ML091140022

PINGP Lr - Draft Turbine Missile Discussion

ML091140022
Person / Time
Site:	Prairie Island
Issue date:	04/16/2009
From:	- No Known Affiliation
To:	Division of License Renewal
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Download: ML091140022 (4)

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PrairieIslandNPEm Resource From: Eckholt, Gene F. [Gene.Eckholt@xenuclear.com]

Sent: Thursday, April 16, 2009 3:19 PM To: Richard Plasse

Subject:

Draft Turbine Missile Discussion Attachments: Turbine rotor inspection supplement (2).doc 1

Hearing Identifier: Prairie_Island_NonPublic Email Number: 1003 Mail Envelope Properties (7A9B2084CC9CEC45828E829CBF20D638033F6C2C)

Subject:

Draft Turbine Missile Discussion Sent Date: 4/16/2009 3:18:34 PM Received Date: 4/16/2009 3:18:38 PM From: Eckholt, Gene F.

Created By: Gene.Eckholt@xenuclear.com Recipients:

"Richard Plasse" <Richard.Plasse@nrc.gov>

Tracking Status: None Post Office: enex02.ft.nmcco.net Files Size Date & Time MESSAGE 0 4/16/2009 3:18:38 PM Turbine rotor inspection supplement (2).doc 34880 Options Priority: Standard Return Notification: No Reply Requested: No Sensitivity: Normal Expiration Date:

Recipients Received:

DRAFT - FOR INFORMATION ONLY In a letter dated April 6, 2009, NSPM provided its conclusion that the analysis of the probability of damage to safeguard equipment from turbine missiles was not a TLAA. In a conference call on April 15, 2009, the NRC requested clarification about how the Current Licensing Basis (CLB) for turbine missiles addresses the turbine rotor inspection interval. NSPM agreed to provide clarifying information in a supplement to the RAI response.

The CLB for turbine valve testing is encompassed by the PINGP Technical Requirements Manual (TRM), Section 3.7.3, Turbine Overspeed Protection. The TRM is a licensee-controlled document that is maintained consistent with the USAR. A licensee's TRM is recognized by the NRC as an element of a plant's licensing bases in NRR Office Instruction LIC-100, Control of Licensing Bases for Operating Reactors. The TRM requirement for turbine valve stroke testing states, "Frequency of testing shall be consistent with the methodology presented in WCAP-11525, "Probabilistic Evaluation of Reduction in Turbine Valve test Frequency", and in accordance with the established NRC acceptance criteria for the probability of a turbine missile ejection incident of 1.0 x 10-5 per year. In no case shall the turbine valve test interval exceed one year." The NRC approved the use of WCAP-11525 methodology to determine valve test frequency in PINGP License Amendments 86 (Unit 1) and 79 (Unit 2) issued by an NRC letter dated February 7, 1989.

WCAP-11525 was prepared to provide a probabilistic basis for selecting a turbine valve testing interval. The WCAP methodology is used to define a valve test interval that, in conjunction with other factors, maintains the overall probability of turbine missile ejection less than 1E-5/year. The valve test interval, in conjunction with the design configuration and valve failure rates, determines the probability that a design, intermediate or destructive overspeed event could occur. However, the valve test interval, in itself, is only one of the factors which contribute to the overall probability of missile ejection. At PINGP turbine valve stroke tests are currently performed at a nominal interval of six months.

While the CLB does not specifically address a turbine rotor inspection frequency, WCAP-11525 recognizes that the probability of a missile ejection at any speed is a function of the type and condition of the rotor. The probability that a missile would be ejected at normal operating speed or overspeed is directly dependent on the elapsed operating time since the last rotor inspection. Each performance of a rotor inspection reduces the probability of missile ejection from rotor failure.

After each rotor inspection, the probability of a missile ejection from a rotor failure increases with time until the next inspection. Since the purpose of WCAP-11525 was to address the valve testing interval, the conditional probabilities of missile ejection were conservatively assumed values. Even though the overall probability of turbine missile ejection from the fully integral turbine rotor type installed at PINGP could be maintained <1E-5/year with rotor inspection intervals of 30 or more years, the WCAP analysis conservatively assumed a nominal inspection interval of ten operating years. The WCAP analysis did not specify 4/16/09

DRAFT - FOR INFORMATION ONLY this ten operating year interval as a limiting requirement; it was simply one of the variables that had to be specified along with the applicable rotor type as an input to the analysis. At PINGP, turbine rotor inspections are performed at a nominal interval of 100,000 operating hours (approximately 11.4 operating years). Low pressure turbine rotor inspections are currently planned at the next refueling outages (1R26 and 2R26) of both Units 1 and 2. The curve that appears as USAR Figure 12.2-38 is not used to determine a rotor inspection interval.

In summary, in order to satisfy the TRM requirement to maintain the overall probability of turbine missile ejection below 1E-5/year, PINGP must utilize both an appropriate turbine valve test interval and an appropriate turbine rotor inspection interval to assure that the assumptions used in the WCAP-11525 analysis methodology continue to be met. Neither the turbine valve test interval nor the rotor inspection interval involves a time-limited assumption defined by the current operating term of the plant. Therefore, the analysis of the probability of damage to safeguard equipment from turbine missiles is not a TLAA 4/16/09