoduction Pursuant to 10 CFR 50.90, Virginia Electric and Power Company requests changes to Technical Specifications Surveillance Requirements 4.7.1.7.2.a.1 and 4.7.1.7.2.a.2 for North Anna Power Station Units 1 and 2. The Technical Specifications surveillance requirements currently require testing and inspection of the Turbine Overspeed Protection System control valves to ensure their ability to prevent overspeeding of the

- turbine. Based on an analysis of _ Westinghouse BB 296 turbines with steam chests, the proposed change would increase the surveillance test interval for the turbine governor and throttle valves from "at least once per 31 days" to *at least once per 92 days." The disassembly inspection intervals for these valves will continue to be in accordance with

the current Technical Specifications surveillance requirements.

The proposed changes to the North Anna Units 1 and 2 Technical Specifications will not adversely affect the safe operation of the plant or result in an unreviewed safety question as defined by the criteria of 10 CFR 50.59.

Rashground Current Licensina Basis The original tecbine overspeed protection system testing requirements were issued as part of the original operating licenses for North Anna Units 1 and 2 dated April 1,1978 and August 21,1980, respectively. The initial testing frequency of once per 7 days was based on the manufacturers instruction book for the operation and control of the North Anna Power Station Westinghouse steam turbines. By letter dated April 16,1984, the NRC amended the North Anna Units 1 and 2 Technical Specifications (Amendment Nos. 56 and 38) allowing the turbine valves to be demonstrated operable by testing the valves once per 31 days instead of once per 7 days. The revised surveillance requirement was based on pas, demonstrated reliability and performance of thu turbine overspeed protection system, a viable ongoing maintenance inspection, and tiio turbine valve test program coupled with an in-place inspection program for the low pressure turbine discs. During plant licensing turbine missiles and the probability of (lamage to equipment that could cause core damage were evaluated by the NRC and the NRC Page 1 of 10

1 estimated the probability for unacceptable damage by a turbine missile (P 4) to be about 2.0E 5 per turbine year.

l The current turbine missile analysis documented in the UFSAR establishes the probability for unacceptable damage by a turbine missile (P-M as 1.1E 6 and 8.67E-7 per turbine year for Units 1 and 2, respectively.

The test and inspection frequency for the turbine intercept and reheat stop valves was recently revised for Nor1h Anna Units 1 and 2 (Amendment Nos.195 and 176 issued February 8,1996). The basis for the revised testing frequencies was a Westinghouse evaluation of the probability of turbine missiles based on reduced testing frequency.

The evaluation focused on the two overspeed events (i.e., design and intermediate overspoed) that are affected by the test intervals of the reheat stop and intercept valves. For the current 18 month Technical Specification test frequency, the probability of turbine missile generation (P 1) due to interceptor and reheat stop valves is 1.24E 7.

CurrentDnjgr1Dmla The turbine overspcod protection system functions to prevent overspeeding the turbine-generator unit. As part of the turbine overspeed protection system, the control valves function to isolate the high pressure and low pressure turbines from the steam supply.

There are four types of turbine control valves: the turbine governor valves, the turbine throttle valves, the turbine reheat stop valves, and the turbine reheat intercept valves.

There are four of each type of these valves per unit. The governor valves and throttle valves control steam flow to the high pressure turbines and the reheat stop and intercept valves control steam flow to the low pressure turbines.

A turbine overspeed condition significantly increases the probability of turbine missile generation relative to operation at normal speed due to the increased stress in the turbine rotor at higher operating speeds. The turbine overspeed protection system is described in Section 10.2 of the North Anna Power Station Updated Final Safety Analysis Report.

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QingusMon Westinghouse Electric Corporation has performed an evaluation of the probability of generating turbine miasiles as a direct function of the reduced testing frequency of the turbine governor valves (GV) and throttle valves (TV) for nuclear plants with Westinghouse DB 296 turbines with steam chests. Design and intermediate overspeed events are not major contributors to the turbine e,hetion probability for BB 296 turbines.

Therefore, the evaluation focused on the one overspeed event (i.e., destructive overspeed) that is affected by the tes; intervals of the turbine throttle and governoi valves. lne North Anna turbinos are also of the heavy hub design which reduces the probability of turbine generated mitsiles from disk failure resulting from a destructive overspeed. The results of the evaluation are discussed in tne attached Westinghouse WCAP 14733, "Probabilistic Analysis of Reduction in Turbine Valve Test Frequency for Nuclear Plants with BB 296 Turbines with Steam Chests." The Westinghouse evaluation calculates the effects of extending the test interval for the turbine governor and throttle valves up to 18 months using the fault tree models and methodology from the Westinghouse report WCAP 11525, "Probabilistic Evaluation of Reduction in Turbine Valve Test Frequency," dated June 1987. In a supplemental safety evaluation issued in their letter to Westinghouse Electric Corporation dated November 2,1989, the NRC accepted this methodology for use in the determination of the probability of turbinc missile generation.

The North Anna turbine governor and throttle 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 vaive stem sticking while the units were carrying load. This high level of reliability is maintained, in part, by the volatile chemical treatment of the feedwater system which essentially eliminates valve failure due to scale buildup on the valvo surfaces.

The proposed Technical Specifications changes are based upon past operational experience, Westinghouse recommendations for inspection intervals, and the results of the Westinghouse evaluation report (WCAP 14733) included with this submittal.

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i Evaluation of Postulated Turbine Missiles '

The turbine overspeed protection specification is provided to ensure that the turbine is i protected from destructive overspeed. Protection from excessive overspeed is required since excessive overspeed of the turbine could generate potentially damaging missiles that could impact and damage safety related components, equipment, or structures.

! The North Anna Updated Final Safety Analysis Report (UFSAR) Section 10.2.1 j describes the evaluation of postulated turbine missiles. Both the probability of missile ejection and the probability of impact to safety related items were considered in the UFSAR as described below.

The probability (P4) of damage to plant structures, systems, and components j important to safety is:

! P4 = P1 x?2 x P3 i where:

P1 = the probability of generation and ejection of a high energy missile, P2 = the probability that a missile strikes a critical plant region, given its generation and ejection, and P3 = the probability that the missile strike damages its target in a manner leading to unacceptable consequences. Unacceptable consequences are defined here as the loss of the capacity to maintain the integrity of the reactor coolant pressure boundary, to shut down the plant, maintain it in a safe shutdown condition, and/or j limit offsite radiation exposures.

The proposed change to the North Anna Technical Specifications does not increase the probability of damage to any plant structures, systems, or components as currently evaluated in the North Anna UFSAR. The current turbine missile analysis documented i in the UFSAR establishes the probability for unacceptable damage by a turbine missile (P 4) as 1.1E 6 and 8.666E 7 per turbine year for Units 1 and 2, respectively. The total probability of turbine missiles (P-1) for proposed quarterly testing of the turbine i

govemor and throttle valves is 8.8E 7.

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i l l Evaluation of Acceptance Criteria j i

The NRC staff considers 1 x 10 7 per year an acceptable risk rate for the loss of an

essential system from a single event (reference NRC Regulatory Guide 1.115,

" Protection Against Low Trajectory Tuibine Missiles"). Thus, the probability (P4) of ,

damage to plant structures, systems, and components important to safety is an j acceptable risk for values less than or equal to 1 x 10 7 per year.  ;

J The NRC staff also accepts a value of 1 x 10 2 for the combined probabilities of strike

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and damage (P2 x P3) for an unfavorably oriented turbine generator. (Reference NRC <

letter from C. E. Rossi to J. A. Martin of Westinghouse Electric Corporation, " Approval for Referencing of Licensing Topical Reports WSTG 1 P, WSTG 2 P, and WSTG 3 P "

dated February 2,1987.)

Based on an evaluation of the probability of damage to plant structures, systems, and components important to safety (P4 = P1 x P2 x P3) considering the acceptance of the values described above for (P4) and (P2 x P3), a general acceptance criterion for probability of turbine missile ejection (P1) can reasonably be established as 1 x 104 per year.

Eighability of MissileDction Resultlag from Extending Test Interval Three overspeed events were considered in evaluating the probability for missile ejection: design overspeed (120 percent of rated speed), intermediate overspeed (132 percent of rated speed), and destructive overspeed. The missile ejection frequency results in WCAP 11525, for BB 296 steam chests, indicate that the design and intermediate overspeed failure probabilities are not major contributors to turbine missile ejection probability for BB 296 turbines. Therefore, this evaluation focused- on establishing the probability of destructive overspeed. Destructive overspeed is

assumed if there is a system separation and the failure to isolate one of the four steam paths to the high pressure turbine.

The Westinghouse evaluation, using updated failure and operating data, presents the probability of turbine missile ejection for the destructive overspeed event as ( ) per year with a quarterly test interval. An allowance of 1.0E 6 is added to account for the probability of overspeed from the design and intermediate overspeed events and any Page 5 of 10

i model uncertainties (as noted above the P-1 for the reheat and inte:w mtn is 1.246 ,

7). The total probability of turbine missile ejection is based r- wnd!"cw p MIEm of missile ejection given the' destructive overspeed occ: . Dv Mr% neuvd  ;

event occurs if there is a system separation (genera % L(9 h 6 , f,1 total probability for a missile ejection must be multiplied by the mmp WW i%ency of system separation for the unit so that they can be mearured against the acceptance i

criteria. Westinghouse established the system separation frequency to be 0.29 and conservatively assumed the bounding annual frequency of system separation to be 0.4.

i Therefore, the total probability of a missile ejection for destructive overopeed is as follows:

[ ] Destructive overspeed events

[ 1 Design and intermediate overspoed events

[ ]-

x_M (annual system separations)

=[ ] per year Total probability of missile ejection (P1)

Conclusion:

The risk associated with quarterly testing is acceptable, maintained within the regulatory acceptance criteria and the UFSAR.

Operating and Testina Experiencs .

P Operating experience and tesang at North Anna Units 1 and 2 were also considered in support of these proposed Technical Specifications changes. There have been no incidents of turbine g'overnor or throttle valve stem sticking while the units were carrying load.

Industry Experience associated with Turbine Overspand The proposed Technical Specification changes were evaluated against industry experience associated with turbine overspeed events to determine if these changes could contribute to an increase in the probability of a turbine overspeed. The items reviewed included INPO Significant Event Repor17 92, ' Turbine Failure Caused By Overspeed" (Salem Unit 2 event of November 9,1991), and NUREG 1275, ' Operating .

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Experience Feedback Report Turbine Generator Overspeed Protection Systems,"

dated April,1995.

This review ,adicated that the primary factors contributing to turt,ine overspeed events .;

were inadequate preventative and corrective maintenance on the Turbine Overspeed Protection System (TOPS) and system components and human factor deficiencies in  !

performing TOPS surveillance testing. None of the referenced overspeed events were l

associated with the failure of turbine control valves (throttle, govemor, reheat stop, and reheat intercept valves) to cycle closed when required to do so because of valve stem -
binding or valve failure.  !
The proposed change to the surveillance interval of the turbine throttle and govemor l valves will extend the freedom of movement test from at least once per 31 days to at '

least once per 92 days.

Th9 proposed change to the surveillance interval for the turbine throttle and governor valves does not modify the physical testing methods or procedures of any portion of the Turbine Overspeed Protection System. Therefore, it has been determined thrit this proposed change will not increase the probability of a turbine overspeed event of the

type evaluated in the above documents.

Conclusion The results of plant operating experience and testing as well as the manufacturars' recommendations do not preclude support of reduced frequency testing of the turbine

, throttle and govemor valves. Together with the favorable turbine missile evaluation discussed above indicate that the turbine system reliability is acceptable with a 92 day test interval for tho turbine throttle and govemor valves. Therefore, the proposed Technical Specification changes will not reduce the margin of safety of the plant.

i Specific Changen The following Technical Specifications change applies to both Units 1 and 2:

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Sality Significant [

Virginia Electric and Power Company has reviewed the proposed Technical Specifications changes and determined that the chenges would not involve an

, unreviewed safety question.

1. The proposed changes do not increase the probability of occurrence or the

! consequences of an acciderst or malfunction of equipment important to safety  !

previously evaluated in the safety analysis report. The proposed changes have no adverse impact upon probability or consequences of any accident previously evaluated. Furthermore, the proposed change does not increase the probability of the generation of missiles or the probability of damage to any plant system,  !

-structure, or component currently evaluated in the UFSAR. Only the surveillance interval for cycling the turbine governor and throttle valves is changed. No new or unique accident precursors are introduced by thic change in surveillance l requirements.

4 The heavy hub design of the turbine rotors provides fur 1her assurance that the probability of the ejection of destructive missiles remains minimal. Based upon the ,

results of the probabilistic risk assessment, the probability of a turbine generated ,

t missile is less than 10 5 per year which the Commission has endorsed as the acceptable level for turbine operation.

Turbine governor 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 stop and intercept valves) will continue to be verified once every 18 months required by the Technical Specifications. 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 Page 8 of 10

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overspeed protection system will operate as designed, if needed. Therefore, the I proposed change does not involve a significant increase in the probability of any -  !

accident previously evaluated.  ;

, Plant design and operations have not changed. Therefore the consequence of accident previously evalue.ted will not increase.

2. The proposed changes would not create the possibility for an accident or  ;

malfunction of a different type than any evaluated previously in the safety analysis  :

repost. The Technical Specifications will continue to require testing and Inspection of the turbine governor and throttle valves to ensure operability of the turbine overspeed protection system. The turbine and the turbine control system will

continue to be operated in the same manner. The Westinghouse evaluation j i supports increasing the surveillance interval for cycling the turbine throttle and governor valves.

l Operating experience at North Anna Units 1 and 2 support the proposed Technical Specifications changes. There have been no incidents of turbine govemor and  ;

throttle valve stem sticking while the units were carrying load.

. The proposed revision to the Technical Specifications will not change 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. Therefore, the proposed change does not create the possibility of a new or different kind of accident from any accident previously evaluated.

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3. The proposed change will not reduce the margin of safety as defined in the basis for any Technical Specifications. The design and operation of the turbine overspeed protection and turbine control systems are not being changed. The probabilistic evaluation performed by Westinghouse, with the updated turbine .

operating experience, has established that thero is no increats in the overall probabilities of turbine overspeed or turbine missile generation (P 1) for quarterly valve testing above that currently evaluated in the UFSAR The results of the accident analyses, which are documented in the UFSAR, continue to bound operation with the proposed testing frequency. Therefore, the proposed change does not involve a reduction in a margin of safety.

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i Based on the above evaluation, the proposed changes to the Technical Specifications will not adversely affect the safe operation of the plant. Therefore, the proposed change request for North Anna Units 1 and 2 does not result in an unroviewed safety question as defined in the criteria cf 10 CFR 50.59. ,

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NON PROPRIETARY l l

Westinghouse Evaluation Report WCAP-14733, Revision 1, June 1997 l

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Probabilistic Analysis of Roduction in Turbine Valve Test Frequency for Nuclear Plants with Westinghouse BB 296 Turbines with Steam Chests i

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