POLICY ISSUE

INFORMATION

March 8, 2002 SECY-02-0041

FOR: The Commissioners

FROM: William D. Travers

Executive Director for Operations

SUBJECT: STATUS OF ACCIDENT SEQUENCE PRECURSOR AND SPAR MODEL

DEVELOPMENT PROGRAMS

PURPOSE:

To inform the Commission of the status of the Accident Sequence Precursor (ASP) Program

and the development of the Standardized Plant Analysis Risk (SPAR) models that support this

programs.

SUMMARY:

In an April 24, 1992, memorandum to the Chairman, the staff committed to report periodically to

the Commission on the status of efforts to improve the ASP Program. In SECY-94-268, dated

October 31, 1994, two changes were made to the report. First, the staff committed to provide

the report annually, and second, the staff began providing annual quantitative ASP results. 

Since the last status report, SECY-00-034, dated March 1, 2001, the staff has:

ï Completed evaluating Fiscal Year (FY) 1999 events for precursors; completed preliminary

analyses of FY 2000 precursors; and continued analyzing FY 2001 events.

ï Evaluated trends in the precursor data.

ï Continued developing SPAR models.

BACKGROUND:

The discussion below provides a brief background of the ASP and SPAR Model Development

Programs and their uses. 

CONTACTS: Don Marksberry, RES, 301-415-6378 (ASP Program)

Patrick OíReilly, RES, 301-415-7570 (SPAR models)
The Commissioners 2

1

One industry trend indicator used in this performance measure was provided by the ASP Program for FY

2000. The Industry Trends Program is discussed in SECY-01-0111, ìDevelopment of an Industry Trends Program

for Operating Power Reactors.î

ASP Program. The ASP Program was established by the NRC in 1979 in response to the Risk

Assessment Review Group report (see NUREG/CR-0400, September 1978). The primary

objective of the ASP Program is to systematically evaluate U.S. nuclear plant operating

experience to identify, document, and rank operating events most likely to lead to inadequate

core cooling and severe core damage (precursors), if additional failures had occurred.

The secondary objectives of the ASP Program are 

- To categorize the precursors by their plant-specific and generic implications, 

- To provide a measure for trending nuclear plant core damage risk, and 

- To provide a partial check on probabilistic risk assessment (PRA)-predicted dominant core

damage scenarios. 

The program is also used to monitor the agencyís performance against the following Strategic

Plan performance goals in the Reactor Safety arena for maintaining safety (see Volume 2, 

Part 1, of NUREG-1614, September 2000):

No more than one event per year identified as a significant precursor (i.e., conditional core damage

probability or importance > 1 x 10-3) of a nuclear reactor accident.

No statistically significant adverse industry trends in safety performance.1

Events and conditions from licensee event reports, inspection reports, and special requests

from NRC staff are reviewed for potential precursors. These potential precursors are analyzed,

and a conditional core damage probability (CCDP) is calculated by mapping failures observed

during the event onto accident sequences in risk models. An event with a CCDP or a condition

with a change in core damage probability (∆CDP or importance) greater than or equal to 

1.0 x 10-6 is considered a precursor in the ASP Program.

Current ASP analyses do not explicitly include uncertainties for calculating CCDPs. Revision 3

of the SPAR models includes uncertainty analysis capability for future ASP analyses. 

Sensitivity analyses are used to assess the impact of uncertainties in key parameters that

could influence the characterization of an event as a precursor (CCDP or ∆CDP > 1.0 x 10-6),

important precursor (CCDP or ∆CDP > 1.0 x 10-4) or significant precursor (CCDP or ∆CDP >

1.0 x 10-3). 

SPAR Model Development Program. SPAR models are the analysis tool used by staff

analysts in many regulatory activities, including the ASP Program and Phase 3 of the

Significance Determination Process (SDP). The SPAR models have evolved from two sets of

simplified event trees that were used initially to perform precursor analyses in the early 1980s. 

One set of event trees was used for boiling-water reactors (BWRs) and one set for

pressurized-water reactors (PWRs) to model plant response to the same set of initiating

events. In 1985, the models were improved to better reflect the operating plant population by
The Commissioners 3

using eight plant classes of event trees that reflected differences among the plant designs. In

1992, the staff conducted a workshop of internal and external stakeholders to discuss the

methods, models, and data needs of the program to better meet the programís objectives. 

The results and recommendations generated at this meeting were documented in NUREG/CP0124 (June 1992). In response to the recommendations, the staff prepared the Integrated

ASP Program Plan (Enclosure 1 to SECY-94-076, March 22, 1994) for improving the models

used for ASP analyses. Among the improvements identified was the development of Level 1,

Revision 1 SPAR models, which included plant-specific fault trees and other improvements to

the existing event tree-based models. Another improvement was the development of feasibility

studies for specialized models, such as low power or shutdown, external events, and Level 2

models. In 1998, Revision ì2QAî SPAR models were issued that more closely reflected plantspecific risk based on comments generated by peer reviews of precursor analyses and

models. 

In 1999, the SPAR Model Users Group (SMUG) assumed coordination of model development

efforts that support the ASP Program and other risk-informed regulatory processes. This

group is comprised of representatives from the Offices of Nuclear Reactor Regulation and

Nuclear Regulatory Research, and regional offices that use reactor plant risk models in

regulatory activities. In August 2000, the SMUG completed the SPAR Model Development

Plan, which conforms to the modeling needs identified by SMUG members and their

management for performing risk-informed regulatory activities. The following models are

addressed in this plan: 

ï Level 1: Internal events during full power operation

ï Level 1: Internal events during low-power and shutdown operations

ï Level 1: External events (including fires, floods, seismic events)

ï Level 2: Large early release frequency (LERF).

Uses. The NRC staff uses the ASP methodology, SPAR models, and results of ASP analyses

to perform the following risk-informed regulatory activities:

(1) Promptly assess the risk significance of operational events to support regulatory decisions

by senior management. 

(2) Evaluate the significance of inspection findings as part of the agencyís reactor oversight

process (ROP), in Phase 3 of the significance determination process (SDP).

(3) Establish plant-specific, risk-informed thresholds to support the development of enhanced

performance indicators for the ROP.

(4) Develop risk-informed thresholds for industry-level indicators in the Industry Trends

Program.

(5) Evaluate the change in risk associated with licensing amendments submitted by licensees

requesting changes in surveillance frequencies or allowed outage times. 

(6) Support decisions to develop generic communications.

(7) Systematically screen, review, and analyze operational experience data for accident

sequence precursors. 
The Commissioners 4

2

Results for FY 1999 are based on final analyses. Results for FY 2000 are based on a combination of final

and preliminary analyses. The preliminary analysis of events in FY 2001 is ongoing.

3

Although, preliminary analyses of events in FY 2001 are ongoing, the screening and review of FY 2001

events for potential significant precursors were completed.

(8) Evaluate the generic implications of precursors; trend industry performance; and compare

results against PRAs. 

(9) Perform regulatory analyses to resolve generic issues.

DISCUSSION:

This section provides summaries of historical trends and insights, documentation and issuance

of ASP results, and status of SPAR model development.

Historical Trends and Insights

A review of the ASP analyses2

 for FYs 1999, 2000, and 2001 and a comparison with analyses

from previous years for insights and trends are summarized in the Attachment. The ASP

results used to monitor the agencyís performance against the two Strategic Plan goals are as

follows: 

Significant Precursors. The Strategic Plan performance goal is ìNo more than one event per

year identified as a significant precursor of a nuclear accident.î A significant precursor is

defined in the Strategic Plan as an event that has a 1/1000 (10-3) or greater probability of

leading to a reactor accident.

No significant precursors were identified during FYs 1999, 2000, and 20013

 with a CCDP or

∆CDP > 1.0 × 10-3. Significant precursors have occurred, on the average, about once every 

4 to 5 years. The events in the significant precursor group involve differing failure modes,

causes, and systems.

Industry Trends. The occurrence rate of precursors has exhibited a decreasing trend that is

almost statistically significant during the 1993ñ2000 period (see Figure 1 of the Attachment). 

The occurrence rate of precursors has decreased over the period by about a factor of 2.

Documentation and Issuance of ASP Results

There are 17 reports in the NUREG/CR series documenting the results of the ASP Program

covering the years 1969ñ1998. The precursor report for FY 1999 was completed and sent to

publications in September 2001. However, the report was subsequently withdrawn from

distribution because of the agencyís heightened awareness of the release of sensitive

information to the public following the September 11th terrorist attacks. In addition, preliminary

and final precursor analyses are not yet being issued to licensees or released to the public

pending Agency guidance (in response to the Staff Requirements Memorandum, ìStaff

Requirements - COMSECY-01-0030 - Guidance to the Staff on Release of Information to the

Public,î dated January 25, 2002) and office procedures for classifying and transmitting sensitive

information involving results of risk analyses. Staff is working to resolve this issue as soon as

possible.
The Commissioners 5

Trends in ASP events are also displayed on the NRC web page maintained by NRR for the

industry trends program. The ASP trends on this page will be updated after this status report is

issued.

SPAR Model Development

The SPAR Model Users Group (SMUG) is comprised of representatives from each of the

organizations within the agencyís program and regional offices that use risk models in

regulatory activities. The SMUG meets on a regular basis to provide technical guidance for

the SPAR Model Development Program consistent with the approved Integrated SPAR Model

Development Plan. This plan conforms to the modeling needs identified by the SMUG

members and their management for performing risk-informed regulatory activities. 

In the last year, the staff completed the following activities in model and methods development:

Level 1: Internal events during full power operation

ï Maintained Revision 2QA SPAR models on an as-needed basis (e.g., add plant features

not previously modeled, correct a deficiency). These models are currently used by the

staff to perform risk-informed regulatory activities.

ï Completed 19 Revision 3i SPAR models (the ìiî stands for interim; most of these models

have not undergone an onsite QA review). To date, 60 Revision 3i SPAR models out of a

total of 70 have been produced. 

ï Completed the onsite QA review of ten Revision 3i SPAR models. These onsite reviews

were performed in conjunction with NRRís benchmarking of the SDP Plant Notebooks. A

total of 20 of the 60 Revision 3i models have undergone onsite review.

Level 1: Internal events during low-power and shutdown operations

ï Completed preliminary templates for developing low-power/shutdown models for all

pressurized-water reactors and for boiling-water reactor models for BWR 4 and BWR 5/6

designs.

Level 2: Large early release frequency (LERF)

ï Began evaluation of the current capabilities to model Level 2/LERF sequences for riskinformed staff activities and to plan future development work.

/RA/

William D. Travers

Executive Director 

 for Operations

Attachment: Results, Trends, and Insights from the ASP Program
The Commissioners 6

SECY PAPER DATED: / /02

SUBJECT: STATUS OF ACCIDENT SEQUENCE PRECURSOR AND SPAR MODEL

DEVELOPMENT PROGRAMS

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DATE 2/9/02 12/05/01 12/05/01 12/11/01

OFFICE NRR* E RES* E DEDR EDO 

NAME SCollins AThadani CPaperiello WTravers 

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1 Attachment

Results, Trends, and Insights from the ASP Program

This section describes the status of Accident

Sequence Precursor (ASP) analysis of Fiscal

Year (FY) 1999, 2000, and 2001 events, the

analysis of historical Accident Sequence

Precursor (ASP) trends, and the evaluation

of insights.

1.0 ASP Event Analyses

The status of analyses is provided in Table 1. 

The results are summarized below.

FY 1999 analyses. The final analyses of

events in FY 1999 were completed in

September 2001. Peer review comments

were received from the respective licensee

and NRC staff and incorporated into the final

analysis. Seven of the initiating events and

conditions that occurred during FY 1999 had

a conditional core damage probability

(CCDP) or a change in core damage

probability (∆CDP or importance) >1 × 10-6. 

Of these seven precursors, all were at-power

precursors. 

The results of final ASP analyses for FY

1999 are presented in Table 2 for precursors

involving initiating events and Table 3 for

precursors involving conditions. 

FY 2000 analyses. The screening, review,

and preliminary analysis of events in FY 2000

were completed in February 2002. The final

analysis of four precursors were completed. 

The issuance of preliminary analyses for

licensee peer review was delayed because of

the agencyís heightened awareness of the

release of sensitive information to the public

following the September 11th terrorist attacks. 

This will delay the completion of the

remaining final analyses.

The ASP analyses identified 14 precursors

for FY 2000 (4 precursors based on final

analyses, 10 precursors based on preliminary

analyses). Of these 14 precursors, all were

at-power precursors. 

The results of ASP analyses for FY 2000 are

presented in Table 4 for precursors involving

initiating events and Table 5 for precursors

involving conditions. 

FY 2001 analyses. The screening and

review of licensee event reports (LERs) of

events in FY 2001 were completed in 

January 2002. The preliminary analyses are

ongoing. 

A combination of LERs and daily event

notification reports (10 CFR 50.72) were

reviewed to identify potential significant

precursors (CCDP or ∆CDP > 1 × 10-3). No

such events were identified for FY 2001. 

2.0 Overall Industry Trends

This section provides the results of trending

analyses for all precursors and for precursors

grouped by the order of magnitude of their

CCDP or ∆CDP (called CCDP bins). 

Note: The trending results cover the 

FY 1993ñ2000 period. Although most of the

results for FY 2000 are preliminary, all

preliminary analyses have undergone inhouse review. However, FY 2000 results are

subject to change when they become

finalized. Results for FY 2001 are not

included in the trends because the

preliminary analyses of all events are not

complete.

2.1 Occurrence Rate of Precursors

The 1993ñ2000 occurrence rate of all ASP

precursors are trended by fiscal year. The

trend plot is shown in Figure 1. 

The mean occurrence rate of precursors has

exhibited a decreasing trend that is almost

statistically significant. The occurrence rate

of precursors has decreased over the period

by a factor of 2.
2 Attachment

2.2 Occurrence Rate of Precursors by

CCDP Bins

In addition to the occurrence rate of all

precursors, the data were analyzed to

determine whether trends exist in the

occurrence rate of precursors with CCDP of

different orders of magnitude. The analysis

method is based on a staff technical paper

reported in Reference 1. 

The objective of this analysis was to

determine whether a trend exists in the ASP

precursor bin data. The results of the

trending analysis of the four probability bins

(>10-3, 10-4, 10-5, 10-6) for the FY 1993ñ2000

period are as follows:

CCDP 

Bin

Trend 

(FY 1993ñ2000a

)

>10-3 No statistically significant

trend

10-4 Decreasingñstatistically

significant

10-5 Decreasingñstatistically

significant

10-6 No statistically significant

trend

a. Results for FY 2000 are preliminary

A histogram of the occurrence rate as a

function of fiscal year for each probability bin

is provided in Figures 2a-d. The trend line of

the mean occurrence rate (with the 90%

confidence band) is shown in the two figures

with statistically significant trends. No trend

line is shown when a statistically significant

trend was not detected.

3.0 Insights

The number of significant precursors

presented in Section 3.1 cover the 

FY 1993ñ2001 period. The result for FY

2001 is based on the screening and review of

a combination of LERs and daily event

notification reports (10 CFR 50.72).

The insights presented in the remaining 

sections cover the FY 1993ñ2000 period. 

Although most of the results for FY 2000 are

preliminary, all preliminary analyses have

undergone in-house review. However, 

FY 2000 results are subject to change when

they become finalized.

3.1 Significant Precursors

The ASP Program is used to monitor the

agencyís performance against the following

Strategic Plan performance goal: ìNo more

than one event per year identified as a

significant precursor of a nuclear accident.î 

A significant precursor is defined in the

Strategic Plan as an event that has a 1/1000

(10-3) or greater probability of leading to a

reactor accident (Ref. 2).

No significant precursors were identified

during FYs 1999, 2000, and 2001 with a

CCDP or ∆CDP > 1.0 × 10-3. Significant

precursors have occurred, on the average,

about once every 4 to 5 years. The events in

this group involve differing failure modes,

causes, and systems.

Two precursors with a CCDP > 1.0 × 10-3

have occurred since 1991óthe Wolf Creek

event involving a reactor coolant system

draindown to the refueling water storage tank

during hot shutdown (1994) and the Catawba

2 event involving an extended plant-centered

loss of offsite power with an emergency

diesel generator out of service for

maintenance (1996). 

3.2 Important Precursors

Precursors with a CCDP or ∆CDP 

> 1.0 × 10-4 are considered important in the

ASP Program. There were no important

precursors in FY 1999. Two important

precursors were identified in FY 2000óthe

Cook 1 and 2 conditions involving potential

high-energy line breaks. 

The review of the ASP data reveals the

following:
3 Attachment

ï The mean occurrence rate of important

precursors has exhibited a decreasing

trend that is statistically significant during

the FY 1993ñ2000 period (Fig. 3). The

mean occurrence rate of precursors

decreased over this period by a factor of 6.

ï During the FY 1993ñ2000 period, 16

important precursors have occurred. Of

these, 38% involved a loss of offsite power

initiating event.

3.3 Initiating Events vs. Conditions

A precursor can be the result of an

operational event involving an actual initiating

event (e.g., loss of offsite power) or a

precursor can be a condition found during a

test, inspection, or engineering evaluation. A

condition involves a reduction in safety

system reliability or function for a specific

duration (and no initiator actually occurred

during this time).

Five of the seven precursors in FY 1999

involved conditions (unavailability of

equipment); two involved initiating events. In

FY 2000, 12 of the 14 precursors involved

conditions; two involved initiating events. 

A review of the data reveals the following:

ï The results for FY 1999 and FY 2000 are

consistent with the FY 1993ñ2000 results. 

Historically, conditions (64%) outnumbered

initiating events (36%). In FY 1999,

conditions contributed to 71% of the

precursors. In FY 2000, conditions

contributed to 86%. This indicates that

risk-significant conditions are being

identified prior to unplanned demands of

the degraded safety systems.

ï The mean occurrence rate of precursors

involving initiating events has exhibited a

decreasing trend that is statistically

significant during the FY 1993ñ2000 period

(Fig. 4). The occurrence rate of precursors

decreased over this period by a factor of 5.

ï No trend that was statistically significant

was detected for precursors involving

conditions (Fig. 5).

3.4 Precursors Involving Loss of Offsite

Power Initiating Events

One precursor involving a loss of offsite

power (LOOP) initiating event occurred in

both FY 1999 and FY 2000óthe Indian 

Point 2 loss of offsite power to safety-related

buses following a reactor trip (1999) and the

Diablo Canyon 1 extended loss of offsite

power (2000). 

The review of the ASP data reveals the

following:

ï The mean occurrence rate of LOOP

precursor events exhibited a decreasing

trend that is almost statistically significant

during the FY 1993ñ2000 period (Fig. 6). 

The occurrence rate of precursors

decreased over this period by a factor of

4. 

ï Over one-third (38%) of the LOOP

precursor events that occurred during 

FYs 1993ñ2000 are considered important

precursors (CCDP >1.0 x 10-4) in the ASP

Program.

ï A simultaneous unavailability of an

emergency power system train and a

LOOP were also involved in 3 of the 15

LOOP precursor events during the 

FY 1993ñ2000 period. Two of the

precursors involving a LOOP event and an

emergency power train unavailability had a

CCDP > 1×10-4. 

ï None of the precursors since 1989 have

involved a grid-related LOOP event.

3.5 Precursors at BWRs vs. PWRs

None of the precursors in FY 1999 and 

FY 2000 occurred at a boiling-water reactor

(BWR); there has been only one such

precursor since 1996. A review of the data

for BWRs and pressurized-water reactors

(PWRs) during the FY 1993ñ2000 period

reveals the following:

ï BWRs. The mean occurrence rate of

precursors at BWRs has exhibited a

decreasing trend that is statistically
4 Attachment

significant during the FY 1993ñ2000

period (Fig. 7). The mean occurrence

rate of precursors has decreased during

this period by a factor of 17.

ï PWRs. No trend that was statistically

significant was detected for the mean

occurrence rate of precursors at PWRs

(Fig. 8). Historically, precursors occur at a

PWR on an average of 10 per year. 

Results for FY 1999 (7 precursors) are

consistent with the historical average. 

The slight increase in the number of

precursors in FY 2000 (14 precursors) over

the historical average was attributed in part

to conditions related to design-basis issues

affecting multi-unit plantsóone condition

affecting all three units at Oconee and two

conditions affecting both units at Cook. 

According to the staffís review of individual

plant examinations (NUREG-1560, Ref. 3),

the core damage frequencies estimated in the

individual plant examinations were generally

lower for BWRs than for PWRs. NUREG1560 attributed the difference to the larger

number of injection systems in the BWR

design along with the ability to rapidly

depressurize to allow the use of low-pressure

injection systems. This may explain, in part,

the lower number of precursors at BWRs.

3.6 Precursors Caused by Unavailability

of Safety-Related Equipment

Most precursors involve the unavailability of

safety-related equipment. These events

occur during periods of extended

unavailability of equipment without a reactor

trip, or in combination with a reactor trip in

which a risk-important component is unable to

perform its safety function due to an

unavailability condition.

A review of the ASP data during the 

FY 1993ñ2000 time period produced the

following insights about the unavailability of

safety-related equipment.

Equipment unavailabilities in BWRs

ï Only 11 precursors occurred in BWRs

during the FY1993ñ2000 period. There

were too few events to detect trends in

equipment unavailabilities.

Emergency core cooling systems (ECCS)

ï An unavailability of safety-related highand/or low-pressure injection trains

contributed to 64% of all precursors in

PWRs. 

Most of these unavailabilities were caused

by failures in the ECCS (33%), emergency

power source (27%), and design-basis

issues of other systems/structures

impacting the ECCS or an ECCS support

system (25%). 

ï Of the 17 precursors that involved a failure

in an ECCS train:

- Sixteen precursors involved a conditional

unavailability found during testing,

inspection, or engineering reviews. This

indicates that conditions are being

identified prior to unplanned demand of

the degraded ECCS. 

- Eleven precursors involved a condition

affecting sump recirculation during

postulated loss-of-coolant accidents of

varying break sizes.

Auxiliary/emergency feedwater systems

(AFW/EFW)

ï The unavailability of AFW/EFW trains

contributed to 43% of all precursors in

PWRs. 

Most of these unavailabilities were caused

by failures in the AFW/EFW system

(29%), emergency power source (43%),

and design-basis issues of other

systems/structures impacting AFW/EFW

or an AFW/EFW support system (26%).

ï Of the 10 precursors that involved a failure

in an AFW/EFW train:

- Five of the train failures occurred

following a reactor trip.
5 Attachment

- Eight of the precursors involved the

unavailability of the turbine-driven

AFW/EFW pump train.

Emergency power sources in PWRs

ï The unavailability of emergency power

sources, such as emergency diesel

generators (EDGs) and hydroelectric

generators (at Oconee), contributed to

25% of all precursors in PWRs. 

Most of these unavailabilities were caused

by failures in the emergency power system

(75%). 

ï A simultaneous EDG unavailability and a

LOOP were also involved in three of the

LOOP-related precursors. Two of the

precursors involving a LOOP event and

EDG unavailability had a CCDP1.0×10-4. 

The third of these precursors was less risk

significant because restoration of the EDG

and/or offsite power were more likely

resulting in a lower CCDP. 

3.7 Annual ASP Index

The annual ASP Index is derived for order of

magnitude comparisons with industry average

core damage frequency (CDF) estimates

derived from probabilistic risk assessments

(PRAs) and individual plant examinations

(IPEs). The index for a fiscal year is the sum

of the conditional core damage probabilities

(CCDPs) divided by the number of reactor

years (RYs). 

Limitations. Using CCDPs from ASP results

to estimate CDF is difficult because (1) the

mathematical relationship requires a great

level of detail, (2) statistics for frequency of

occurrence of specific precursor events are

sparse, and (3) events and conditions that did

meet ASP precursor criteria also need to be

accounted for in the assessment. 

The ASP models and process do not explicitly

cover all core damage frequency scenarios,

such as fires, flooding, and external events,

and are therefore incomplete for estimating

total CDF. Also, using CCDP to estimate

CDF can overestimate the frequency because

of double counting. 

Because of these and other limitations, the

CCDPs have been used primarily as a

relative trending indication. Despite these

limitations, ASP results can be linked to CDF

by using an Annual ASP Index. The IPEs

also give incomplete estimates of total CDF,

but IPEs are reasonably similar in scope to

the current ASP Program. 

Results. For the last eight fiscal years, the

annual ASP index is as follows:

Fiscal

Year

Annual ASP

Index

(per RY)

1993

1994

1995

1996

1997

1998

1999

2000a

Average

1.4 x 10-5

3.1 × 10-5

2.1 × 10-6

2.9 × 10-5

4.5 x 10-7

5.9 x 10-6

9.6 x 10-7

1.1 x 10-5

1.2 x 10-5

a. Result for FY 2000 is preliminary 

The estimated CDFs in the IPEs range from

1.2 x 10-6/RY to 3.7 x 10-4/RY, with an

average value of 6.2 x 10-5/RY. On an order

of magnitude basis, the ASP Index over the

last eight fiscal years is consistent with the

order of magnitude of estimates of CDFs

from the IPEs. However, because of the

limitations discussed above, the ASP results

are not sufficient to verify the IPE results. 

3.8 Consistency with PRAs/IPEs

Most of the precursor events are consistent

with failure combinations identified in

probabilistic risk assessments (PRAs) and

individual plant examinations (IPEs). A

review of the precursor events for the period

1994ñ2000 shows that 19% of the precursors

involved event initiators or conditional

availability of equipment not typically
6 Attachment

modeled in PRAs or IPEs. 

Precursors not typically modeled in PRAs and

IPEs are listed in Table 6.

4.0 References

1. Rasmuson, D.M., and P.D. OíReilly. 

ìAnalysis of Annual Accident Sequence

Precursor Occurrence Rates for

1984ñ94,î in Proceeding of the

International Topical Meeting on

Probabilistic Safety Assessment. 

American Nuclear Society (ANS), 

Park City, Utah. 29 Septemberñ

3 October 1996. Vol. III, pp. 1645-1652. 

ANS: LaGrange Park, Illinois. 1994.

2. U.S. Nuclear Regulatory Commission. 

NUREGñ1614, Vol. 2, Part 1, ìStrategic

Plan, Fiscal Year 2000 - Fiscal Year

2005.î NRC: Washington, D.C.

September 2001.

3. U.S. Nuclear Regulatory Commission. 

NUREGñ1560, ìIndividual Plant

Examination Program: Perspectives on

Reactor Safety and Plant Performance,

Summary Report.î 

NRC: Washington, D.C. October 1996.
7 Attachment

Table 1. Status of ASP analysis as of 02/02/02.

Precursor analysis status FY 1999 FY 2000 FY 2001

Final analysis issued 7 4 0

Preliminary analyses completed 0 10 3

Preliminary analysis underway (includes

events that will be rejected as precursors)

0 0 18

Table 2. FY 1999 at-power precursors involving initiating events.

Plant Description/Event Identifier

Plant

type

Event

date CCDP

Event

type

Davis -

Besse

Manual reactor trip while recovering from

a component cooling system leak and

de-energization of safety-related bus D1

and non-safety bus D2 

(LER 346/98-011)

PWR 10/14/98 1.4 × 10-5 Transient

Indian

Point 2

Loss of offsite power to safety-related

buses following a reactor trip and an

emergency diesel generator output

breaker trip (LER 247/99-015)

PWR 08/31/99 2.8 × 10-6 Loss of

offsite

power

Table 3. FY 1999 at-power precursors involving conditional unavailabilities.

Plant Description/Event identifier

Plant

type

Event

date CCDP

Importance

(CCDP ñ CDP)

Event

type

Oconee 

1, 2, and 

3 a

Postulated high-energy line

leaks or breaks leading to

failure of safety-related 4 kV

switchgear 

(LER 269-99-001)

PWR 2/24/99 Unit 1

3.4 × 10-5 8.2 × 10-6

Unavailability

Unit 2

3.2 × 10-5 5.6 × 10-6

Unit 3

3.1 × 10-5 5.2 × 10-6

Cook 

1 and 2 a

Lack of capability to operate

emergency service water

following a seismic event

(NRC Inspection reports

50-315/316/97-024 and

50-315/316/99-010)

PWR 6/11/99 5.2 x 10-5 

 

3.2 × 10-5 Unavailability

Note:

a. Multiple precursorsóone for each unit.
8 Attachment

Table 4. FY 2000 at-power precursors involving initiating events as of 02/02/02. (Preliminary

results are subject to change.)

Plant Description/Event Identifier

Plant

type

Event

date CCDP

Event

type

Indian

Point 2

Manual reactor trip following a steam

generator tube failure (LER 247/00-001)

PWR 2/15/00 8.0 x 10-5

(Preliminary)

Steam

Generator

Tube

Rupture

Diablo

Canyon 1

Extended loss of offsite power to safetyrelated buses due to 12-kV bus fault

(LER 275/00-04)

PWR 5/15/00 9.7 × 10-5

(Preliminary)

LOOP

Table 5. FY 2000 at-power precursors involving conditional unavailabilities as of 02/02/02.

(Preliminary results are subject to change.)

Plant Description/Event identifier

Plant

type

Event

date CCDP

Importance

(CCDP ñ CDP)

Event

type

Cook 

1 and 2 a

Potential high-energy line

break conditions affecting the

operability of mitigating

systems (LER 315/99-026)

PWR 10/22/99 4.5 × 10-4 4.3 × 10-4 Unavailability

Salem 2 Degraded CO2 fire

suppression system in the

4160Vac switchgear room

(LER 272/99-011)

PWR 11/15/99 4.1 × 10-6

(Preliminary)

3.7 × 10-6

(Preliminary)

Unavailability

Cook 

1 and 2 a

Valves required to operate

post-accident could fail to

open due to pressurelocking/thermal binding 

(LER 315/99-031)

PWR 12/30/99 5.7 × 10-5 3.7 × 10-5 Unavailability

Oconee 

1, 2, and

3 a

Potential inoperability of a

HPI pump following a

tornado of F3, F4, or F5

severity (NRC Inspection

Report 269/00-11)

PWR 4/1/00 Unit 1

4.2 × 10-5

(Preliminary)

5.0 × 10-6

(Preliminary)

Unavailability

Units 2, 3

2.8 × 10-5

(Preliminary)

3.0 × 10-6

(Preliminary)

Harris Degraded fire barriers and

inadequate fire brigade

practice for switchgear 

room B (NRC Inspection

Report 400/00-16)

PWR 6/26/00 3.3 x 10-6

(Preliminary)

3.0 x 10-6

(Preliminary)

Unavailability

Harris Charging/SI Pump ìCî

inoperable for a time in

excess of the TS LCO action

statement requirements 

(LER 400/00-007)

PWR 9/4/00 7.7 x 10-6

(Preliminary)

4.2 x 10-6

(Preliminary)

Unavailability

Table 5. (Continued)
9 Attachment

Plant Description/Event identifier

Plant

type

Event

date CCDP

Importance

(CCDP ñ CDP)

Event

type

Summer Discharge valve to turbinedriven auxiliary feedwater

pump locked closed for 48

days (LER 395/00-006)

PWR 9/21/00 5.1 x 10-6

(Preliminary)

4.2 x 10-6

(Preliminary)

Unavailability

Millstone

2

Failure of turbine-driven

auxiliary feedwater pump

during testing (NRC

Inspection Report 336/00-11)

PWR 9/20/00 7.9×10-6

(Preliminary)

7.2 × 10-6

(Preliminary)

Unavailability

Note:

a. Multiple precursors---one for each unit.

Table 6. Precursors not typically modeled in PRAs or IPEs.

Year Plant(s) Event description

1994 Wolf Creek Blowdown of the reactor coolant system to the refueling water

storage tank during hot shutdown

1996 Wolf Creek Reactor trip with the loss of one train of emergency service water

due to the formation of frazil ice on the circulating water traveling

screens and the unavailability of the turbine-driven auxiliary

feedwater pump

1996 LaSalle 1 and 2 Fouling of the cooling water systems due to concrete sealant

injected into the service water tunnel

1996 Haddam Neck Potentially inadequate residual heat removal pump net positive

suction head following a large- or medium-break loss-of-coolant

accident due to design errors

1998 Oconee 1, 2, and 3 Incorrect calibration of the borated water storage tank (BWST) level

instruments resulted in a situation where the emergency operating

procedure (EOP) requirements for BWST-to-reactor building

emergency sump transfer would never have been met; operators

would be working outside the EOP

1998 Cook 2 Potential failure of all component cooling water pumps due to

steam intrusion resulting from a postulated high-energy line break

1999 Oconee 1, 2, and 3 Postulated high-energy line leaks or breaks in turbine building

leading to failure of safety-related 4 kV switchgear 

1999 Cook 1 and 2 Postulated high-energy line leaks or breaks in turbine building

leading to failure of multiple safety-related equipment
10 Attachment

Figure 1. All precursorsóoccurrence rate, by fiscal year. The

decreasing trend is almost statistically significant (p-value1

 = 0.083). 

The result for FY 2000 is preliminary.

_________________

1

 The p-value is the probability of observing a trend as a result of chance alone. A p-value is considered

statistically significant if the p-value is smaller than 0.05. 
11 Attachment

Figure 2a. Precursors in CCDP bin 10-3óoccurrence rate, by fiscal

year. No trend line shown because no trend detected that is

statistically significant (p-value = 0.3556). The result for FY 2000 is

preliminary.

Figure 2b. Precursors in CCDP bin 10-4óoccurrence rate, by fiscal

year. The decreasing trend is statistically significant (p-value =

0.0204). The result for FY 2000 is preliminary.

Figure 2c. Precursors in CCDP bin 10-5óoccurrence rate, by fiscal

year. The decreasing trend is statistically significant (p-value =

0.0170). The result for FY 2000 is preliminary.
12 Attachment

Figure 3. ìImportantî precursors (CCDP > 10-4)óoccurrence rate,

by fiscal year. The decreasing trend is statistically significant 

(p-value = 0.0126). The result for FY 2000 is preliminary.

Figure 2d. Precursors in CCDP bin 10-6óoccurrence rate, by fiscal

year. No trend line shown because no trend detected that is

statistically significant (p-value = 0.2091). The result for FY 2000 is

preliminary.
13 Attachment

Figure 4. Precursors involving initiating eventsóoccurrence rate,

by fiscal year. The decreasing trend is statistically significant 

(p-value = 0.0025). The result for FY 2000 is preliminary.

Figure 6. Precursors involving loss of offsite power initiating

eventsó occurrence rate, by fiscal year. The decreasing trend is

almost statistically significant (p-value = 0.0699). Result for FY 2000 is

preliminary.

Figure 5. Precursors involving conditional unavailability of

equipmentóoccurrence rate, by fiscal year. No trend line shown

because no trend detected that is statistically significant 

(p-value = 0.9495). The result for FY 2000 is preliminary.
14 Attachment

Figure 8. Precursors involving PWRsóoccurrence rate, by fiscal

year. No trend line shown because no trend detected that is

statistically significant (p-value = 0.4295). The result for FY 2000 is

preliminary.

Figure 7. Precursors involving BWRsóoccurrence rate, by fiscal

year. The decreasing trend is statistically significant (p-value =

0.0024). Result for FY 2000 is preliminary.