ML20246J835

Safety Evaluation Re Low Pressure Turbine Maint Program. Program Acceptable

ML20246J835
Person / Time
Site:	Limerick
Issue date:	05/09/1989
From:	Office of Nuclear Reactor Regulation
To:
Shared Package
ML20246J821	List: ML20246J816 ML20246J835
References
NUDOCS 8905170180
Download: ML20246J835 (3)

Text

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4 ENCLOSURE SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION 4

LOW PRESSURE TURBINE SYSTEM MAINTENANCE PROGRAM PHILADELPHIA ELECTRIC COMPANY LIMERICK GENERATING STATION UNIT 2 DOCKET NO. 50-353

1. 0 INTRODUCTION fly letter dated August 19, 1988 Philadelphia Electric Company submitted the Limerick Unit 2 low pressure turbine maintenance program to satisfy the requirement in section 3.5.1.3.3 of the Limerick Unit 2 Safety Evaluation Report, NUREG-0991.

Although large steam turbines and their auxiliaries are not safety related systems as definea by NRC regulations, failures that occur in these turbines can produce large, high energy missiles. General Design Criterion 4, " Environmental and Missile Design Bases," of Appendix A to 10 CFR Part 50 requires, in part, that structures, systems, and components important to safety be appropriately protected against the effects of missiles that might result from such failures.

In the past, evaluation of the effects of turbine failure on the public health and safety followed Regulatory Guide 1.115, " Protection Against Low-Trajectory Turbine Missiles," and three essentially independent Standard Review Plan (SRP) sections,10.2 " Turbine Generator," 10.2.3 " Turbine Disk Integrity," and 3.5.1.3

" Turbine Missiles." According to NRC guidelines stated in section 2.2.3 of the SRP and Regulatory Guide 1.115, the probability of unacceptable damage from tur-bine missiles (P4) should be less than or equal to about I chance in 10 million per year for an individual plant. The probability of unacceptable damage result-ing from turbine missiles is generally expressed as the product of (1) the prob-ability of turbine failure resulting in the ejection of turbine disc (or internal structure) fragments through the turbine casing (P 3

); (2) the probability of ejected missiles perforating intervening barriers and striking safety-related structures, systems, or components 2(P ); and (3) the probability of struck structures, systems, or components failing to perform their safety function (P3 )*

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A L_______________--__-__ _ _ _ - _ - . __ _ _ __

In recent years, the staff has shifted review emphasis from P and P to P . I 2 3 7 Applicants and licensees are required to show that the turbine missile generation probability, P3 , satisfies turbine reliability requirements criteria. For an unfavorably oriented turbine, such as Limerick Unit 2 turbines, P y is required to be less than 10-5 per year. This is the general, minimum reliability require-ment (Criterion A) for loading the turbine and bringing the system on line.

Criterion B states that if the probability exceeds 10-5 per year but less than 10-4 per year, the turbine may be kept in service until the next scheduled outage, at which time the licensee is to take action to reduce the probability to meet the 10-5 per year limit before returning the turbine to service. In order to assure that the turbine missile probability follows the turbine reliability '

requirements criteria, the staff requires applicants and licensees to submit, for approval, a turbine inspection program including inspection intervals that are based on turbine missile generation analysis.

2.0 EVALUATION Limerick Unit 2 has three General Electric low pressure turbines - LPA, LPB, and LPC. The licensee calculated the turbine missile generation probability of each low pressure turbine using the General Electric turbine reliability methodology. The staff has approved the GE's methodology as documented in Safety Evaluation Report related to the operation of Hope Creek Generating Station Supplement No. 6, NUREG-1048, July 1986.

The GE's methodology considers turbine wheel operating conditions such as temper-ature, pressure, steam quality, material properties, inspection intervals and other factors. The results of the licensee's calculation show that the turbine missile generation probability of each low pressure turbine is less than 10-5 per year if each low pressure turbine is inspected every six operating years. The licensee has also committed to include the turbine system maintenance program and the revision to the methodology of projecting turbine missile generation in FSAR sections 10.2.3.6 and 3.5.1.3, respectively, during a future update of the FSAR.

3.0 CONCLUSION

The staff, concludes that the turbine maintenance program at Limerick Unit 2 is acceptable because the calculated turbine missile generation probability satisfies the staff's turbine reliability requirement criteria specified in section~3.5.1.3.3 of Limerick Unit 2 Safety Evaluation Report.

- Principal Contributor: John Tsao

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