ML070670090

E-mail Re Palisades: Revised Draft RAI Questions

ML070670090
Person / Time
Site:	Palisades
Issue date:	03/08/2007
From:	Tam P NRC/NRR/ADRO/DORL/LPLIII-1
To:	Hazelhoff A, Lahti L Nuclear Management Co
chawla M
References
TAC MD3059
Download: ML070670090 (3)

Text

From:

Peter Tam To:

Amy C. Hazelhoff; Laurie A. Lahti Date:

03/08/2007 7:39:23 AM

Subject:

Palisades: Revised Draft RAI Questions (TAC MD3059)

Amy:

The subject relief request involves a proposed alternative to the American Society of Mechanical Engineers requirements. Regulatory Guide 1.174 includes quantitative guidelines that can be used to determine if the change in risk arising from the requested alternative is sufficiently small to support a determination that the requested action provides an acceptable level of quality and safety.

We held a teleconference with you and your contractor on 3/1/07 to discuss 2 draft RAI questions that were transmitted to you by my e-mail of 2/20/07 (Accession No. 070510257).

During the teleconference, licensee personnel proposed to respond to the 2 questions using values estimated in a topical report proposed by Westinghouse in WCAP-16168, "Risk-Informed Extension of Reactor Vessel In-Service Inspection Interval," to estimate the change in risk associated with extending the interval. WCAP-16168 (Accession No. ML060330504), currently under NRC staff review, proposes a methodology to be used to support relief requests under 50.55a(3)(i) to extend the reactor vessel welds inspection interval from 10 to 20 years.

The discussion in the teleconference centered on one of the staffs upcoming RAI questions on WCAP-16168, i.e., the precise definition of the conditional probability of reactor vessel failure produced by Westinghouses FAVOR code calculations and how to use that value consistent with its definition to estimate the change in risk associated with reactor vessel weld inspection interval extensions.

The teleconference discussion also included reference to the staffs letter to Westinghouse dated January 27, 2005 (Accession No. ML050250410), that discusses a framework which could be used to support one-cycle extensions of reactor vessel weld inspections. In that letter, the staff concluded that, [a]bsent having the WCAP-16168 quantitative technical basis, however, this risk determination would need to be made on a qualitative basis. The discussion during the teleconference indicated that the quantitative technical basis for the change in risk estimate is still not available because some staff questions about WCAP-16168 remain unresolved. Therefore, the staff is retracting the 2 questions previously sent to you by my e-mail of 2/20/07, as follows:

(1) Please provide an estimate of the increase in core damage frequency (CDF) and large early release frequency (LERF) associated with extending the reactor vessel weld inspection interval from ten years to ten years plus two operating cycles.

(2) Please describe the process used to develop the above estimates, including the process relied upon to provide confidence that the risk/reliability models correspond with the as built, as-operated plant.

In place of the 2 previous questions, the staff now has the following question:

As discussed in a letter to Westinghouse dated January 27, 2005, the staff expects that one-time requests to extend the inspection interval of the reactor pressure vessel (RPV) welds by one cycle should include a discussion indicating that the likelihood of a significant pressurized thermal shock (PTS) event over the next operating cycle is very low. Your submittals dated March 31 and October 11, 2005, described Palisades' response to three of the most significant PTS sequences identified in the ongoing PTS rulemaking work. To support the conclusion that the request for relief for this second one-cycle extension satisfies the risk-informed principal that any proposed increase in risk is small, please provide an estimate of the annual frequency of these more severe PTS sequences and describe the process used to

evaluate the frequency of these events which could challenge the integrity of the RPV, if a flaw was present.

Please call me to set up a conference call to discuss this new question. This e-mail aims solely to prepare you and others for the proposed conference call. This e-mail does not convey an official NRC staff position, and does not formally request for additional information.

Peter S. Tam, Senior Project Manager Plant Licensing Branch III-1 Division of Operating Reactor Licensing Office of Nuclear Reactor Regulation e-mail: pst@nrc.gov Tel.: 301-415-1451 CC:

Stephen Dinsmore Mail Envelope Properties (45F003FB.E94 : 12 : 35330)

Subject:

Palisades: Revised Draft RAI Questions (TAC MD3059)

Creation Date 03/08/2007 7:39:23 AM From:

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PST@nrc.gov Recipients Action Date & Time nmcco.com Transferred 03/08/2007 7:39:56 AM AMY.HAZELHOFF (Amy C. Hazelhoff)

LAURIE.LAHTI (Laurie A. Lahti) nrc.gov OWGWPO03.HQGWDO01 Delivered 03/08/2007 7:39:31 AM SCD1 CC (Stephen Dinsmore)

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