ML20248E036
| ML20248E036 | |
| Person / Time | |
|---|---|
| Site: | Byron, Braidwood  |
| Issue date: | 05/26/1998 |
| From: | NRC (Affiliation Not Assigned) |
| To: | |
| Shared Package | |
| ML20248E027 | List: |
| References | |
| NUDOCS 9806030166 | |
| Download: ML20248E036 (4) | |
Text
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UNITED STATES g
,j NUCLEAR REGULATORY COMMISSION e
WASHINGTON, D.C. 30666 0001 o
+9.....,o SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION RELATED TO AMENDMENT NO.103 TO FACILITY OPERATING L) CENSE NO. NPF-37.
AMENDMENT NO.103 TO FACILITY OPERATING LICENSE NO. NPF 66.
J AMENDMENT NO. 93 TO FACILITY OPERATING LICENSE NO. NPE-72, AND AMENDMENT NO. 93 TO FACILITY OPERATING LICENSE NO. NPF-77 COMMONWEALTH EDISON COMPANY BYRON STATION. UNIT NOS.1 AND 2 BRAIDWOOD STATION. UNIT NOS 1 AND R DOCKET NOS. STN 50-454. STN 50-455. STN 50-456 AND STN 50-457
1.0 INTRODUCTION
By letter dated September 24,1997, Commonwealth Edison Company (Comed, the licensee) proposed changes to the Technical Specifications (TSs) for Byron Station, UnXs 1 and 2, and Braidwood Station, Units 1 and 2. The proposed changes revise surveillance requirements (SRs) related to the turbine overspeed protection system. On the basis of past operational j
experience, Westinghouse Electric Corporation's (Westinghouse) recommendations for inspection intervals, and the results of the Westinghouse evaluation proprietary report WCAP-14732 (nonproprietary version is WCAP-14733), dated June 1997, the licensee proposes to I
increase the surveillance test interval for the turbine govemor valves and turbine throttle valves from "at least once per 31 days" to "at least once per 92 days." The disassembly and inspection 1
intervals for these valves will continue to be in accordance with the current TS SRs.
2.0 BACKGROUND
Byron, Units 1 and 2, and Braidwood, Units 1 and 2, are each equipped with Westinghouse BB-296 turbine generators with steam chests. The Westinghouse turbines are conventional 1800-rpm, tandem-compound units consisting of one double-flow, high-pressure cylinder and three double-flow, low-pressure cylinders. The turbines are provided with two moisture separator reheaters located between the high-pressure and the low-pressure cylinders.
Each high-pressure steamline to the high-pressure cylinder contains a throttle valve and a govemor valve. A reheat stop valve and an intercept valve are provided in the crossover piping between each moisture separator reheater and the low-pressure turbine cylinders. The function of these valves is to control and limit the turbine speed and, in case of a loss ofload, stop steam supply to the turbine.
The turbine generator system is equipped with overspeed protection to minimize the probability of turbine missiles and meet the requirements of General Design Criterion (GDC) 4,
" Environmental and Dynamic Effects Design Bases." To demonstrate the operability of the turbine overspeed protection system, TS 4.3.4.2.a currently requires that the four turbine throttle 9906030166 990526 PDR ADOCK 05000454 P
, valves, the four turbine govemor valves, the six turbine reheat stop valves and 'the six turbine reheat intercept valves be tested every 31 days.
The licensee proposes that the throttle valves and the govemor valves be demonstrated operable by testing the valves once every 92 days instead of once every 31 days. NUREG-1366,
" improvements to Technical Specifications Surveillance Requirements," dated December 1992, identified turbine throttle valve and govemor valve testing as an example where plant safety could be improved by decreasing the frequency of testing. NUREG-1366 recommends that where the turbine manufacturer agrees, the turbine valve testing frequency should be changed to l
quarterly. The guidance for implementing the recommendations in NUREG-1366 is provided in
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Generic Letter (GL) 93-05, "Line-Item Technical Specifications improvements to Reduce Survei!!ance Requirements for Testing During Power Operation." The reheat stop valves and the reheat intercept valves would continue to be tested every 31 days.
I Comed proposes to modify TS SR 4.3.4.2.a by deleting the turbine throttle valves and govemor valves and re-numbering the remaining items. TS SR 4.3.4.2.b will be added to include the throttle and govemor valve testing at the new frequency of 92 days, and to also require direct observation of the movement of the valves through one complete cycle from the running position.
The remaining surveillance requirements in TS SR 4.3.4.2 will be re-designated c, d, e, and f.
Other associated administrative changes will also be made. Finally, Comed proposes to change the Bases to refer to the most recent Westinghouse guidance, " Operation and Maintenance Memo 093."
The licensee stated that, since plant modifications were completed on Braidwood in the October 1989 and spring 1990 refueling outages and on Byron in the fall 1988 and spring 1989 outages, the turbine govemor and throttle valves have been tested monthly with no failures. In addition, operational experience has shown no failures of these valves to close in response to turt>ine trip demands.
To establish the effects of turbine valve testing frequency, Westinghouse performed an evaluation of the probability of generating turbine missiles as a direct function of reducing the testing frequency of the turbine govemor valves and throttle valves. The results are discussed in Westinghouse report WCAP-14732, "Probabilistic Analysis of Reduction in Turbine Valve Test Frequency for Nuclear Plants with Westinghouse BB-296 Turbines with Steam Chests," dated June 1997.
The Ws;tinghouse evaluation focusses on the probability of turbine missile ejection due to destructive overspeed (runaway speed in excess of approximately 18G percent). ' Destructive overspeed is assumed to occur when at least one govemor valve and throttle valve in the same steam chest fail to close after a system separation (sudden and total loss of load on the generator, such as the load loss that is experienced if the generator output breakers opened whi!e the plant is at full power). Dodgn overspeed (approximately 120 percent of rated turbine speed) and intermediate overspeed (approximately 130 percent) were not explicitly calculated because previous evaluations have indicated that they were not major contributors to turbine missile ejection probability for BB-296 turbines (Westinghouse report WCAP-11525, "Probabilistic Evaluation of Reduction in Turbine Valve Test Frequency," 1987). The turbine missile ejection frequencies in WCAP-14732 were calculated following the same basic methodology as is described in WCAP-11525. In a supplemental safety evaluation dated November 2,1989, issued to Westinghouse, the NRC staff accepted this methodology for use in the determination of the probability of turbine missile generation.
. WCAP-14732 used BB-296 failure rates for turbine govemor and throttle valves based on plant operating experience over a data collection period from 1990 through and including 1995. This time period provided failure rates based on current valve design and maintenance practices while retaining adeounte time for rare events to occur. Westinghouse added an allowance to cover -
any model uncertainties and to secount for the probability of missile ejection from design and intermediate overspeed events. The destructive overspeed model was constructed assuming that a loss of load or system separation occurrod. The frequency of system separation was calculated to be 0.29 per year; however, a more conservative value of 0.4 per year was used in the analysis. The conditiona.1 probability of missiie ejection (e.g., the probability of valve failures) was then multiplied by the frequency of system separation to obtain the probability of missile ejection per year from destructive overspeed. The probability of turbine missile ejection che to l
destructive overspeed was calculated for turbine valve test intervals of 1 week,1 month, 3 months,6 months, and 12 months.
3.0 EVALUATION For determining maintenance and testing schedules for turbine control and overspeed protection systems, the NRC staff recommended that the annual probability of turbine missile ejection not exceed 1.0E-5 a year for unfavorably oriented turbines and 1.0E-4 for favorably oriented turbines.
Byron and Braidwood turbines are unfavorably oriented. For all test intervals analyzed (i.e.,
1 week,1 month,3 months,6 months, and 12 months), the missile ejection frequency from destructive overspeed met the acceptance criteria of 1.0E-5 a year. However, since the govemor and throttle valve failure rates are based on plant operating experience (primarily l
monthly testing), sufficient failure information for longer test intervals does not currently exist.
Westinghouse supports quarterly testing until reasonable failure rate data can be accumulated.
For quarterly testing, the total probability of turbine missile ejection from destructive overspeed was determined to be 8.8E-7 per year.
The staff found that safety could be improved, equipment degradation decreased, and an l
unnecessary burden on personnel eliminated by reducing the amount of testing that the TSs j
require during power operation. GL 93-05 provided guidance to implement these l
recommendations as line-item TS improvements. These line-item TS improvements are reported in NUREG-1366. The proposed changes to the TS are in accordance with the above guidance.
Section 5.13 of NUREG-1366 provides a comprehensive evaluation of Turbine Overspeed Protection System Testing and contains NRC recommendations for the frequency of testing of l
l turbine valves. NUREG-1366 recommends that where the turbine manufacturer agrees, the l
l turbine valves testing frequency should be changed to one test done quarterly (i.e., the j
surveillance interval could be extended to up to 3 months if such a change is supported by the
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turbine manufacturer's generic data and the licensee follows the manufacturer's methodology j
using plant-specific data to justify the new test frequency). Westinghouse report WCAP-14732 l
recommended that all plants with BB-296 nuclear turbines with steam chests, such as the j
turbines at Byron and Braidwood, change the survs;llance frequency to quarterly.
Section 10.2 of the Standard Review Plan (SRP), NUREG-0800, provides guidance on evaluating the surveillance testing of steam valves. The purpose of the guidance is to ensure that the turbine overspeed protection system will perform in a manner that meets the requirements of GDC 4 of Appendix A to 10 CFR Part 50 with regard to the protection of structures, systems, and components import $nt to safety from the effects of turbine missiles.
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4 With quarterly testing of the turbine throttle valves and govemor valves, the total probability of tuitine missile ejection from destructive overspeed was determined to be 8.8E-7 per year. This is within the NRC guidance of 1.0E-5 per year for unfavorably oriented turbines.
4.0 SUMM E(
The proposed TS amendment to increase the surveillance test interval for the turbine govemor and throttle valves from "at least once every 31 days" to "at least once every 92 days' was found to be within the boundary of the guidance provided in NUREG-1366. The amendment complies with the requirements of GDC 4 of Appsndix A to 10 CFR Part 50 and the intent of the guidance of Section 10.2 of 'tw CRP with regard to the protection of structures, systems, and components important to safek from the effects of turbine missiles. Since plant modifications were completed, the turbine govemor and throttle valves have not failed to close in response to turbine trip demands or surveillance tests. Furthermore, the probability of turbine-generated missiles is within NRC limits. On the basis of the staff's evaluation of the licensee's amendmeret request, the ptoposed changes to the surveillance requirements for the turbine overspeed protection system are acceptable.
5.0 STATE CONSULTATION
in accordance with the Commission's regulations, the lilinois State official was notified of the proposed issuance of the amendments. The State official had no comments.
6.0 P, ENVIRONMENTAL CONSIDERATION The amendments change a surveillance requirement. The NRC staff has determined that the amendments involve no significant increase in the amounts, and no significant change in the types, of any effluents that may be reieased 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 amendments involve no significant hazards consideration, and there has been no public comment on such finding (63FR11917). Accordingly, the amendments meet 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 amendments.
7.0 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 conduc%d in compliance with the Commission's regulations, and (3) the issuance of the amendments will not be inimical to the common defense and security or to the health and safety of the public.
Principal Contributor: S. Bailey Date:
May 26, 1996 t
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