[Federal Register Volume 70, Number 191 (Tuesday, October 4, 2005)]
[Pages 57901-57903]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 05-19790]



Notice of Availability of Draft Report for Comment: ``Estimating 
Loss-of-Coolant Accident (LOCA) Frequencies Through the Elicitation 
Process,'' NUREG-1829

AGENCY: Nuclear Regulatory Commission.

ACTION: Notice of availability and request for comments.


DATES: Written comments must be provided by November 30, 2005.
    Background: In support of an effort to develop a risk-informed 
revision of the emergency core cooling system (ECCS) requirements for 
commercial nuclear power plants, estimates of loss-of-coolant accident 
(LOCA) frequencies have been developed which will enable redefinition 
of the design-basis break size for these requirements. These LOCA 
frequency estimates have been developed using an expert elicitation 
process by consolidating service history data and insights from 
probabilistic fracture mechanics (PFM) studies with knowledge of plant 
design, operation, and material performance. This expert elicitation to 
develop LOCA frequency estimates is described in draft NUREG-1829, 
``Estimating Loss-of-Coolant Accident (LOCA) Frequencies Through the 
Elicitation Process'' (June 2005).
    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

[[Page 57902]]

instantaneous break with a flow rate equivalent to a double-ended 
guillotine break (DEGB) of the largest primary piping system in 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.
    Solicitation of Comments: 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 
    Comment Period: The NRC will consider all written comments received 
before November 30, 2005. To facilitate the comment process the NRC 
will conduct a workshop on October 31, 2005, to be held in room O4B6 at 
NRC Headquarters, 11545 Rockville Pike, Rockville, Maryland. 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. A separate notice will be published in the Federal 
Register announcing the public workshop. Comments received after 
November 30, 2005, will be considered if time permits. Comments should 
be addressed to the contact listed below.
    Availability: An electronic version of the report and the 
accompanying experts' raw data files, are available electronically at 
http://www.nrc.gov/reading-rm/doc-collections/nuregs/staff/sr1829/ and 
through the NRC's Electronic Reading Room at http://www.nrc.gov/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 title                   ADAMS accession No.                      File format
NUREG-1829............................  ML051520574                   Adobe Acrobat Document.
BWR Non-piping Raw Data for NUREG-1829  ML051580341                   Microsoft Excel Worksheet.
BWR Piping Raw Data for NUREG-1829....  ML051580344                   Microsoft Excel Worksheet.
PWR Non-piping Raw Data for NUREG-1829  ML051580346                   Microsoft Excel Worksheet.
PWR Piping Raw Data for NUREG-1829....  ML051580347                   Microsoft Excel Worksheet.

    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 [email protected].
    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.

T10E10, U.S. Nuclear Regulatory Commission, 11545 Rockville Pike, 
Rockville, MD 20852, telephone (301) 415-6177, facsimile number: (301) 
415-5074, e-mail [email protected].

    Dated at Rockville, Maryland, this 19th day of September 2005.

    For the Nuclear Regulatory Commission.
Andrea Lee,
Acting Branch Chief, Materials Engineering Branch, Division of 
Engineering Technology, Office of Nuclear Regulatory Research.

Attachment--Preliminary Agenda

Public Workshop on Draft Report for Comment: ``Estimating Loss-of-
Coolant Accident (LOCA) Frequencies Through the Elicitation Process,'' 

October 31, 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

[[Page 57903]]

9:45 a.m.-10:15 a.m.--Discussion of 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

[FR Doc. 05-19790 Filed 10-3-05; 8:45 am]