POLICY ISSUE 

INFORMATION 

April 9, 2012 SECY-12-0056

FOR: The Commissioners 

FROM: Eric J. Leeds, Director

Office of Nuclear Reactor Regulation 

SUBJECT: FISCAL YEAR 2011 RESULTS OF THE INDUSTRY TRENDS 

PROGRAM FOR OPERATING POWER REACTORS 

PURPOSE: 

The purpose of this paper is to inform the Commission of the results of the U.S. Nuclear 

Regulatory Commission’s (NRC’s) Industry Trends Program (ITP) for fiscal year (FY) 2011. 

This paper does not propose any new actions or commitments. 

BACKGROUND: 

The NRC staff implemented the ITP in 2001 to monitor for adverse trends in safety performance 

based on industry-level indicators. After the NRC assesses adverse trends for safety 

significance, it responds as necessary to any identified safety issues, including adjustments to 

the inspection and licensing programs. One important output of the ITP is the annual agency 

performance measures reported to Congress on the number of statistically significant adverse 

industry trends in safety performance. This outcome measure is part of the NRC Performance 

and Accountability Report. In addition, the NRC annually reviews the results of the ITP and any 

actions taken or planned during the Agency Action Review Meeting. The NRC reports the 

findings of this review to the Commission. This paper is the 11th annual report to the 

Commission on the ITP. 

NRC Inspection Manual Chapter (IMC) 0313, “Industry Trends Program,” dated May 29, 2008, 

contains details of the ITP, including definitions of monitored indicators and program 

descriptions. 

CONTACT: Michael F. Balazik, NRR/DIRS 

 301-415-2856
The Commissioners - 2 - 

DISCUSSION: 

Using the ITP, the staff monitors industry safety performance to identify and address adverse 

industry trends. The indicators are comprehensive and based on the best available data. An 

adverse trend exists if the slope of the regression line fitted to the long-term indicator data has a 

positive value. 

The ITP also uses precursor events identified by the Accident Sequence Precursor (ASP) 

Program to assess industry performance. The staff analyzes the occurrence rate of precursors 

to determine if an adverse trend exists. The staff uses the ASP results as one of the agency’s 

monitored indicators. 

 

In addition to the long-term indicators, the ITP uses a statistical approach based on prediction 

limits to identify potential short-term, year-to-year emergent issues before they become 

long-term trends. 

The ITP complements the Reactor Oversight Process (ROP); the ITP monitors industry-level 

performance, whereas the ROP provides oversight of individual plant conditions and events. 

The Office of Regulatory Research (RES) provides indirect support to the ITP in the areas of 

operating experience data and models that are developed and budgeted under other RES 

programs, such as the Standardized Plant Analysis Risk Model Development Program, the ASP 

Program, and the Reactor Operating Experience Data Collection and Analysis Program. The 

ITP uses the results of RES work in the ASP Program to assess industry performance, although 

the funding and performance of RES work are completely separate from the ITP. 

Outside of the normal ITP process, internal stakeholders noted an increase in the number of 

special inspections (SI) in calendar year (CY) 2011 as compared to CY 2009 and CY 2010. The 

staff reviewed SI data from 2006-2011 to evaluate whether a trend existed. The staff concluded 

that there was no trend over the period. Although there was an increase in the number of 

reactive inspections performed in calendar year (CY) 2011 compared to the number 

performed in CY 2009 and CY 2010, this is a reflection of fewer reactive inspections in CY 2009 

and CY 2010 than in previous years. The number of reactive inspections performed in CY 2011 

was comparable to the numbers from CY 2007 and CY 2008, and lower than those performed in 

CY 2006. 

FY 2011 LONG-TERM INDUSTRY TRENDS: 

Based on the ITP indicators and the ASP Program results, the staff did not identify any 

statistically significant adverse trends in industry safety performance through the end of 

FY 2011. The graphs in Enclosure 1 show the long-term ITP indicator trends and the ASP data. 

The ASP Program considers an event with a conditional core damage probability (CCDP) or an 

increase in core damage probability (ΔCDP) greater than or equal to 1x10-6 to be a precursor. 

The RES staff evaluated precursor data from FY 2001 to FY 2010 and identified no statistically 

significant trends for the occurrence rate of all precursors during that period (Figure 14 of 

Enclosure 1). Additionally, the staff identified statistically significant decreasing trends for 

precursors with a CCDP or ΔCDP greater than or equal to 1x10-4 and for precursors that 
The Commissioners - 3 - 

occurred at pressurized-water reactors during this same period. The data period for ASP 

trending analysis is a rolling 10-year period. 

The ASP Program also provides the basis for the safety performance measure of zero 

significant accident sequence precursors of a nuclear reactor accident. This is one measure 

that is associated with the safety goal that the NRC established in its Strategic Plan. 

A significant precursor is an event that has a probability of at least 1 in 1,000 (i.e., CCDP or 

ΔCDP greater than or equal to 1x10-3) of leading to a reactor accident. 

The one event evaluated as a potential significant precursor in FY 2010 involved an electrical 

fire at H.B. Robinson Steam Electric Plant that led to a reactor trip with a subsequent loss of 

reactor coolant pump seal cooling and additional complications. The potential for the event to 

be a significant precursor was identified after new information became available in 

December 2010 during followup inspection activities. The NRC staff issued a preliminary ASP 

analysis and transmitted it to the licensee in accordance with established procedures. This 

prompted the licensee to perform an additional thermal-hydraulic analysis, which resulted in 

changes to modeling assumptions and reduced the CCDP of the event to 4×10-4, which is lower 

than the CCDP threshold value of a significant precursor. 

No significant precursors were identified in FY 2011. However, the staff will continue to 

evaluate the flooding episode at Fort Calhoun Station and the earthquake at North Anna Power 

Station, Units 1 and 2, to identify any significant precursors. 

FY 2011 SHORT-TERM INDUSTRY PERFORMANCE: 

The staff uses a statistical approach based on prediction limits to identify potential short-term, 

year-to-year emergent issues before they become long-term trends. Enclosure 2 shows the 

short-term results and the prediction limits for each of the ITP indicators. Based on current 

information, none of the indicators exceeded its prediction limit in FY 2011. Short-term FY 2011 

data did not reveal any emerging trends that warranted additional analysis or significant 

adjustments to the nuclear reactor safety inspection or licensing programs. 

FY 2011 RESULTS OF BASELINE RISK INDEX FOR INITIATING EVENTS: 

In 2008, the NRC staff implemented the Baseline Risk Index for Initiating Events (BRIIE) as part 

of the ITP. The BRIIE (1) tracks several types of events that could potentially start (“initiate”) a 

challenge to a plant’s safety systems, (2) assigns a value to each initiating event according to its 

relative importance to the plant’s overall risk of damage to the reactor core, and (3) calculates 

an overall indicator of industry safety performance. 

The BRIIE concept provides a two-level approach to industry performance monitoring. The first 

level (referred to as Tier 1 performance monitoring) tracks and counts the number of times the 

initiating events that have an impact on plant safety occur in nuclear power plants during the 

year. Nine initiating event categories are monitored for boiling-water reactors and 10 for 

pressurized-water reactors. The number of times that each event occurs is compared to a 

predetermined number of occurrences for that event. If the predetermined number is exceeded, 

one can infer the possible degradation of industry safety performance. This annual tracking 
The Commissioners - 4 - 

allows the NRC to intervene and engage the nuclear industry before any long-term adverse 

trends in performance emerge. 

The second level (referred to as Tier 2 performance monitoring) addresses the risk to plant 

safety and core damage that each of the initiating events contributes. Each event is assigned 

an importance value, a ranking based on its relative contribution to overall risk to plant safety. 

The greater the contribution of the event to overall risk, the higher the importance value it is 

assigned. Using statistical methods, the importance values are combined with the number of 

times the events occur during the year to calculate a number that indicates how much the 

overall industry risk of damage to the reactor core has changed from a baseline value. If the 

BRIIE-combined industry value reaches or exceeds a threshold value of 1x10-5 per reactor 

critical year, the NRC informs Congress of this performance outcome, along with actions that 

already have been taken or are planned in response, in the NRC Performance and 

Accountability Report. 

Enclosure 3 provides the Tier 1 and Tier 2 BRIIE results. The NRC staff identified one initiator, 

Loss of Offsite Power (LOOP), for which the 2011 occurrence rate exceeded the Tier 1 

prediction limit. The seven LOOP events representing essentially three occurrences at three 

different multiunit sites are as follows: 

• Surry Units 1 and 2, lost offsite power when a tornado damaged the switchyard on 

April 16, 2011. 

• Browns Ferry Units 1, 2, and 3, lost offsite power when high winds and tornadoes 

damaged the area’s transmission lines on April 27, 2011. 

• North Anna Units 1 and 2, lost offsite power during an earthquake on 

August 23, 2011. 

These events all resulted from natural phenomena and were outside of the licensee’s control. 

As such, they are not indicative of degraded industry performance. In each instance, the 

LOOPs occurred on a particular day and depended on a single initiator. The prediction limit is 

determined assuming independent event occurrences. Tier 1 activities are intended to reveal 

degrading industry performance before any long-term adverse trends emerge. These LOOP 

events were accounted for in other ITP long-term performance indicators with no statistically 

significant adverse trends identified. Nevertheless, the staff will share information with other 

Office of Nuclear Reactor Regulation (NRR) staff involved in updates to Title 10 of the Code of 

Federal Regulations (10 CFR) Section 50.63 (Loss of All Alternating Current Power-Station 

Blackout Rule) for their consideration. No program adjustments are recommended at this time; 

however the staff will continue to monitor this occurrence for future program adjustments if 

warranted. For all other Tier 1 initiators, the FY 2011 occurrence rates are below the 

associated prediction limits. Figure 15 of Enclosure 3 shows that the BRIIE combined industry 

value in FY 2011 (2.98x10-7 per reactor critical year) indicates above baseline industry 

performance and is below the established reporting threshold of ∆CDF = 1.0x10-5 per reactor 

critical year. 
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RESOURCES: 

For ITP activities, resources are included in the FY 2012 Budget of 0.5 full-time equivalent 

(FTE) and $535K; and in the FY 2013 President’s Budget of 0.5 FTE and $575K. These 

resources are to conduct ongoing ITP implementation in FY 2012 and FY 2013. Any additional 

resources required in the future years will be addressed during the Planning, Budgeting, and 

Performance Management process. 

COORDINATION: 

The Office of the Chief Financial Officer has reviewed this paper and concurs. The Office of the 

General Counsel has reviewed this paper and has no legal objection. 

 /RA/ 

 Eric J. Leeds, Director 

Office of Nuclear Reactor Regulation 

Enclosures: 

1. Fiscal Year 2011 Long-Term Industry 

 Trend Results 

2. Fiscal Year 2011 Short-Term Industry 

 Performance 

3. Summary of Baseline Risk Index for 

 Initiating Events: Annual Graphs 

 through Fiscal Year 2011
The Commissioners -5- 

RESOURCES: 

For ITP activities, resources are included in the FY 2012 Budget of 0.5 full-time equivalent 

(FTE) and $535K; and in the FY 2013 President’s Budget of 0.5 FTE and $575K. These 

resources are to conduct ongoing ITP implementation in FY 2012 and FY 2013. Any additional 

resources required in the future years will be addressed during the Planning, Budgeting, and 

Performance Management process. 

COORDINATION: 

The Office of the Chief Financial Officer has reviewed this paper and concurs. The Office of the 

General Counsel has reviewed this paper and has no legal objection. 

 

 Eric J. Leeds, Director 

Office of Nuclear Reactor Regulation 

Enclosures: 

1. Fiscal Year 2011 Long-Term Industry 

 Trend Results 

2. Fiscal Year 2011 Short-Term Industry 

 Performance 

3. Summary of Baseline Risk Index for 

 Initiating Events: Annual Graphs 

 through Fiscal Year 2011 

WITS 200100034/EDATS: SECY-2011-0208 (G20110755) 

ADAMS Accession Number: ML12065A340 *concurred via email 

OFFICE NRR/DIRS/IPAB BC:NRR/DIRS/IPAB Tech Editor* BC:NRR/DIRS/IOEB BC:NRR/PMDA/BFEB*

NAME MBalazik RFranovich JDougherty HChernoff HLe 

(KNewell for) 

DATE 02/27/12 03/08/2012 02/24/2012 03/09/2012 03/14/2012 

OFFICE D:NRR/DIRS OCFO* OGC (NLO) RES* D:NRR 

NAME FBrown DPelton MSpencer BSheron ELeeds 

DATE 03/08/2012 03/22/2012 03/20/2012 03/27/2012 04/09/2012 

OFFICIAL RECORD COPY
ENCLOSURE 1 

FISCAL YEAR 2011 LONG-TERM INDUSTRY TREND RESULTS 

The staff of the U.S. Nuclear Regulatory Commission did not observe any statistically significant 

adverse trends in the Industry Trends Program performance indicator data from the most recent 

10 years (fiscal years 2002–2011), as indicated by the figures below.

Figure 1 Automatic scrams while critical

Scrams per Unit
-2- 

Figure 2 Safety system actuations

Figure 3 Significant events 

While the slope of the trend line model is in the adverse direction (Figure 3), the statistical 

analysis of the Significant Events data indicates that the trend is not statistically significant. 

0.05

0.07

0.04

0.05

0.03

0.02

0.03

0.02

0.10

0.06

0

0.04

0.08

0.12

2002 2003 2004 2005 2006 2007 2008 2009 2010 2011

Events per Unit

Fiscal Year

Industry Average

Industry Average Trend

Events 

per Unit Actuations per Unit
-3-

Figure 4 Safety system failures 

Figure 5 Forced outage rate

Failures per Unit Forced Outage Rate (%)
-4-

Figure 6 Equipment forced outages 

Figure 7 Collective radiation exposure

Equipment Force Outage Rate per 1,000 

Commerical Critical Hours Average Exposure per Unit (Person-REM)
-5-

Figure 8 Unplanned power changes

Figure 9 Reactor coolant system activity

Power Changes per 7,000 Critical Hours Maximum Percentage of Technical 

S

pecification Limit
-6-

Figure 10 Reactor coolant system leakage

Figure 11 Drill and exercise performance

Percentage Timely and Accurate Maximum Percentage of Technical 

Specification Limit
-7-

Figure 12 Emergency response organization drill participation

Figure 13 Alert and notification system reliability

Percentage of Key Personnel Percentage of Successful Tests
-8-

Figure 14 Accident sequence precursors

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

2001 2002 2003 2004 2005 2006 2007 2008 2009 2010

All Precursors Occurrence Rate

Fiscal Year

Mean (FYs 2001 –2010)

Precursors Mean Occurrence Rate
ENCLOSURE 2 

FISCAL YEAR 2011 SHORT-TERM INDUSTRY PERFORMANCE

The annual industry trend analysis compares data for the most recent year to established 

short-term “prediction limits.” The prediction limits are 95th percentiles of predictive distributions 

for the data. The predictive distributions are statistical probability distributions that describe 

expected future performance. They are derived from performance during “baseline” periods for 

each performance indicator (PI). Baseline periods are periods for each PI during which the data 

can be regarded as fairly constant and indicative of “current” performance. 

Based on current information and using established prediction limits, the staff’s evaluation of the 

fiscal year (FY) 2011 Industry Trends Program PIs indicates that no PI exceeded its associated 

prediction limit in FY 2011, as shown in the following figures. However, the staff has not 

finalized the probabilistic risk analyses for several events that occurred in FY 2011. These 

events have the potential to be classified as Significant Events as defined by IMC 0313. The 

events include the earthquake that affected North Anna and the LOOPs that occurred at Browns 

Ferry and Surry. The Significant Events indicator may exceed its short-term prediction limit 

based on the finalized data associated with these events. The staff currently attributes the 

events to external phenomena that were beyond the licensee’s control and unrelated to safety 

performance. Additionally, these external events affected three multiunit sites for a total of 

seven reactor units, which drives the indicator value up. If the Significant Events indicator 

exceeds the prediction limit based on finalized data analysis, the staff will evaluate the factors 

contributing to the events, determine if changes to the Reactor Oversight Process are 

warranted, and provide an overview of its evaluation and conclusions in an addendum to this 

SECY paper. 

Figure 1 Automatic scrams while critical

0.00

0.25

0.50

0.75

1.00

2002 2003 2004 2005 2006 2007 2008 2009 2010 2011

Scrams per Unit

Fiscal Year

Industry Average

Prediction Limit (0.673)

Baseline Industry Average (0.534), 1997-2002
-2- 

Figure 2 Safety system actuations 

Figure 3 Significant events

0.00

0.20

0.40

0.60

2002 2003 2004 2005 2006 2007 2008 2009 2010 2011

Actuations per Unit

Fiscal Year

Industry Average

Excluded August 2003 Blackout Event

Prediction Limit (0.375)

Baseline Industry Average (0.275), 1998-2005

0.00

0.03

0.06

0.09

0.12

0.15

2002 2003 2004 2005 2006 2007 2008 2009 2010 2011

Events per Unit

Fiscal Year

Industry Average

Prediction Limit (0.106)

Baseline Industry Average (0.524), 2000-2004
-3- 

Figure 4 Safety system failures 

Figure 5 Forced outage rate 

0.00

0.50

1.00

1.50

2002 2003 2004 2005 2006 2007 2008 2009 2010 2011

Failures per Unit

Fiscal Year

Industry Average

Prediction Limit (1.04)

Baseline Industry Average (0.859), 2001-2004

0

2

4

6

2002 2003 2004 2005 2006 2007 2008 2009 2010 2011

Forced Outage Rate % 

Fiscal Year

Industry Average

Prediction Limit (4.28)

Baseline Industry Average (2.40), 2001-2004
-4- 

Figure 6 Equipment forced outages 

Figure 7 Collective radiation exposure

0.00

0.05

0.10

0.15

0.20

0.25

2002 2003 2004 2005 2006 2007 2008 2009 2010 2011

Equipment Forced Outage Rate per

1,000 Commerical Critical Hours

Fiscal Year

Industry Average

Prediction Limit (0.180)

Baseline Industry Average (0.142), 1998-2004

0

50

100

150

200

2002 2003 2004 2005 2006 2007 2008 2009 2010 2011

Average Exposure per Unit (Person-REM)

Fiscal Year

Industry Average

Prediction Limit (139)

Baseline Industry Average (119), 1999-2002
-5- 

Figure 8 Unplanned power changes per 7,000 critical hours

Figure 9 Reactor coolant system activity

0.00

0.25

0.50

0.75

1.00

2002 2003 2004 2005 2006 2007 2008 2009 2010 2011

Maximum Percentage of Tech. Spec. Limits

Fiscal Year

Industry Average

Prediction Limit (0.646)

Baseline Industry Average (0.462), 2001-2004

0

2

4

6

8

10

2002 2003 2004 2005 2006 2007 2008 2009 2010 2011

Maximum Percentage of Tech. Spec. Limits

Fiscal Year

Industry Average

Prediction Limit (6.32)

Baseline Industry Average (5.14), 2000-2004

Maximum Percentage of Technical 

Specification Limit

Maximum Percentage of Technical 

S

pecification Limit
-6- 

Figure 10 Reactor coolant system leakage

Figure 11 Drill and exercise performance

0

2

4

6

8

10

2002 2003 2004 2005 2006 2007 2008 2009 2010 2011

Maximum Percentage of Tech. Spec. Limits

Fiscal Year

Industry Average

Prediction Limit (6.32)

Baseline Industry Average (5.14), 2000-2004

92

94

96

98

100

2002 2003 2004 2005 2006 2007 2008 2009 2010 2011

Percentage Timely and Accurate

Fiscal Year

Industry Average

Prediction Limit (95.21)

Baseline Industry Average (95.62), 2000-2004

Percenta

ge Timel

y and Accurate Maximum Percentage of Technical 

Specification Limits
-7- 

Figure 12 Emergency response organization drill participation

Figure 13 Alert and notification system reliability 

94

96

98

100

2002 2003 2004 2005 2006 2007 2008 2009 2010 2011

Percentage of Key Personnel

Fiscal Year

Industry Average

Prediction Limit (97.50)

Baseline Industry Average (97.87), 2003-2007

97

98

99

100

2002 2003 2004 2005 2006 2007 2008 2009 2010 2011

Percentage of Tests Successful

Fiscal Year

Industry Average

Prediction Limit (98.35)

Baseline Industry Average (98.87), 2001-2004

Percenta

ge of Successful Tests Percenta

ge of Ke

y Personnel
-8- 

Note that the 2003 blackout event in the safety system actuations graph (Figure 2) and the 

2000 steam generator tube rupture event at Indian Point Nuclear Generating Unit 2 in the 

reactor coolant system leakage graph (Figure 10) were not included in the short-term data for 

determining prediction limits. They were excluded from the development of the prediction limit 

models because they are considered outlier events that overly influenced the statistical analysis 

of the industry-wide data. This treatment results in a more conservative prediction limit. 
ENCLOSURE 3 

SUMMARY OF BASELINE RISK INDEX FOR INITIATING EVENTS: 

ANNUAL GRAPHS THROUGH FISCAL YEAR 2011 

The Baseline Risk Index for Initiating Events (BRIIE) addresses the initiating event (IE) 

cornerstone in the U.S. Nuclear Regulatory Commission’s Reactor Oversight Process for 

monitoring commercial nuclear power plants. It is based on plant performance for the 

10 initiator events listed in the table below. 

 INITIATOR ACRONYM APPLICABLE PLANTS 

 General transient TRAN Both plant types, separately 

 Loss of condenser heat sink LOCHS Both plant types, separately 

 Loss of main feedwater LOMFW Both plant types 

 Loss of offsite power LOOP Both plant types 

Loss of vital alternating 

current bus LOAC Both plant types 

Loss of vital direct current 

bus LODC Both plant types 

Stuck-open safety/relief 

valve SORV Both plant types, separately 

 Loss of instrument air LOIA Both plant types, separately 

Very small loss-of-coolant 

accident VSLOCA Both plant types 

Steam generator tube 

rupture SGTR Pressurized-water reactors 

(PWRs) only 

The BRIIE program, described in NUREG/CR-6932, “Baseline Risk Index for Initiating Events 

(BRIIE),” issued June 2007, consists of two levels or tiers. The first tier considers individual IEs 

and evaluates performance based on statistical prediction limits. This evaluation is for the 

ongoing monitoring and early detection of possible industry-level deficiencies. A second tier is a 

risk-based integrated measure evaluated for each plant type. Because four of the initiators 

have separate data for each plant type, there are a total of 14 Tier 1 graphs. 

The units for the Tier 1 IE frequency graphs are event counts for a fiscal year divided by the 

industry critical time for the year. The Tier 1 graphs also show the average frequency for an 

established “baseline period” and 95-percent prediction limits for a future year if occurrences 

continue at the same rate as in the baseline period. If industry data shift as time progresses, 

the baseline periods used to determine the prediction limits may no longer be relevant. The 

periods originally were developed to describe, roughly, calendar years 1998–2002. 

The events were reviewed in early 2010. Several events in the loss of condenser heat sink and 

loss of main feedwater categories were reclassified to reflect more accurately the actual effect 

on the plant. After the data were reclassified, the existing baseline periods were checked to see 

if any trends were present that would make the periods no longer appropriate for describing the 

ongoing data. Because such trends were not found, the baseline periods were not changed. 

However, new prediction limits were identified for these categories with reduced data that are 

more appropriate for the way ongoing events are now classified and that allow the Tier 1 BRIIE 

assessment to remain realistic and not overly conservative. 

The prediction limits depend on the expected critical years of reactor operation in the upcoming 

year and the baseline occurrence rate for each indicator. A rate can exceed a limit by having 
-2-

more events than expected or by having the same number of events and less critical time than 

expected. In recent years, U.S. nuclear power plant availability has been approximately 

90 percent at the industry level. This figure enters into the calculations that determine the 

bounds on the number of events that might be expected. 

The NRC staff identified that for one initiator, LOOP, for which the FY 2011 occurrence rate 

exceeded the Tier 1 prediction limit. The seven LOOP events representing essentially three 

occurrences at three different multiunit sites are as follows: 

• Surry Units 1 and 2, lost offsite power when a tornado damaged the switchyard on 

April 16, 2011. 

• Browns Ferry Units 1, 2, and 3, lost offsite power when high winds and tornadoes 

damaged the area’s transmission lines on April 27, 2011. 

• North Anna Units 1 and 2, lost offsite power during an earthquake on 

August 23, 2011. 

These events all resulted from natural phenomena and were outside of the licensee’s control. 

As such, they are not indicative of degraded industry performance. In each instance, the 

LOOPs occurred on a particular day and depended on a single initiator. The prediction limit is 

determined assuming independent event occurrences. Tier 1 activities are intended to reveal 

degrading industry performance before any long-term adverse trends emerge. These LOOP 

events were accounted for in other ITP long-term performance indicators with no statistically 

significant adverse trends identified. Nevertheless, the staff will share information with other 

NRR staff involved in updates to 10 CFR 50.63 (Loss of All Alternating Current Power-Station 

Blackout Rule) for their consideration. No program adjustments are recommended at this time; 

however the staff will continue to monitor this occurrence for future program adjustments if 

warranted. For all other Tier 1 initiators, the FY 2011 occurrence rates are below the 

associated prediction limits. 

The Tier 2 integrated index includes for each plant type the relative contribution of each initiator 

to the risk of core damage, based on the events that occurred in each fiscal year. The event 

frequencies are converted to core damage frequency (CDF) estimates by multiplying by 

Birnbaum risk coefficients. These coefficients are industry averages of the contribution to core 

damage from each initiator as reflected in the industry standardized plant analysis risk models. 

Figure 15 shows annual differences in estimated industry CDF compared to the established 

baseline levels of these quantities. The combined industry BRIIE value for FY 2011 (2.98x10-7

per reactor critical year) indicates a shift from the last several years of below baseline industry 

performance to slightly above baseline industry performance. This shift is mainly attributed to 

LOOP events dominating the estimated core damage frequency for both plant types. The 

combined industry BRIIE value is below the established reporting threshold of ∆CDF = 1.0x10-5 

per reactor critical year. 
-3-

Figure 1 Pressurized-water reactor (PWR) general transients

Figure 2 Boiling-water reactor (BWR) general transients 

Events 

per Reactor Critical Year Events 

per Reactor Critical Year
-4-

Figure 3 PWR loss of condenser heat sink

Figure 4 BWR loss of condenser heat sink 

Events 

per Reactor Critical Year Events 

per Reactor Critical Year
-5-

Figure 5 Loss of main feedwater

Figure 6 Loss of offsite power 

The prediction limit for loss of offsite power was calculated under the assumption that the eight 

at power events that occurred during the 2003 blackout were a single event. This treatment 

results in a more conservative prediction limit. Events 

per Reactor Critical Year Events 

per Reactor Critical Year
-6-

Figure 7 Loss of vital alternating current bus

Figure 8 Loss of vital direct current bus

Events 

per Reactor Critical Year Events 

per Reactor Critical Year
-7-

Figure 9 PWR stuck-open safety/relief valve

Figure 10 BWR stuck-open safety/relief valve

Events 

per Reactor Critical Year Events 

per Reactor Critical Year
-8-

Figure 11 PWR loss of instrument air

Figure 12 BWR loss of instrument air

Events 

per Reactor Critical Year Events 

per Reactor Critical Year
-9-

Figure 13 Very small loss-of-coolant accident

Figure 14 PWR steam generator tube rupture

Events 

per Reactor Critical Year Events 

per Reactor Critical Year
-10-

Figure 15 BRIIE Tier 2 (ΔCDF)

ΔCDF 

per Reactor Critical Year