ML20044G361
| ML20044G361 | |
| Person / Time | |
|---|---|
| Site: | South Texas  |
| Issue date: | 05/18/1993 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML20044G357 | List: |
| References | |
| NUDOCS 9306020400 | |
| Download: ML20044G361 (4) | |
Text
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SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION RELATED TO THE TURBINE MAINTENANCE PROGRAM HOUSTON LIGHTING & POWER COMPANY CITY PUBLIC SERVICE BOARD OF SAN ANTONIO CENTRAL POWER AND LIGHT COMPANY CITY OF AUSTIN. TEXAS DOCKET NOS. 50-498 AND 50-499 l
SOUTH TEXAS PROJECT. UNITS 1 AND 2
1.0 INTRODUCTION
In Section 3.5.1.3 of the Safety Evaluation Report related to the operation of South Texas Project, Units 1 and 2 (NUREG-0781), the staff concluded that the probabilit, of unacceptable damage to safety-related systems and components by turbine missile generation is acceptably low provided the turbine missile generation probability is maintained at 1.0E-4 or less by an acceptable maintenance program.
In a letter dated October 28, 1992, the licensee submitted for NRC approval a revised turbine maintenance program based on the manufacturer's calculations of missile generation probabilities (Refs. 1, 4, and 5).
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2.0 BACKGROUND
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General Design Criterion 4, " Environmental and Missile Design Bases," of Appendix A to 10 CFR Part 50 requires that structures, systems, and components
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important to safety are protected against the effects of missiles that might j
result from such failures.
j In the past, the staff evaluation of the effects of turbine missiles on the l
public health and safety followed Regulatory Guide (RG) 1.115, " Protection Against Low-Trajectory Turbine Missiles," and Standard keview Plan (SRP),
2.2.3, 10.2, 10.2.3, and 3.5.1.3.
According to SRP 2.2.3 and RG 1.115, the probability of unacceptable damage from tutine missiles should be less than or equal to 1.0E-6 per year for an individu 1 plant. This probability is the product of (1) the probability of turbine failure resulting in the ejection of turbine disc fragments through the turbine casing; (2) the probability of ejected missiles that penetrate intervening barriers and strike safety-related structures, systems, or components; and (3) the probability of struck structures, systems, or components failing to perform their safety function.
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- The staff has shifted review emphasis from mitigation of the effects of turbine missiles to the prevention of missile generation.
Licensees are required to show that the turbine missile generation probability satisfies turbine reliability requirements criteria. For the favorable turbine orientation at South Texas, the turbine missile generation probability should be less than 1.0E-4 per year. This is the general minimum reliability requirement for loading the turbine and bringing it on line.
If the missile generation probability is greater than 1.0E-4 per year, then the NRC requires that the licensee take certain actions to reduce the probability (Table 1 and Ref. 4).
In order to assure that the licensee's turbine missile probabilities satisfy q
the NRC turbine reliability requirement, the staff requires licensees to submit, within 3 years of operation, a turbine maintenance program that includes maintenance activities and inspection intervals, which are based on the manufacturer's calculations of turbine missile generation probabilities.
3.0 EVALUATION Each main turbine at South Texas was manufactured by Westinghouse and consists of a high pressure turbine, three low pressure turbines (LP1, LP2, and LP3),
and a spare low pressure turbine. The licensee's schedule calls for inspection of the Unit I turbine rotors every 60 months, 22 months, and 45 months of operation for turbines LP1, LP2, and LP3, respectively.
For Unit 2, the inspection schedule is 53 months, 47 months, and 17 months for turbines LP1, LP2, and LP3, respectively. The intervals were calculated based on the Westinghouse method (Ref. 1) which has baen approved by the NRC staff (Ref. 4). The probability calculation showed that with the above inspection schedule, the probability of missile generation for each low pressure turbine will be less than or equal to 3.33E-5 per year. This satisfies 1.0E-4 per i
year specified in SER Section 3.5.1.3.
The licensee's turbine maintenance
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program also requires that the turbine rotor assembly be subjected to a full-non-destructive examination accordir3 to the above schedule.
Maintenance of the turbine overspeed protection system is also a major part of the overall program (Refs. 3 and 5).
In accordance with Technical Specification 3/4.3.4.2, the licensee tests the turbine valves (stop, governor, reheat stop, and intercept valves) once per 31 days in Modes 1 and 2 to verify operability. One of each type of these valves is disassembled and inspected at least once per 40 months. The licensee also does surface and visual inspection on valve seats, discs, and stems.
If unacceptable flaws or excessive corrosion are found, all other valves of that type will be i
inspected. The electrical overspeed protection device is calibrated at least once every 18 months and the mechanical overspeed trip is tested following each major turbine outage (Ref. 5).
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4.0 CONCLUSION
The staff concludes that the turbine maintenanca program at South Texas Units 1 and 2 is acceptable because the licensee's turbine missile generation probability satisfies the staff's requirements of 1.0E-4 per year for a favorable orientation. The maintenance program of the turbine overspeed 2
protection system (Refs. 3 and 5) satisfies the surveillance requirements as specified in Technical Specification 3/4.3.4.2.
5.0 REFERENCES
1.
" Analysis of the Probability of the Generation and Strike of Missiles from a Nuclear Turbine," Westinghouse Steam Turbine Generation Division, March 1974.
e 2.
" Missile Energy Analysis Methods for Nuclear Steam Turbines," Westinghouse Steam Turbine Generator Division, WSTG-2-P, May 1981.
(Proprietary) 3.
" Analysis of the Probability of a Nuclear Turbine Reaching Destructive Overspeed," Westinghouse Steam Turbine Generator Division, WSTG-3-P, July 1984.
(Proprietary) l 4.
USNRC correspondence from C.E. Rossi to J.A. Martin, Safety Evaluation i
Report February 2,1987, Approval for Referencing of Licensing Topical Reports: March 1974 Report; WSTG-2-P, May 1981; and WSTG-3-P, July 1984.
5.
" Revised Turbine System Maintenance Program," Houston Lighting & Power Company, ST-HL-AE-4183, October 28, 1992.
Attachc.ent:
Table i
Principal Contributor: M. Griggs Date: May 18, 1993 4
5
Table 1.
Turbine System Reliability Criteria" Probability, yr
Favorably Unfavorably Oriented Oriented Turbine Turbine Required Licensee Action (A) P3< 10
P, < 10'3 This is the general, minimum reliability requirement for loading the turbine and bringing the system on line.
(B) 10 < P3< 10'3 10'5 < P3 < 10
If this condition is reached during operation, the turbine may be kept in service until the next scheduled outage, at which time the licensee is to take action to reduce P meet the appropriate criterio, to n
before returning the turbine to service.
(C) 10'3 < P < 10'2 10 < P, < 10'3 If this condition is reached during 3
operation, the turbine may be isolated from the steam supply within 60 days, at which time the licensee is to take action to reduce P, fore returning the turbine to to meet the appropriate criterion be i
service.
(D) 10'2 < P, 10'3 < P If this condition is reached at any 3
time during operation, the turbine is to be isolated from the steam supply within 6 days, at which time the licensee is to take action to reduce P to meet the appropriate criterio,n before returning the turbine to service.
i Taken from Reference 4.
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