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Notice of Public Workshop on Draft Report for Comment: “Estimating Loss-of-Coolant Accident (LOCA) Frequencies Through the Elicitation Process,” NUREG-1829

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Nuclear Regulatory Commission.


Announcement of public workshop. Revision of workshop date.


Workshop date, November 9, 2005.

Background: The Nuclear Regulatory Commission (NRC) issued draft NUREG-1829, “Estimating Loss-of-Coolant Accident (LOCA) Frequencies Through the Elicitation Process,” for public comment in June 2005. The report is available under ADAMS Accession Number ML051520574 and on the NRC Web site at​reading-rm/​doc-collections/​nuregs/​staff/​sr1829/​. A separate notice was published in the Federal Register on October 4, 2005, announcing the availability of this report (70 FR 57901). This report describes LOCA frequency estimates developed using an expert elicitation process in support of an effort to develop a risk-informed revision of the emergency core cooling system (ECCS) requirements for commercial nuclear power plants by redefinition of the design-basis break size. The expert elicitation process consolidates service history data and insights from probabilistic fracture mechanics (PFM) studies with knowledge of plant design, operation, and material performance to arrive at the LOCA frequency estimates.

The ECCS requirements in the United States are contained in 10 CFR 50.46, Appendix K to part 50, and General Design Criterion (GDC) 35. Specifically, ECCS design, reliability, and operating requirements exist to ensure that the system can successfully mitigate postulated LOCAs. Consideration of an instantaneous break with a flow rate equivalent to a double-ended guillotine break (DEGB) of the largest pipe in the primary piping system of the plant generally provides the limiting condition in the required 10 CFR part 50, Appendix K analysis. However, the DEGB is widely recognized as an extremely unlikely event, so NRC staff is performing a risk-informed revision of the design-basis break size requirements.

A central consideration in selecting a risk-informed design basis break size is an evaluation of the LOCA frequency as a function of break size. The most recent NRC-sponsored study of pipe break failure frequencies is contained in NUREG/CR-5750 (Poloski, 1999). Unfortunately, these estimates are not sufficient for design basis break size selection because they do not address all current passive-system degradation concerns (e.g., primary water stress corrosion cracking) and they do not discriminate among breaks having effective diameters greater than 6 inches.

There have been two approaches traditionally used to estimate LOCA frequencies and their relationship to pipe size: (i) Estimates based on statistical analysis of service experience data, and (ii) PFM analysis of specific postulated failure mechanisms. Neither approach is fully suitable for evaluating LOCA event frequencies due to the rarity of these events and the modeling complexity. This study used an expert elicitation process, which is well-recognized for quantifying phenomenological knowledge when data or modeling approaches are insufficient. Elicitation responses from a panel of 12 experts determined individual LOCA frequency estimates for the 5th percentile, median, mean and 95th percentile of the frequency distribution for each of six LOCA categories. Group estimates were determined by aggregating the individual estimates using the geometric mean of the individual estimates for each frequency parameter (i.e., median, mean, 5th and 95th percentiles). Group variability was estimated by calculating 95% confidence bounds for each of the group frequency parameters. A number of sensitivity analyses were conducted to examine the effects on the quantitative results from variation of the assumptions, structure and techniques of the baseline analysis procedure.

Public Workshop: The NRC will conduct a public workshop on Wednesday, November 9, 2005, to be held in room O6B4 at NRC Headquarters, 11545 Rockville Pike, Rockville, Maryland. This is a revision to the October 31, 2005, workshop date announced in the Federal Register Notice published, October 4, 2005; 70 FR 57901. The purpose of the workshop is to facilitate the comment process. In the workshop, the staff will provide an overview of the report and address clarification of items identified by the public. A preliminary agenda is attached. Persons planning to attend this meeting are urged to contact the below named individual at least two working days prior to the meeting to be advised of any potential changes to the agenda. The NRC seeks comments on the report and is especially interested in comments on the following questions:

1. Is the structure of the expert elicitation process appropriate for the stated problem and goals of the study?

2. Are the assumptions and methodology of the analysis framework used to process the panel responses appropriate and reasonable? Are they consistent with the type of information provided by the expert panel and the goals of the study?

3. Is the geometric mean aggregation methodology appropriate for the panel responses and the study goals? Should other aggregation methodologies be considered and what are their advantages and disadvantages?

As previously published in the Federal Register, October 4, 2005; 70 FR 57901, the NRC will consider all written comments on draft NUREG-1829, “Estimating Loss-of-Coolant Accident (LOCA) Frequencies Through the Elicitation Process,” received before November 30, 2005. Comments received after November 30, 2005, will be considered if time permits. Comments should be addressed to the contact listed below. An electronic version of the report and the accompanying experts' raw data files, are available electronically at​reading-rm/​doc-collections/​nuregs/​staff/​sr1829/​ and through the NRC's Electronic Reading Room at​reading-rm/​adams.html. From the latter site, you can access the NRC's Agencywide Document Access and Management System (ADAMS), which provides text and image files of NRC's public documents. The ADAMS accession numbers for the documents related to this notice are:

Document titleADAMS Accession No.File format
NUREG-1829ML051520574Adobe Acrobat Document.
BWR Non-piping Raw Data for NUREG-1829ML051580341Adobe Acrobat Document.
BWR Piping Raw Data for NUREG-1829ML051580344Adobe Acrobat Document.
PWR Non-piping Raw Data for NUREG-1829ML051580346Adobe Acrobat Document.
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PWR Piping Raw Data for NUREG-1829ML051580347Adobe Acrobat Document.

If you do not have access to ADAMS or if there are problems in accessing the documents located in ADAMS, contact the NRC Public Document Room (PDR) Reference staff at 1-800-397-4209, 301-415-4737, or by e-mail to

These documents may also be viewed electronically on the public computers located at the NRC's PDR, O1F21, One White Flint North, 11555 Rockville Pike, Rockville, MD 20852. The PDR reproduction contractor will copy documents for a fee.

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Dr. Charles A. Greene, Mail Stop T10E10, U.S. Nuclear Regulatory Commission, 11545 Rockville Pike, Rockville, MD 20852, telephone (301) 415-6177, facsimile number: (301) 415-5074, e-mail

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Dated at Rockville, Maryland, this 25th day of October 2005.

For the Nuclear Regulatory Commission.

Jennifer Uhle,

Chief, Materials Engineering Branch, Division of Engineering Technology, Office of Nuclear Regulatory Research.

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Attachment—Preliminary Agenda

Public Workshop on Draft Report for Comment: “Estimating Loss-of-Coolant Accident (LOCA) Frequencies through the Elicitation Process,” NUREG-1829

November 9, 2005*—9 a.m.-12 p.m., Room O-4B6

Preliminary Agenda

9 a.m.-9:15 a.m.—Introduction

9:15 a.m.-9:45 a.m.—Overview of NUREG-1829

9:45 a.m.-10:15 a.m.—Clarification of items identified by the public

10:15 a.m.-10:30 a.m.—Break

10:30 a.m.-12 noon—Clarification of items identified by the audience

12 noon—Adjourn

*Revised date

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[FR Doc. 05-21650 Filed 10-28-05; 8:45 am]