UNITED STATES

NUCLEAR REGULATORY COMMISSION

OFFICE OF NUCLEAR REACTOR REGULATION

WASHINGTON, D.C. 20555-0001

December 6, 2006

NRC REGULATORY ISSUE SUMMARY 2006-24

REVISED REVIEW AND TRANSMITTAL PROCESS FOR ACCIDENT

SEQUENCE PRECURSOR ANALYSES

ADDRESSEES

All holders of licenses for nuclear power reactors, except those who have permanently ceased

operations and have certified that fuel has been permanently removed from the reactor vessel.

INTENT

The U.S. Nuclear Regulatory Commission (NRC) is issuing this regulatory issue summary (RIS)

to inform addressees that a revised process for accident sequence precursor (ASP) analyses

has been implemented. This includes a revised process for licensee review of the ASP

analyses. This RIS requires no action or written response on the part of an addressee.

BACKGROUND INFORMATION

The NRC established the ASP program in 1979 in response to the Risk Assessment Review

Group report (see NUREG/CR-0400, dated September 1978). The ASP program

systematically evaluates U.S. commercial nuclear power plant operating experience to identify,

document, and rank the operating events and conditions that were most likely to have led to

inadequate core cooling and severe core damage (precursors), considering the likelihood of

additional failures.

To identify potential precursors, the NRC staff reviews plant events based on information

contained in licensee event reports and NRC inspection reports. The staff then analyzes the

identified potential precursors by calculating the probability of an event leading to a core

damage state. A plant event can be one of two types: (1) an occurrence of an initiating event,

such as a reactor trip or a loss of offsite power, with any subsequent equipment unavailability or

degradation or (2) a degraded plant condition (e.g., unavailability or degradation of equipment)

without an occurrence of an initiating event. 

An initiating event with a conditional core damage probability (CCDP) greater than or equal to

1×10-6 (one in a million) or a degraded plant condition resulting in an increase in core damage

probability (ΔCDP) greater than or equal to 1×10-6 is considered a precursor in the ASP

program. An initiating event with a CCDP greater than or equal to 1×10-3 or a degraded plant

condition resulting in a ΔCDP greater than or equal to 1×10-3 is considered a significant

precursor in the ASP program.

ML060900007
RIS 2006-24

Page 2 of 3

The ASP program is one of three agency programs that assess the risk significance of issues

and events. The other two programs are the Significance Determination Process (SDP) and

the event response evaluation process, as defined in Management Directive (MD) 8.3, “NRC

Incident Investigation Program.” Since these programs serve different functions, there are

some inherent differences in the processes. For example, the SDP, which is used to determine

the safety significance of inspection findings, analyzes each finding individually and screens out

events where there are no licensee performance deficiencies. In contrast, the ASP program

evaluates all potentially significant plant events and degraded conditions, and analyzes

concurrent multiple degraded conditions together. 

In the past, the ASP program would perform risk evaluations of all events that had not been

eliminated by the screening criteria to determine if these events were potential precursors to

core damage accidents. The preliminary ASP analyses were then transmitted to the licensees

for review and comment. After these reviews were completed and the comments resolved, the

final analyses would be transmitted to all stakeholders. This ASP evaluation process was

sometimes duplicative in that, in some cases, there would also be evaluations of the same

events produced by the SDP or MD 8.3 programs. Also, the review process could be resource

consuming, and the licensees have generally limited their comments to high-profile and/or highrisk analyses, commenting on only a few low-risk analyses (i.e., CCDP or ΔCDP less than

1×10-5). 

The NRC staff has developed a more efficient ASP process to reduce the number of

evaluations performed, and a more risk-informed approach to the review and transmittal

process to reduce administrative and review burdens on the NRC staff and the licensees.

SUMMARY OF THE ISSUE

New ASP evaluation process. The new ASP process has been revised to utilize the results of

the SDP or MD 8.3 programs, when appropriate, to achieve better efficiencies. 

• Selection of precursors with an SDP or documented MD 8.3 evaluation. For

degraded conditions or significant operational occurrences for which there is an SDP or

documented MD 8.3 quantitative risk evaluation, the ASP program will utilize the results

of these evaluations, where applicable, without performing a separate ASP analysis. 

• Selection of precursors with no SDP or documented MD 8.3 evaluation. The ASP

program will continue to perform analyses for those events for which there is no SDP or

MD 8.3 evaluation. Examples of these types of events include most initiating events,

and plant conditions for which there are no performance deficiencies or where there are

concurrent multiple degraded conditions. 

• Potentially significant precursors. For all events (including those being evaluated by

the SDP or MD 8.3 processes) that, based on preliminary evaluations, could be

significant precursors, the ASP program will perform an expedited analysis, in order to

support the reporting requirements in the annual NRC Performance and Accountability

Report to Congress, NUREG-1542, and to support the proposed new Abnormal

Occurrence criteria described in SECY-05-0137. 
RIS 2006-24

Page 3 of 3

New ASP review process. The new ASP review process has been revised to be more 

risk-informed, as described below. 

• Lower risk events. If an ASP analysis results in a CCDP or ΔCDP of less than 1x10-4,

formal review comments by the licensee will no longer be requested. A summary of the

analysis and results will be issued to the pertinent licensee and other stakeholders for

information. However, if the licensee or other stakeholders choose to comment on

these analyses, the NRC will continue to address their comments. 

• Higher risk events. If an ASP analysis results in a CCDP or ΔCDP greater than 1x10-4,

formal review comments by the licensee will continue to be requested. These ASP

analyses will be issued as final to all stakeholders after resolution of the review

comments. 

BACKFIT DISCUSSION

This RIS requires no action or written response and is; therefore, not a backfit under

10 CFR 50.109. Consequently, the staff did not perform a backfit analysis.

FEDERAL REGISTER NOTIFICATION

A notice of opportunity for public comment on this RIS was not published in the Federal

Register because it is informational and requires no specific action or written response.

CONGRESSIONAL REVIEW ACT

This RIS is not a rule as designated by the Congressional Act (5 U. S. C. §§ 801-808) and;

therefore, is not subject to the Act.

PAPERWORK REDUCTION ACT STATEMENT

This RIS does not contain any information collections and; therefore, is not subject to the

requirements of the Paperwork Reduction Act of 1995 (44 U.S.C. 3501 et seq.).

CONTACT

Please direct any questions about this matter to the technical contact listed below.

/RA/

Michael J. Case, Director

Division of Policy and Rulemaking

Office of Nuclear Reactor Regulation

Technical Contact: Anne-Marie Grady, RES

Phone: 301-415-7645

E-mail: axg5@nrc.gov

Note: NRC generic communications may be found on the NRC public Web site,

http://www.nrc.gov, under Electronic Reading Room/Document Collections.
RIS 2006-24

Page 3 of 3 New ASP review process. The new ASP review process has been revised to be more risk-informed, as described below. 

• Lower risk events. If an ASP analysis results in a CCDP or ΔCDP of less than 1x10-4, formal review comments by the licensee will no longer be requested. A summary of the 

analysis and results will be issued to the pertinent licensee and other stakeholders for

information. However, if the licensee or other stakeholders choose to comment on

these analyses, the NRC will continue to address their comments. 

• Higher risk events. If an ASP analysis results in a CCDP or ΔCDP greater than 1x10-4, formal review comments by the licensee will continue to be requested. These ASP

analyses will be issued as final to all stakeholders after resolution of the review

comments. 

BACKFIT DISCUSSION

This RIS requires no action or written response and is; therefore, not a backfit under

10 CFR 50.109. Consequently, the staff did not perform a backfit analysis.

FEDERAL REGISTER NOTIFICATION

A notice of opportunity for public comment on this RIS was not published in the Federal

Register because it is informational and requires no specific action or written response.

CONGRESSIONAL REVIEW ACT

This RIS is not a rule as designated by the Congressional Act (5 U. S. C. §§ 801-808) and;

therefore, is not subject to the Act.

PAPERWORK REDUCTION ACT STATEMENT

This RIS does not contain any information collections and; therefore, is not subject to the

requirements of the Paperwork Reduction Act of 1995 (44 U.S.C. 3501 et seq.).

CONTACT

Please direct any questions about this matter to the technical contact listed below. /RA/

Michael J. Case, Director

Division of Policy and Rulemaking

Office of Nuclear Reactor Regulation

Technical Contact: Anne-Marie Grady, RES Phone: 301-415-7645

E-mail: axg5@nrc.gov

Note: NRC generic communications may be found on the NRC public Web site,

http://www.nrc.gov, under Electronic Reading Room/Document Collections. DISTRIBUTION:

RIS File

* Previous concurrence via email/hard copy

ADAMS ACCESSION NUMBER: ML060900007 TAC# MC9661

OFFICE OEGIB:DRASP TechEd BC:OEGIB:DRASP DD:OERA:DRASP D:DRASP

NAME *AGrady *PKleene *MCheok *PBaranowsky *FEltawila

DATE 10 / 12 /06 03 / 31 /06 10 / 12 /06 10 / 18 /06 10 / 18 /06

OFFICE D:DORL D:DRA D:DIRS OE OGC (NLO) OGC (SBREFA)

NAME *JLubinski for

CHaney

*CHolden for

JLyons

*SRichards for

ECollins

*DSolorio for

CCarpenter

*JMartin *NSanchez

DATE 10 / 31 /06 11 / 2 /06 11 / 6 /06 11 / 7 /06 11 /13 /06 11 / 17 /06

OFFICE PMAS:NRR OIS LA:PGCB:DPR PGCB:DPR BC:PGCB D:DPR

NAME *RMitchell *EBaker CHawes AMarkley CJackson MCase

DATE 11 / 7 /06 11 / 8 /06 11/29/06 12/01/06 12/01/06 12/06/06

OFFICIAL RECORD COPY