NUCLEAR 

REGULATORY 

COMMISSION

Natural Hazard 

Assessments Could Be 

More Risk-Informed

Report to Congressional Requesters

April 2012

GAO-12-465

United States Government Accountability Office

GAO
United States Government Accountability Office

Highlights of GAO-12-465, a report to 

congressional requesters

April 2012

NUCLEAR REGULATORY COMMISSION 

Natural Hazard Assessments Could Be More Risk- Informed

Why GAO Did This Study

On March 11, 2011, an earthquake 

triggered a tsunami wave that 

exceeded the seawall at Japan’s

Fukushima Daiichi nuclear power 

plant, leading to the release of 

radioactive material into the 

environment. The disaster raised 

questions about the threats that natural 

hazards, such as earthquakes and 

floods, may pose to U.S. commercial 

nuclear power reactors. NRC licenses 

and regulates U.S. nuclear power 

reactors. NRC criteria for licensees to 

assess natural hazards were 

developed using an approach that

required reactors to be designed 

according to a set of potential

accidents using deterministic analysis. 

Since the 1990s, NRC has been 

encouraging the use of PRA as part of 

a risk-informed, performance-based 

approach. 

GAO was asked to (1) determine the 

extent to which PRA is applied to 

natural hazards at operating U.S. 

reactors and (2) describe expert views 

on and suggested changes, if any, to 

NRC processes for assessing natural 

hazards at such reactors. GAO 

reviewed documents; analyzed 

responses from 15 experts in 

assessing nuclear reactor risks and/or 

natural hazards; visited five selected 

nuclear power plants; and interviewed 

NRC officials and industry and public 

interest group representatives.

What GAO Recommends

GAO recommends that NRC analyze 

whether licensees of operating 

reactors should be required to develop 

PRAs that address natural hazards. 

NRC agreed with the recommendation 

and stated it will conduct the analysis 

in the context of ongoing initiatives.

What GAO Found

The Nuclear Regulatory Commission (NRC) and companies licensed to operate 

nuclear power reactors (or licensees) apply probabilistic risk assessment (PRA) 

to natural hazards at operating U.S. nuclear reactors to a limited extent. When 

the 104 operating reactors were originally licensed before 1997, NRC required 

licensees to assess natural hazards using deterministic analysis, which—

informed by historical experience, test results, and expert judgment—considers a 

specific set of potential accidents and how the consequences of those accidents 

can be prevented and mitigated. Subsequent to most of these initial licenses 

being issued, NRC, through policy statements and other documents, has 

endorsed PRA—a systematic method for assessing what can go wrong, its 

likelihood, and its consequences, resulting in quantitative estimates of risk—as a 

means to enhance and extend traditional deterministic analysis. In 1991, NRC 

requested that licensees voluntarily examine their reactors’ vulnerability to 

natural hazards and suggested PRA as one of several possible methods for 

licensees to use in their examinations. However, most licensees opted to use 

other methods. According to NRC officials and nuclear power industry 

representatives—and reflected in data GAO obtained from five licensees that 

together operate 25 reactors—few licensees are likely to have developed or 

updated since the 1990s PRAs that address natural hazards. NRC would have to 

conduct an analysis to determine whether or not to require licensees to develop 

PRAs that address natural hazards. According to agency officials, NRC has not 

conducted such an analysis. 

The experts in assessing natural hazards and/or nuclear reactor risks that GAO 

interviewed offered a range of views on (1) the overall adequacy of NRC 

processes for assessing the threats that natural hazards pose to operating U.S. 

nuclear power reactors and (2) what, if any, changes to those processes are 

warranted. Several experts said they believe NRC processes are generally 

adequate for assessing the threats that natural hazards pose to operating 

reactors. However, more than half of the experts GAO interviewed suggested 

expanding the use of PRA for assessing natural hazards as a complement to 

traditional deterministic analyses to provide a more robust approach. Those 

experts cited a number of advantages to doing so, including that PRA can help 

identify vulnerabilities that might otherwise be overlooked by relying on traditional 

deterministic analyses alone. Several experts also identified challenges to 

expanding the use of PRA for assessing natural hazards, including the limited

number of experts qualified to develop PRAs and the costs of doing so. 

View GAO-12-465. For more information, 

contact Frank Rusco at (202) 512-3841 or 

ruscof@gao.gov.
Page i GAO-12-465 Nuclear Regulatory Commission

Letter 1 

Background 6

PRA Is Applied to Natural Hazards to a Limited Extent 15

Experts Offered a Range of Views on NRC Assessment Processes 22

Conclusions 28

Recommendation for Executive Action 29

Agency Comments and Our Evaluation 29

Appendix I Objectives, Scope, and Methodology 31

Appendix II List of Experts 34

Appendix III Key Developments in NRC’s Approach to Probabilistic Risk 

Assessment 35

Appendix IV Information on Developing a Probabilistic Risk Assessment 41

Appendix V NRC Actions on Natural Hazard Assessment in Response to the 

Fukushima Daiichi Disaster 43

Appendix VI Comments from the Nuclear Regulatory Commission 47

Appendix VII GAO Contact and Staff Acknowledgments 49

Tables

Table 1: Natural Hazard PRAs at Selected Nuclear Power Reactors 18

Table 2: Selected Techniques Used in PRA 42

Contents
Page ii GAO-12-465 Nuclear Regulatory Commission

Figure

Figure 1: U.S. Operating Commercial Nuclear Power Reactors by 

Power Plant Location 7

Abbreviations

IPEEE Individual Plant Examination of External Events

NRC Nuclear Regulatory Commission

PRA probabilistic risk assessment

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Page 1 GAO-12-465 Nuclear Regulatory Commission

United States Government Accountability Office

Washington, DC 20548

April 26, 2012

The Honorable Barbara Boxer

Chairman

Committee on Environment and Public Works

United States Senate

The Honorable Edward J. Markey

House of Representatives

On March 11, 2011, a 9.0-magnitude earthquake and subsequent 

tsunami devastated northeast Japan and led to the most extensive 

release of radioactive material at a nuclear power plant since the 1986 

Chernobyl disaster. The Fukushima Daiichi nuclear power plant suffered 

extensive damage when a tsunami wave that exceeded the plant’s 

seawall flooded the site and caused a prolonged loss of electrical power 

at several of its reactors. As a result of the loss of power, plant operators 

were unable to keep three of the reactors cool, which led to fuel melting, 

hydrogen explosions, and the release of radioactive material into the 

environment. The disaster displaced tens of thousands of residents and 

contaminated the surrounding area. The Japanese government expects 

recovery to take years and cost billions of dollars. In light of the disaster, 

Japan and other countries have decided to reduce their reliance on 

nuclear power, which could affect their electricity costs and output of 

carbon emissions.

In the United States, the Nuclear Regulatory Commission (NRC), an 

independent federal agency headed by five commissioners, licenses 

commercial nuclear power reactors and regulates and oversees their safe 

operation and security.1

 

1

Congress created NRC in 1974 to take over the regulatory duties of its predecessor, the 

Atomic Energy Commission. 

An NRC task force has reviewed the Fukushima 

Daiichi disaster and determined that the continued operation of existing 

U.S. nuclear power reactors and the licensing of new reactors do not 

pose an imminent risk to public health and safety. Nevertheless, the 

disaster and its origins in a natural hazard—a tsunami—that was more 

severe than the plant was designed to withstand has raised questions 

about whether a similar event could happen here. These questions were 

 
 

Page 2 GAO-12-465 Nuclear Regulatory Commission

further highlighted by natural hazards that affected the sites of several 

U.S. commercial nuclear power plants and their reactors in 2011,2

including flooding near two power plants in Nebraska, severe storms at a 

plant in Alabama, and an earthquake at a plant in Virginia.3

NRC’s design criteria for nuclear power reactors require that systems, 

structures, and components important to safety be designed to withstand 

the effects of natural hazards, such as earthquakes, tornados, hurricanes, 

floods, and tsunamis, without losing the ability to perform their safety 

functions.4

Most of NRC’s regulatory framework—including its criteria for assessing 

natural hazards for currently-operating reactors—was developed 

according to a deterministic approach. The deterministic approach 

establishes a specific set of potential accidents, the consequences of 

which a nuclear power reactor must be designed to prevent or mitigate to 

protect public health and safety. It also establishes requirements for 

engineering safety margins and quality assurance standards for the 

design, manufacture, and construction of nuclear power reactors. Using 

this approach, NRC developed regulatory requirements primarily based 

on historical experience, test results, and expert judgment without 

The companies licensed to operate nuclear power reactors, or 

licensees, are responsible for protecting their reactors against natural 

hazards by assessing the hazards their reactors may face according to 

these criteria and designing their reactors to withstand such hazards. 

NRC is responsible for reviewing assessments and the resulting reactor 

designs as part of its process for issuing licenses for the construction and 

operation of nuclear power reactors, as well as for providing continuing 

oversight of operating reactors.

 

2

In this report, when we use the term power plant, we are referring to an entire site, and 

nuclear power reactors are the individual units at each site.

3

Each plant successfully withstood these events, although the ground accelerations of the 

earthquake that affected the North Anna Power Station in Virginia exceeded the plant’s 

design at several frequencies for a short period of time. This was the first time an 

operating reactor in the United States exceeded its design limit for ground acceleration. 

NRC and licensee inspections found minimal earthquake damage to the plant. 

4

10 C.F.R. Part 50, Appendix A, General Design Criterion 2—Design Bases for Protection 

Against Natural Phenomena. According to an NRC document, all currently operating 

reactors were licensed to or meet the intent of the General Design Criteria, which include 

General Design Criterion 2.
 

Page 3 GAO-12-465 Nuclear Regulatory Commission

considering quantitative estimates of risk.5 According to NRC documents, 

in developing those requirements, NRC considered the concept of 

“defense-in-depth”—a way of designing and operating nuclear power 

reactors that focuses on creating multiple independent and redundant 

layers of defense to compensate for potential human and mechanical 

failures so that no single layer, no matter how robust, is exclusively relied 

upon.6 The agency has been moving since the 1990s toward a riskinformed, performance-based approach to regulatory decision making 

that is being implemented in phases according to NRC documents.7 This 

approach extends the traditional deterministic approach in part by 

incorporating probabilistic risk assessment (PRA). PRA is a systematic 

method for assessing what can go wrong, its likelihood, and its potential 

consequences to determine quantitative estimates of risk in order to 

provide insights into the strengths and weaknesses of the design and 

operation of a nuclear power reactor. We have previously reported on 

NRC’s use of PRA, its adoption of a risk-informed, performance-based 

approach, and the challenges involved in implementing that approach.8

According to several experts in assessing nuclear reactor risks, risk 

analysts face challenges in assessing the threats posed by natural 

hazards, particularly for extreme natural hazards with a low likelihood of 

occurring but potentially high consequences. These challenges include 

uncertain knowledge about particular natural hazards due to the limited 

 

5

According to NRC, the deterministic approach considers implied but unquantified 

elements of probability in the selection of accidents to be analyzed.

6

According to NRC, the “defense-in-depth” concept is not defined in NRC regulations, and 

there is no single, agency-accepted description of the concept. However, it includes the 

use of access controls, physical barriers, redundant and diverse key safety functions, and 

emergency response measures. 

7

NRC differentiates between “risk-informed” and “risk-based” regulation, noting that the 

former uses risk analysis to augment other information used to support regulatory 

decisions, while the latter approach relies solely on quantitative results of risk 

assessments. NRC does not endorse a risk-based approach.

8

GAO, Nuclear Safety: NRC’s Oversight of Fire Protection at U.S. Commercial Nuclear 

Reactor Units Could Be Strengthened, GAO-08-747 (Washington, D.C.: June 30, 2008); 

GAO, Nuclear Regulatory Commission: Oversight of Nuclear Power Plant Safety Has 

Improved, but Refinements Are Needed, GAO-06-1029 (Washington, D.C.: Sept. 27, 

2006); GAO, Nuclear Regulation: Strategy Needed to Regulate Safety Using Information 

on Risk, GAO/RCED-99-95 (Washington, D.C.: Mar. 19, 1999); GAO, Probabilistic Risk 

Assessment: An Emerging Aid to Nuclear Power Plant Safety Regulation, 

GAO/RCED-85-11 (Washington, D.C.: June 19, 1985).
 

Page 4 GAO-12-465 Nuclear Regulatory Commission

historical record and the rarity of extreme natural hazards, the developing 

state of knowledge about natural hazards that varies by hazard, and 

changes to natural hazards over time potentially due to climate change 

and other causes.

Natural hazard assessments are only one component of NRC’s 

processes for ensuring that nuclear power reactors are protected against 

such hazards. In addition to requiring licensees to design their reactors 

using information from such assessments, NRC also requires licensees to 

have the ability to mitigate the consequence of accidents if they occur to 

prevent core damage and the uncontrolled release of radioactive material 

into the environment and, if that fails, to have emergency preparedness 

procedures in place to mitigate the effects of a radiological release on the 

public and the environment. Operating a nuclear power reactor is never 

entirely free of risk; assessments do not eliminate all risk no matter how 

well the hazards have been assessed but are an important source of 

information for identifying and mitigating vulnerabilities to events that can 

occur as a result of natural hazards.

In this context, you asked us to review how NRC assesses the threats 

that natural hazards pose to operating U.S. commercial nuclear power 

reactors. This report examines (1) the extent to which PRA is applied to 

natural hazards at operating U.S. nuclear power reactors and (2) expert 

views on and suggested changes, if any, to NRC processes for assessing 

natural hazards at operating U.S. nuclear power reactors.

To describe the extent to which PRA is applied to natural hazards at 

operating U.S. nuclear power reactors, we reviewed relevant documents 

and data obtained from NRC and selected licensees. We interviewed 

knowledgeable officials about the data and found the data to be 

sufficiently reliable for the purposes of our report. We reviewed NRC 

policies and procedures, as well as NRC initiatives related to the 

assessment of natural hazards. We also reviewed the NRC task force 

report on lessons learned from the Fukushima Daiichi disaster and 

documented actions NRC has taken on recommendations related to the 

assessment of natural hazards. In addition, we visited a nonprobability 

sample of five nuclear power plants to interview licensees about the 
 

Page 5 GAO-12-465 Nuclear Regulatory Commission

actions they have taken to assess natural hazards.9 The five nuclear 

power plants we visited were the Browns Ferry Nuclear Plant in Alabama, 

the Diablo Canyon Power Plant in California, the North Anna Power 

Station in Virginia, the San Onofre Nuclear Generating Station in 

California, and the Turkey Point Nuclear Plant in Florida. We selected 

these sites to capture a variety of characteristics, including reactor and 

containment vessel type, operating license issuance date, license 

renewal status, and natural hazard activity level. We also interviewed 

NRC officials and representatives from the nuclear power industry,10

To obtain expert views on and suggested changes, if any, to NRC 

processes for assessing natural hazards at operating U.S. nuclear power 

reactors, we held semistructured interviews with 15 experts in assessing 

natural hazards, risks to nuclear power reactors, or both. We identified 

these experts through a literature search, a review of prior GAO reports, 

and congressional and NRC hearings, as well as expert 

recommendations. The experts we interviewed included representatives 

from academia, government, industry, and public interest groups. We 

analyzed experts’ responses across a standard set of questions and 

summarized the results. We did not independently evaluate the quality of 

NRC processes for assessing natural hazards and the threats they pose 

to nuclear power reactors. Appendix I presents a more detailed 

description of our scope and methodology and appendix II lists the names 

and affiliations of the 15 experts we interviewed.

public interest groups, and the insurance industry to discuss NRC 

processes for assessing natural hazards. Further, we interviewed officials 

and reviewed documents from the National Oceanic and Atmospheric 

Administration, the U.S. Army Corps of Engineers, and the U.S. 

Geological Survey on the current state of knowledge on natural hazards, 

how that knowledge has changed over time, and the related uncertainty.

We conducted this performance audit from May 2011 to April 2012 in 

accordance with generally accepted government auditing standards. 

Those standards require that we plan and perform the audit to obtain 

 

9

Because this was a nonprobability sample, the information we gathered from these site 

visits is not generalizable to all 65 operating nuclear power plants but provides important 

illustrative information.

10We interviewed officials from the Nuclear Energy Institute, the policy organization of the 

nuclear energy and technologies industry. 
 

Page 6 GAO-12-465 Nuclear Regulatory Commission

sufficient, appropriate evidence to provide a reasonable basis for our 

findings and conclusions based on our audit objectives. We believe that 

the evidence obtained provides a reasonable basis for our findings and 

conclusions based on our audit objectives.

This section discusses U.S. commercial nuclear power reactors, NRC’s 

approach to natural hazard assessments, NRC’s endorsement of PRA, 

and actions NRC has taken on natural hazard assessments since the 

Fukushima Daiichi disaster.

Currently, 104 commercial nuclear power reactors operate in the United 

States. Together, these reactors generated almost 20 percent of our 

nation’s electricity in 2011. These reactors are located at 65 power plants 

across the country (see fig. 1) and are operated by 26 different 

companies. All 104 operating reactors received their construction permits 

in the 1960s and 1970s, with most receiving their operating licenses in 

the 1970s and 1980s.11 Many reactors are reaching or have reached the 

end of their initial 40-year license. As of March 2012, NRC had renewed 

71 reactor licenses for an additional 20 years and was currently reviewing 

11 license renewal applications.12

 

11One operating reactor received its operating license in 1969, and 6 operating reactors 

received their operating licenses in the 1990s, with the last of those reactors receiving its 

operating license in 1996. The other 97 operating reactors received their operating 

licensees between 1970 and 1989. 

In addition, NRC authorized two new 

reactors in Georgia in February 2012 and two new reactors in South 

Carolina in March 2012 and is considering 10 applications for the building 

and operation of new commercial nuclear power reactors.

12NRC issues licenses for commercial nuclear power reactors to operate for 40 years. 

Under current regulations, licensees may renew their licenses for up to 20 years. License 

renewal focuses on evaluating and managing the adverse effects of aging on a nuclear 

power reactor. Current regulations do not include a requirement that licensees of 

operating reactors reevaluate their design bases pertaining to natural hazards as part of 

the license renewal process.

Background 

Commercial Nuclear 

Power Reactors in the 

United States 
 

Page 7 GAO-12-465 Nuclear Regulatory Commission

Figure 1: U.S. Operating Commercial Nuclear Power Reactors by Power Plant Location

NRC required licensees of the 104 operating reactors to use deterministic 

analysis to assess the natural hazards their reactors might face as part of 

their construction permit and operating license applications.13

 

1310 C.F.R. Part 50, Appendix A and 10 C.F.R. Part 100—Reactor Site Criteria. NRC 

requires new nuclear power reactors licensed after January 1997 to be sited and designed 

with respect to geological and seismic determinations based on an appropriate analysis, 

such as a probabilistic seismic hazard analysis. 10 C.F.R. Part 100, subpart B. 

Among 

other things, NRC required the designs of structures, systems, and 

NRC’s Approach to 

Assessing Natural Hazards
 

Page 8 GAO-12-465 Nuclear Regulatory Commission

components important to safety to reflect appropriate consideration of the 

most severe natural hazards that had been historically reported for a 

reactor site and the surrounding area, with sufficient safety margin to 

account for the limited accuracy, quantity, and period over which historical 

data on natural hazards have been accumulated. NRC staff 

independently reviewed applicants’ natural hazard assessments as part 

of the application review process to determine whether the assessments 

were acceptable. When NRC issued an operating license for a nuclear 

power reactor, these natural hazard assessments became part of the 

reactors’ licensing basis––that is, the collection of documents or technical 

criteria upon which NRC issues licenses for the construction and 

operation of nuclear power reactors. NRC does not require licensees to 

reevaluate their natural hazard assessments on a periodic basis, but 

through several pieces of regulation it requires that licensees consider 

new information on natural hazards as they become aware of it to 

determine if the information may necessitate additional licensee action 

under NRC requirements. NRC staff also continually evaluate new 

information on natural hazards through research and oversight processes 

to assess potential impacts on reactor safety, according to NRC officials, 

and the agency has initiated several discrete efforts over time that have 

examined the threats that natural hazards pose to operating nuclear 

power reactors. For example, in 1991, NRC requested that each licensee 

identify and report plant-specific vulnerabilities to severe accidents 

caused by external events, including natural hazards, as part of its 

Individual Plant Examination of External Events (IPEEE) program.14 More 

recently, NRC initiated the Generic Issue-199 program in 2005 to 

examine the implications of new seismic hazard estimates for the central 

and eastern United States for operating nuclear power reactors.15

 

14NRC, Individual Plant Examination of External Events (IPEEE) for Severe Accident 

Vulnerabilities—10 CFR 50.54(f), Generic Letter No. 88-20, Supplement 4 (Washington, 

D.C.: June 28, 1991). Acts of sabotage or terrorism were not included in the set of events 

considered.

15NRC, Implications of Updated Probabilistic Seismic Hazard Estimates in Central and 

Eastern United States, Generic Issue 199 (Washington, D.C.: June 9, 2005). 
 

Page 9 GAO-12-465 Nuclear Regulatory Commission

NRC’s approach to risk assessment has evolved over time.16

• In 1986, NRC issued a policy statement calling for the use of PRA to 

measure achievement of the agency’s individual and societal safety 

goals.

NRC began 

endorsing the use of PRA to enhance and extend traditional deterministic 

analysis as part of the agency’s move toward a risk-informed, 

performance-based regulatory approach. Specifically,

17 The policy statement aimed to express NRC policy on “how 

safe is safe enough” at U.S. nuclear power plants by broadly defining 

an acceptable level of radiological risk that might be imposed on the 

public as a result of plant operations.18

• In 1995, NRC issued a policy statement encouraging the increased 

use of PRA in all regulatory matters to the extent supported by the 

state-of-the-art in PRA methods and data, and in a manner that 

complemented NRC’s deterministic approach and supported NRC’s 

defense-in-depth philosophy.19

 

16Appendix III describes key developments in NRC’s approach to probabilistic risk 

assessment, including its application to natural hazards.

17NRC, Safety Goals for the Operations of Nuclear Power Plants; Policy Statement, 51 

Fed. Reg. 30,028 (Aug. 21, 1986).

18The policy statement established two qualitative safety goals—one addressing individual 

risk and the other addressing societal risk—each supported by a quantitative health 

effects objective by which the safety goal could be measured. NRC staff subsequently 

adopted subsidiary numerical objectives of less than 1 core damage event expected per 

10,000 years of reactor operation and less than 1 large early radiation release expected 

per 100,000 years of reactor operation to serve as surrogates for the quantitative health 

effects objectives. Both the quantitative health effects objectives and the subsidiary 

numerical objectives are calculated using probabilistic risk assessment. The safety goals, 

quantitative health effects objectives, and subsidiary numerical objectives are applied at 

the individual reactor level. NRC officials indicated that NRC does not consider aggregate, 

industry-wide probabilities that core damage or a large early radiation release would occur 

somewhere in the United States or set total aggregate, industry-wide objectives because 

NRC licenses nuclear power reactors on an individual reactor basis and because they do 

not believe such probabilities would be appropriate.

19NRC, Use of Probabilistic Risk Assessment Methods in Nuclear Regulatory Activities; 

Final Policy Statement, 60 Fed. Reg. 42,622 (Aug. 16, 1995).

NRC’s Endorsement of 

PRA
 

Page 10 GAO-12-465 Nuclear Regulatory Commission

• In 2007, NRC issued regulations requiring applicants for combined 

licenses20 for new nuclear power reactors to submit a description and 

the results of a reactor-specific PRA to NRC as part of their 

application.21 NRC also began requiring licensees of new nuclear 

power reactors that will operate those reactors under a combined 

license to develop, maintain, and periodically upgrade a reactorspecific PRA. Among other things, NRC required the PRA to cover 

initiating events––that is, events that can lead to a reactor accident—

for which there were NRC-endorsed consensus standards for PRA 

quality.22

• In 2009, NRC issued a regulatory guide that endorsed national 

consensus standards for PRA quality.23

• In 2011, the Chairman of the NRC commissioned a task force to 

develop a strategic vision and assess options for a more holistic riskinformed, performance-based regulatory approach, with one option 

being better incorporation of risk management concepts into NRC’s 

regulatory programs.

Effective April 2010, licensees 

of operating reactors who choose to submit risk-informed licensing 

applications are expected to meet the NRC-endorsed guidelines, 

including a quality standard that addresses the assessment of natural 

hazards.

 

20In 1989, NRC promulgated 10 C.F.R. Part 52, which established a new combined 

license for electric power companies to obtain a license to build and operate a new 

reactor. 54 Fed. Reg. 15,372 (Apr. 18, 1989). While NRC has revised its regulatory 

process, the technical bases for its decisions to make findings have generally remained 

the same. The combined license is NRC’s response to the nuclear industry’s concerns 

about the length and complexity of NRC’s former two-step process of issuing a 

construction permit followed by an operating license. The combined license process 

provides a one-step approval process that authorizes a licensee to construct and operate 

a nuclear power reactor; as such, it is intended to provide predictability and early 

resolution of issues in the review process. 

2110 C.F.R. § 52.79(a)(46). 

2210 C.F.R. § 50.71(h). 

23NRC, An Approach for Determining the Technical Adequacy of Probabilistic Risk 

Assessment Results for Risk-Informed Activities, Regulatory Guide 1.200 (Washington, 

D.C.: March 2009).
 

Page 11 GAO-12-465 Nuclear Regulatory Commission

NRC policy statements, training manuals, and other documents cite 

numerous advantages of using PRA to enhance the agency’s traditional 

deterministic approach. Specifically, PRA

• can enhance safety decision making with risk insights, allowing for the 

more efficient use of NRC resources and reducing unnecessary 

burdens on licensees;

• explicitly considers a broader set of potential challenges to safety;

• provides a logical means for prioritizing challenges based on risk 

significance, operating experience, and engineering judgment;

• explicitly identifies, evaluates, and quantifies sources of uncertainty;

• provides a means to test the sensitivity of assessment results to key 

assumptions;

• provides a rigorous, systematic tool for analyzing complex systems 

and enables information integration; and

• provides qualitative insights into plant vulnerabilities and quantitative 

results for use in decision making.

NRC has encouraged the increased use of PRAs in all regulatory matters 

but generally does not require licensees to develop or maintain PRAs for 

currently operating reactors. However, licensees may choose to submit 

license amendment requests so they can transition to less prescriptive, 

risk-informed approaches for particular issues allowed by NRC 

regulations, such as NRC’s risk-informed approach to fire protection.24

 

24In 2004, NRC issued a regulation that allowed the transition of nuclear reactors from its 

existing, prescriptive fire safety regulations to a less prescriptive, risk-informed, 

performance-based approach, under which licensees can use tools, such as fire modeling 

and risk analysis, to determine which areas of the nuclear power plant are most at risk 

from fire. According to NRC officials, these analyses could enable units to focus their 

resources on addressing these higher-risk areas and reduce the number of future 

exemptions in areas that are no longer considered to be at high risk from fire. Reductions 

in exemptions would, thus, simplify the units’ licenses. For more information, see 

According to NRC officials, if licensees choose to apply to use a riskinformed approach, licensees must develop a PRA to the extent required 

GAO-08-747. 
 

Page 12 GAO-12-465 Nuclear Regulatory Commission

for the application.25 In contrast, applicants for combined licenses to build 

and operate new nuclear power reactors are required to develop a PRA in 

addition to a deterministic analysis.26 For combined licenses, NRC 

expects those PRAs to address natural hazards except for natural 

hazards that licensees determine are an insignificant contributor to risk, 

based on NRC-endorsed screening criteria. For example, a nuclear 

power reactor that is distant from a coast will likely be able to omit 

tsunami hazards from its PRA. Further, NRC requires licensees of new 

nuclear power reactors who will operate their reactors under a combined 

license to develop, maintain, and periodically upgrade their PRAs over 

the operating life of their reactors.27

 

25Regulatory activities that use PRA include (1) risk-informed licensing change 

applications; (2) categorization and treatment of structures, systems, and components 

under 10 C.F.R. § 50.69; (3) fire protection programs under 10 C.F.R. § 50.48(c); and (4) 

licenses, certifications, and approvals for nuclear power plants under 10 C.F.R. Part 52.

NRC staff have prepared guidance to assist licensees in developing risk-informed 

changes to their licensing bases. See NRC, An Approach to Using Probabilistic Risk 

Assessment in Risk-Informed Decisions on Plant-Specific Changes to the Licensing Basis, 

Regulatory Guide 1.174 (Washington, D.C.: July 1998). 

PRAs are used to evaluate several 

categories of initiating events, including internal events, which start inside 

the nuclear power plant or the electric system serviced by the power plant 

(e.g., random hardware failures and operator actions), and external 

events, which typically start outside the nuclear power plant, such as 

natural hazards (e.g., earthquakes, external floods, external fires, and 

high-wind events). According to NRC documents, PRA can estimate three 

different levels of risk—one focused on reactor core damage, one 

focused on containment release, and one focused on radiological 

2610 C.F.R. § 52.79(a)(46). 

27Specifically, NRC requires licensees who will operate reactors under a combined license 

to develop, maintain, and periodically upgrade a Level 1 and Level 2 PRA. 10 C.F.R. § 

50.71(h).
 

Page 13 GAO-12-465 Nuclear Regulatory Commission

consequences to the public.28 For information on developing a PRA, see 

appendix IV.

In March 2011, NRC chartered a staff task force to review its processes 

and regulations in light of the Fukushima Daiichi disaster and to 

recommend whether NRC should make near-term improvements to its 

regulatory system.29

 

28Specifically, Level 1 PRAs evaluate events that can lead to plant accidents and examine 

reactor systems and operators’ responses to calculate “core damage frequency”—the 

frequency of the combinations of initiating events, hardware failures, and human errors 

leading to the uncovering of the reactor core such that reflooding of the core is not 

imminent. Level 2 PRAs start with Level 1 core damage accidents and assess the 

frequencies of various categories of containment releases. The results for operating 

reactors are typically reported in terms of “large early release frequency”—the frequency 

of those accidents leading to significant, unmitigated releases from the reactor’s 

containment in a time frame prior to effective evacuation of the nearby population such 

that there is a potential for early health effects. Level 3 PRAs start with Level 2 radiological 

release accidents and assess the public health consequences of a radiological release in 

terms of injury to the public and damage to the environment. Because core damage and 

large early release estimates are easier to calculate than public health consequences, the 

results of Level 1 and Level 2 PRAs are often used as surrogates for the results of a Level 

3 PRA. NRC has plans to perform a new Level 3 PRA (including Level 1 and Level 2 

PRAs) for a single operating power plant that will include assessment of natural hazards 

and be completed within 4 years. 

The task force issued its final report in July 2011. 

Among other things, the task force found that NRC regulations and 

guidance provide a robust approach for assessing natural hazards, but it 

noted that NRC’s processes for assessing natural hazards had evolved 

over time. As a result, the licensing bases, design, and level of protection 

from natural hazards differ among operating reactors depending on when 

the reactors were built and when they were licensed for operation. Based 

on these findings, the task force recommended, among other things, that 

NRC order licensees to reevaluate the seismic and flooding hazards at 

their sites against current NRC requirements and guidance and, if 

necessary, update their reactor design bases. The task force further 

recommended that NRC initiate rulemaking to require licensees to 

confirm seismic and flooding hazards every 10 years, address any new 

and significant information and, if necessary, update their reactor design 

bases. In addition, the task force recommended that, while these longer 

term actions are being completed, NRC order licensees to check their 

seismic and flood protection features to identify and address any plant29Appendix V describes actions that NRC has taken in response to the Fukushima Daiichi 

disaster that relate to natural hazard assessments. 

Actions NRC Has Taken on 

Natural Hazard 

Assessments Since the 

Fukushima Daiichi 

Disaster
 

Page 14 GAO-12-465 Nuclear Regulatory Commission

specific vulnerabilities and verify the adequacy of monitoring and 

maintenance for protection features such as watertight barriers and seals.

In response to the task force report, in August 2011, NRC directed 

agency staff to identify the task force recommendations that could and, in 

the staff’s view, should be implemented without unnecessary delay. The 

staff submitted a report to NRC in October 2011 with their proposed 

prioritization of the recommendations. In this report, the staff 

recommended that NRC move forward with requiring licensees to 

reevaluate the seismic and flooding hazards at their sites and to check 

their seismic and flood protection features. However, the staff 

recommended that NRC wait to initiate rulemaking requiring licensees to 

confirm their sites’ seismic and flooding hazards every 10 years in order 

to gain experience from the implementation of the initial reevaluation. In 

December 2011, NRC approved the staff’s proposals and supported 

actions on several recommendations. That same month, Congress 

passed, and the President signed, the 2012 Consolidated Appropriations 

Act, which included a provision directing NRC to require that licensees 

reevaluate external hazards at their sites, including seismic and flooding 

hazards, as expeditiously as possible.30 In February 2012, NRC staff 

proposed issuing a request for information that, among other things, 

would ask licensees to reevaluate the seismic and flooding hazards at 

their sites.31

 

30Consolidated Appropriations Act, 2012, Pub. L. No. 112-74, Div. B, § 402 (Dec. 23, 

2011). 

The staff suggested PRA as one of two acceptable methods 

for licensees to use in reevaluating seismic hazards. The staff did not 

exclude, but also did not suggest, PRA as a method for assessing 

flooding hazards because, according to NRC officials, NRC currently uses 

a deterministic approach for assessing flooding hazards. NRC staff also 

proposed addressing other natural hazards, such as wind and missile 

loads from tornados and hurricanes, at a later date once sufficient 

resources are available for the reevaluations. According to NRC officials, 

it is not clear yet whether agency staff will suggest licensees use PRA or 

other methods to reevaluate those other natural hazards. In March 2012,

31NRC, Proposed Orders and Requests for Information in Response to Lessons Learned 

from Japan’s March 11, 2011, Great Tohoku Earthquake and Tsunami, SECY-12-0025 

(Washington, D.C.: Feb. 17, 2012). 
 

Page 15 GAO-12-465 Nuclear Regulatory Commission

NRC issued the request for information proposed by agency staff in 

February.32

Licensees and NRC apply PRA to natural hazards at operating U.S. 

nuclear power reactors to a limited extent, according to information 

provided by nuclear power industry representatives, NRC officials, and 

several experts in assessing nuclear reactor risks that we interviewed and 

data we obtained. While NRC has endorsed PRA as a means to enhance 

and extend traditional deterministic assessments, the agency has not 

conducted the analyses to determine whether or not it should require 

licensees of operating reactors to develop and maintain PRAs that 

address natural hazards.

Nuclear power industry representatives, NRC officials, and several 

experts in assessing nuclear reactor risks we interviewed, as well as data 

we obtained, indicate that licensees and NRC apply PRA to natural 

hazards at operating nuclear power reactors to a limited extent. Prior to 

its response to the Fukushima Daiichi disaster, the last time NRC

requested licensees to assess natural hazards was in 1991 when the 

agency initiated the IPEEE program. For the IPEEE, NRC requested each 

licensee to voluntarily identify and report plant-specific vulnerabilities to 

severe accidents caused by external events, including natural hazards. 

Although PRA was suggested by NRC as one of several possible 

methods for licensees to use in their examinations, most licensees opted 

to use methods other than PRA. Specifically, NRC received 70 IPEEE 

submissions from licensees covering all operating reactors. According to 

NRC’s summary of the 70 submissions, about 40 percent applied PRA to 

seismic hazards, and about 15 percent applied PRA to high winds, floods, 

and other external events. NRC reported that almost no licensees 

identified vulnerabilities with respect to seismic hazards, and none 

identified vulnerabilities related to high winds, flooding, or other external 

events. Nonetheless, according to NRC’s summary, 70 percent of the 

IPEEE submissions identified plant improvements related to seismic 

hazards, and about 50 percent identified plant improvements related to 

high winds and flooding. NRC performed a review of all IPEEE 

 

32NRC, Staff Requirements—SECY-12-0025—Proposed Orders and Requests for 

Information in Response to Lessons Learned From Japan’s March 11, 2011, Great 

Tohoku Earthquake and Tsunami (Washington, D.C.: Mar. 9, 2012). 

PRA Is Applied to 

Natural Hazards to a 

Limited Extent

Licensees and NRC Apply 

PRA to Natural Hazards 

to a Limited Extent
 

Page 16 GAO-12-465 Nuclear Regulatory Commission

submissions to assess the technical adequacy of the methods and data 

used and noted that the quality of licensees’ submissions varied. NRC did 

not attempt to verify or validate licensees’ results. NRC staff proposed in 

2011 that licensees reevaluate seismic hazards under the agency’s 

Generic Issue-199 program. Similar to the IPEEE, the proposal cited PRA 

as one—but not the only—possible method for assessing seismic 

hazards. The NRC staff has now proposed to pursue the reevaluation of 

seismic hazards as part of the implementation of the Fukushima Daiichi 

task force recommendations and has identified PRA as one of the 

methods that licensees can choose to use.

Nuclear Energy Institute representatives told us that, if data were to be 

collected on licensees’ current application of PRA to natural hazards, they 

believe the data would likely show little difference since the IPEEE in the 

1990s. They also stated that they believe few of those PRAs developed 

for the IPEEE would have been updated since then. Likewise, senior 

NRC officials told us that were NRC to obtain current data on licensees’ 

application of PRA to natural hazards, the results would likely show 

limited application, substantial variability, and few new or updated 

assessments since the IPEEE. According to several experts in assessing 

nuclear reactor risks we spoke with, few of the nation’s operating reactors 

currently have PRAs in place that address natural hazards. For example, 

one expert stated that some reactors have PRAs that address seismic 

hazards, and a few have PRAs that address external floods, but the rest 

do not have PRAs that address natural hazards at all.

NRC does not collect comprehensive data on the extent of licensees’ 

voluntary application of PRA to natural hazards at operating reactors. 

NRC officials stated that, because PRAs are not required for operating 

reactors, NRC does not have comprehensive data on their use. NRC 

officials told us that where licensees voluntarily undertake risk-informed 

applications or license amendments, the information licensees submit to 

NRC may include results on natural hazards. For example, when a 

licensee chooses to use a risk-informed approach to request a change to 

its licensing basis, the licensee must determine whether natural hazards 
 

Page 17 GAO-12-465 Nuclear Regulatory Commission

are a significant contributor to risk and provide that information to NRC.33

To make this determination, the licensee may use a PRA. Apart from 

these circumstances, however, NRC does not have data on operating 

reactor licensees’ PRA use. NRC officials could not tell us, therefore, 

which licensees of operating reactors have voluntarily undertaken PRAs 

that address natural hazards, what natural hazards are addressed, when 

existing PRAs were first developed and last updated, or the extent to 

which they meet NRC-endorsed quality standards. Nuclear Energy 

Institute representatives similarly told us that they do not collect data on 

PRA use and do not have a plant-by-plant listing of which reactors have 

PRAs in place that address natural hazards. NRC officials said that they 

do not collect comprehensive PRA information because licensees of 

operating reactors are not required to have a PRA. Further, they told us 

that, if NRC decided to collect this information, the agency would likely 

issue a “generic communication”––NRC’s primary method of 

communicating information to licensees and interested stakeholders or 

requesting information from them. NRC officials told us that a generic 

communication to obtain comprehensive PRA information would take 

NRC a significant amount of time to develop and issue.34

According to representatives of five licensees we interviewed, 12 of the 

25 reactors those licensees operate have PRAs in place that address 

seismic hazards, and none have PRAs in place that address high winds 

or external floods (see table 1).35

 

33NRC licenses nuclear power reactors on an individual reactor basis. Therefore, 

licensees’ natural hazard assessments—including voluntary PRAs—and NRC’s review of 

those assessments are at the individual reactor level and generally do not take into 

account other nuclear power reactors on the same site, or other nuclear power plants in 

the region or nationwide. An event that affects multiple reactors can potentially have 

greater consequences than an event that affects a single reactor.

The representatives told us that, in 

some instances, a reactor does not have a PRA in place for a given 

natural hazard because a screening analysis indicated that the hazard’s 

contribution to risk was sufficiently low that it could be eliminated from 

34Some generic communications request information or actions relative to NRC 

regulations and require responses. Others suggest methods for meeting NRC 

requirements on which licensees may voluntarily act or communicate related technical or 

regulatory information.

35We requested data from the companies that operate the five nuclear power plants we 

visited regarding their application of PRA to seismic hazards, floods, high winds, and other 

external events at the reactors we toured, as well as the other reactors operated by the 

companies. 
 

Page 18 GAO-12-465 Nuclear Regulatory Commission

further consideration; in other instances, licensees have performed 

margin studies instead or rely on traditional deterministic assessments of 

the hazard. Of the seismic PRAs in place at 12 of the 25 reactors for 

which we obtained information, 10 have not been updated since they 

were first developed. The seismic PRAs at 6 of the 12 reactors were 

developed in the 1990s under NRC’s IPEEE program. The seismic PRAs 

at the other reactors were developed between 1983 and 2010 for a 

variety of reasons. For example, two were developed as part of voluntary 

licensee efforts to better understand seismic risk. Another was developed 

as part of a plant’s long-term seismic monitoring program created as a 

condition of its operating license due to discovery during construction of a 

new fault line.

Table 1: Natural Hazard PRAs at Selected Nuclear Power Reactors

Natural hazards included

Licensee

Plant name 

(state) 

Reactor 

numbera Seismic

High

winds

External 

floods Developed

Last 

updated

Dominion Kewaunee Power 

Station (WI)

Data not 

applicable

Yes No No 1994 1994

Millstone Power 

Station (CT)

2 No No No Data not 

applicable

Data not 

applicable

3 Yes No No 1995 1995

North Anna Power 

Station (VA)

1 No No No Data not 

applicable

Data not 

applicable

2 No No No Data not 

applicable

Data not 

applicable

Surry Nuclear 

Power Station 

(VA) 

1 Yes No No 2010 2010

2 Yes No No 2010 2010

NextEra Energy Duane Arnold 

Energy Center (IA)

Data not 

applicable

Yes No No 1997 2005

Point Beach 

Nuclear Plant (WI)

1 Yes No No 1995 1995

2 Yes No No 1995 1995

Seabrook Station 

(NH)

1 Yes No No 1983 2004

St. Lucie Plant 

(FL) 

1 No No No Data not 

applicable

Data not 

applicable

2 No No No Data not 

applicable

Data not 

applicable

Turkey Point 

Nuclear Plant (FL) 

3 No No No Data not 

applicable

Data not 

applicable

4 No No No Data not 

applicable

Data not 

applicable
 

Page 19 GAO-12-465 Nuclear Regulatory Commission

Natural hazards included

Licensee

Plant name 

(state) 

Reactor 

numbera Seismic

High

winds

External 

floods Developed

Last 

updated

Pacific Gas & 

Electric

Diablo Canyon 

Power Plant (CA)

1 Yes No No 1988 1988

2 Yes No No 1988 1988

Southern 

California Edison

San Onofre 

Nuclear 

Generating Station 

(CA) 

2 Yes No No 1995 1995

3 Yes No No 1995 1995

Tennessee Valley 

Authority

Browns Ferry 

Nuclear Plant (AL)

1 No No No Data not 

applicable

Data not 

applicable

2 No No No Data not 

applicable

Data not 

applicable

3 No No No Data not 

applicable

Data not 

applicable

Sequoyah Nuclear 

Plant (TN)

1 No No No Data not 

applicable

Data not 

applicable

2 No No No Data not 

applicable

Data not 

applicable

Watts Bar Nuclear 

Plant (TN)

1 No No No Data not 

applicable 

Data not 

applicable

Sources: GAO analysis of data provided by Dominion, NextEra Energy, Pacific Gas & Electric, Southern California Edison, and 

Tennessee Valley Authority.

a

One or more reactors may be located at a single power plant. Generally, individual reactors are 

numbered. Where only one reactor is located at a plant, however, the reactor may or may not have a 

number associated with it.

NRC has also applied PRA to natural hazards to a limited extent. For 

example, NRC officials told us that the agency has developed 16 external 

event risk analysis models of limited detail that address some natural 

hazards at 19 operating reactors.36

 

36In 1995, NRC began developing these models, called Standardized Plant Analysis Risk 

models. As of December 2011, NRC had 78 Standardized Plant Analysis Risk models that 

address internal events at all 104 operating reactors and 16 Standardized Plant Analysis 

Risk models that address some external events at 19 operating reactors. Specifically, of 

the 16 external event Standardized Plant Analysis Risk models, all 16 include seismic 

events among the natural hazards assessed, 5 include high winds, and 2 include external 

floods. 

These models are available to NRC 

staff for internal use in providing risk insights to various agency initiatives 

such as NRC’s inspection and incident investigation programs. According 

to NRC officials, NRC staff use the models to a limited extent. The 

models rely on licensee information, such as that derived from the IPEEE, 

that could be out-of-date. NRC officials told us they have plans to validate 
 

Page 20 GAO-12-465 Nuclear Regulatory Commission

the models in the future to improve confidence among agency staff in 

their use. According to the officials, there has also recently been renewed 

interest in the agency in resuming development of these models for the 

remaining operating reactors. The officials told us NRC is finalizing the 

first in a series of such models that include more recent plant information 

and improved validation.37 In a separate initiative, according to NRC 

documents and officials, NRC has plans to perform a new full-scope PRA 

for a single operating power plant that will calculate off-site risk and its 

contributors, including natural hazards.38 NRC staff identified several 

reasons for doing so, including technical advances that had taken place 

since the last similar NRC effort in the late 1980s,39 scope considerations 

that had not previously been addressed such as multiunit site effects, and 

potential future uses for these models such as verifying or revisiting 

regulatory requirements and guidance and developing NRC’s in-house 

PRA technical capability.

While NRC has endorsed PRA as a means to enhance and extend 

traditional deterministic assessments, the agency has not conducted the 

analyses to determine whether or not it should require licensees of 

operating reactors to develop and maintain PRAs that address natural 

hazards. NRC’s “backfit rule” generally requires NRC to assure licensees 

that requirements placed on them will change only when warranted from 

 

37NRC officials told us the 19 reactors that the existing Standardized Plant Analysis Risk 

external event models were developed for were selected on the basis of convenience and 

availability of information. NRC’s initial plans to develop additional external event 

Standardized Plant Analysis Risk models for the remaining reactors were not carried out 

due to an agency determination that resources would be better allocated to internal event 

Standardized Plant Analysis Risk model development. 

38In September 2011, the NRC directed agency staff to plan for and perform a new fullscope comprehensive site Level 3 PRA for a single operating power plant that will include 

assessment of natural hazards and be completed within 4 years. A full-scope 

comprehensive site Level 3 PRA is a PRA that includes a quantitative assessment of the 

public risk from accidents involving all site reactor cores and spent nuclear fuel that can 

occur during any plant operating state, and that are caused by all initiating event hazards 

(internal events, fires, flooding, seismic events, and other site-specific external hazards). 

39NRC, Severe Accident Risks: An Assessment for Five U.S. Nuclear Power Plants, 

NUREG-1150 (Washington, D.C.: December 1990).

NRC Has Not Analyzed 

Whether It Should Require 

PRAs That Address 

Natural Hazards
 

Page 21 GAO-12-465 Nuclear Regulatory Commission

a public health and safety standpoint.40 As such, to require that licensees 

of operating reactors apply PRA to natural hazards, NRC officials told us 

NRC would have to determine that the advantages of doing so are 

necessary to ensure that protection of the public health and safety is 

adequate or that the added increase in overall protection is at least 

substantial and justifies the cost of implementation. Senior NRC officials 

told us that it may be difficult to meet either of these criteria in order to 

justify the imposition of a new PRA requirement on licensees of operating 

reactors.41

 

40NRC’s backfit rule requires that NRC determine that new requirements would result in a 

substantial increase in the overall protection of public health and safety or common 

defense and security and that this increased protection justifies the cost of 

implementation. 10 C.F.R. § 50.109(a)(3). Backfit is defined in 10 C.F.R. § 50.109(a)(1) as 

“the modification of or addition to systems, structures, components, or design of a facility; 

or the design approval or manufacturing license for a facility; or the procedures or 

organization required to design, construct or operate a facility; any of which may result 

from a new or amended provision in the Commission’s regulations or the imposition of a 

regulatory staff position interpreting the Commission’s regulations that is either new or 

different from a previously applicable staff position.” The backfit rule requires a backfit 

analysis demonstrating that the new or changed requirement is a substantial increase in 

overall protection unless NRC finds that a backfit is needed to ensure that protection of 

public health and safety is adequate, or in other limited situations. 10 C.F.R. § 

50.109(a)(4).

NRC officials we spoke with were unaware of any formal 

determinations by the agency regarding whether or not requiring PRA of 

operating reactors meets the backfit rule or exceptions thereto. 

Additionally, an official from the agency’s Office of General Counsel told 

us that the NRC has the authority to “administratively except” a new 

41We reported in 1999 that similar difficulties had been expressed by the NRC staff in the 

1990s, during which time NRC was considering whether and to what extent it should 

revise its regulations pertaining to operating reactors to make them risk-informed. In 1998, 

agency staff proposed to the NRC a phased approach to risk-informing its regulations. 

The staff also recommended that implementation of revised regulations be voluntary. The 

staff noted that it would be difficult to make the backfit determination needed to require 

implementation and further that doing so could create the impression that current plants 

were less safe. In response, the Chairman suggested a more aggressive approach that 

would entail a risk-informed approach for all relevant regulations across the board and 

stated that the revised regulations should be mandatory, unless the staff found that backfit 

provisions could not be met. Also, the Commissioners expressed concern about a 

voluntary approach, believing that it would create two classes of plants operating under 

two different sets of regulations. Ultimately, however, the NRC adopted the staff’s 

proposed phased approach to risk-informing its regulations of operating reactors and 

determined that compliance would be voluntary. 
 

Page 22 GAO-12-465 Nuclear Regulatory Commission

requirement from the backfit rule.42 NRC officials stated that it generally 

takes several years to conduct and document the regulatory analyses to 

determine whether a new requirement should be implemented and then 

to complete the rulemaking process necessary before a requirement can 

become law.43

The 15 experts in assessing natural hazards and/or nuclear reactor risks 

that we interviewed offered a range of views on (1) the adequacy of NRC 

processes for assessing the threats that natural hazards pose to 

operating U.S. nuclear power reactors and (2) what, if any, changes to 

those processes are warranted. Experts had varied views on the overall 

adequacy of NRC natural hazard assessment processes and identified a 

number of strengths and limitations of those processes. More than half of 

the experts we interviewed suggested expanding the use of PRA for 

assessing natural hazards as a complement to traditional deterministic 

analyses, and several experts cited a number of challenges to doing so. 

Some experts we interviewed suggested other changes to NRC 

processes for assessing natural hazards. Appendix II lists the names and 

affiliations of the 15 experts we interviewed.

The 15 experts we interviewed had varied views on the overall adequacy 

of NRC processes for assessing the threats that natural hazards pose to 

operating U.S. nuclear power reactors and identified a number of 

strengths and limitations of those processes. Several experts said they 

believe NRC processes are generally adequate. One of those experts 

noted that, in his view, NRC’s processes for assessing natural hazards 

work well due to the conservative approach NRC takes to assessing and 

designing against natural hazards. Another expert who agreed that NRC 

processes are adequate overall explained that natural hazards are well 

 

42The official told us that the NRC did this when it adopted the Aircraft Impact Assessment 

Rule in 2009. See 74 Fed. Reg. 28,112, 28,143 (June 12, 2009). The official stated that an 

administrative exception must be the subject of notice and public comment for 

rulemakings, but is not required with respect to the issuance of orders.

43Officials from NRC’s Office of General Counsel believe that a requirement to perform a 

PRA would not be subject to the backfit rule because such a requirement would constitute 

an information collection (and possibly reporting) requirement that NRC has deemed not 

to be subject to the backfit rule. However, according to the same officials, if the 

requirement is implemented by rule, the NRC would prepare—as part of that rulemaking—

a regulatory analysis to support the adoption of such a requirement.

Experts Offered a 

Range of Views on 

NRC Assessment 

Processes 

Experts Noted Strengths 

and Limitations of NRC 

Assessment Processes
 

Page 23 GAO-12-465 Nuclear Regulatory Commission

accounted for in reactor designs and that NRC does as good of a job as 

any other federal agency in ensuring that its regulated community 

protects against natural hazards. Several other experts expressed mixed 

views on the adequacy of NRC processes. Some of these experts stated 

that they had more confidence in NRC’s approach for assessing the 

threats that natural hazards pose to proposed new reactors than its 

approach for operating nuclear power reactors because the former 

requires licensees to use PRA in addition to traditional deterministic 

assessments when assessing natural hazards. Two experts stated that 

NRC processes for assessing natural hazards are inadequate. One of 

those experts noted that NRC and licensees need to consider other 

possibilities that may affect natural hazard assessments, such as the 

potential effects of climate change on the natural hazards a reactor may 

face over its operating life.

Experts we interviewed identified a number of strengths of NRC 

processes for assessing the threats posed by natural hazards to 

operating nuclear power reactors. Four strengths were identified by more 

than two experts. First, about half of the experts identified as a strength 

the structured and thorough process NRC follows in reviewing licensee 

assessments of natural hazards. A few of these experts noted the 

stringency of NRC’s review of licensee assessments, and a few others 

said that the agency has done an effective job defining a comprehensive 

set of natural hazards that licensees are responsible for assessing. 

Second, several experts identified as a strength what they said was 

NRC’s conservative approach to licensee assessments and reactor 

designs for natural hazards, including NRC’s emphasis on (1) defense-indepth principles, (2) engineering safety margins, and (3) reactor designs 

based on the worst credible natural hazards a site is expected to face. 

Some of these experts said that, because NRC’s approach to assessing 

natural hazards is conservative, they believe that U.S. nuclear power 

reactors are well-designed and well-protected from natural hazards. 

Third, a few experts pointed to the expertise of NRC’s staff in assessing 

natural hazards as another strength of the agency’s processes. These 

experts said that NRC staff are diligent in trying to understand natural 

hazards and are proactive in identifying new information for assessing 

natural hazards. Several experts indicated that NRC is effective at 

identifying and considering new natural hazard data, although one of 

those experts also noted that it can take the agency time to incorporate 

new data into agency regulations. Fourth, a few experts identified as a 

strength what they said was NRC’s global leadership in developing the 

latest tools and methods for assessing the threats posed by natural 

hazards to nuclear power reactors.
 

Page 24 GAO-12-465 Nuclear Regulatory Commission

Experts we interviewed also identified a number of limitations of NRC 

processes for assessing the threats posed by natural hazards to 

operating nuclear power reactors. Three limitations were identified by 

more than two experts. First, about half of the experts identified as a 

limitation that NRC has not required licensees of operating reactors to 

assess natural hazards using PRA and that licensees generally have not 

done so. One of these experts noted that while NRC generally has good 

information on natural hazards, the use of PRA would allow NRC to better 

quantify the adequacy and uncertainty of that information. Another of 

these experts stated that deterministic assessments of natural hazards 

alone can provide a false sense of security with regard to the certainty of 

the information that has been assessed. Second, several experts 

identified as an additional limitation of NRC processes for assessing 

natural hazards that some existing assessments are not as robust or upto-date as they could be. One of these experts noted that NRC generally 

does not require licensees of operating reactors to systematically 

reevaluate their natural hazard assessments, and some experts 

highlighted external flooding and high-wind hazards in particular as 

warranting further review. Third, a few experts identified as a limitation 

that NRC’s processes for assessing natural hazard threats are overly 

bureaucratic and slow and can lead to overly conservative results. One of 

those experts noted that NRC’s conservatism can potentially lead to the 

costly overdesign of nuclear power reactors. Another of these experts 

stated that NRC processes are bureaucratic and time-consuming and 

undermine the agency’s ability to identify actions that need to be taken 

promptly in response to new information.

More than half of the 15 experts we interviewed suggested expanding the 

use of PRA for assessing natural hazards at operating nuclear power 

reactors as a complement to traditional deterministic assessments, and 

several experts cited a number of advantages, as well as challenges to 

doing so. The experts’ suggestion was similar to a lesson learned from 

the Fukushima Daiichi disaster identified by a team from the International 

Atomic Energy Agency, which found a need for the nuclear power 

community to increase efforts to develop PRAs for external events.44

 

44International Atomic Energy Agency, Mission Report: The Great East Japan Earthquake 

Expert Mission (Vienna, Austria: June 16, 2011). 

Of 

the experts who suggested expanding the use of PRA, more than half 

More Than Half of Experts 

Suggested Expanded 

Use of PRA
 

Page 25 GAO-12-465 Nuclear Regulatory Commission

noted that such risk assessments should not be the sole basis for 

decision making but should be used to complement traditional 

deterministic assessments in order to provide a more robust approach to 

assessing natural hazards. A few experts noted that NRC has already 

moved in this direction by requiring that licensees of new reactors 

develop and maintain PRAs that address natural hazards. Some experts 

emphasized the importance of NRC better integrating PRA into agency 

processes for assessing natural hazards, and others said that such risk 

assessments of natural hazards should be comprehensive, periodically 

updated, and integrated with licensees’ PRAs of internal events.

The experts who suggested expanding the use of PRA for natural 

hazards cited a number of advantages of using it to complement 

traditional deterministic assessments, including the following:

• PRA takes a broader look at hazards than traditional deterministic 

assessment by systematically examining all potential causes of an 

accident for any particular hazard, including events beyond those 

accounted for in a reactor’s original design.

• PRA considers and quantifies the likelihood of events and can help 

inform regulatory priorities by identifying more significant contributors 

to a reactor’s risk profile.

• PRA provides a structure for explicitly addressing and presenting 

uncertainty and attempts to account for unknowns.

• PRA can help identify design vulnerabilities that might otherwise be 

overlooked by relying on traditional deterministic assessments alone.

• PRA can support improved safety decision making by providing risk 

insights.

Several experts also identified challenges to expanding the use of PRA 

for natural hazards, including the following:

• More than half of the experts we interviewed noted that uncertain 

knowledge about natural hazards, particularly the frequency of large 

hazards, presents a challenge in evaluating those hazards. Some 

experts noted the difficulty of identifying the frequency of large natural 

hazards for use in a PRA given the limited data resulting from the brief 

historical record and rarity of the hazards. One of these experts said 

he believes the uncertainties associated with low-probability, high-
 

Page 26 GAO-12-465 Nuclear Regulatory Commission

consequence natural hazards are too large for a PRA to produce an 

accurate risk estimate, and two other experts said the information 

generated by a PRA would be useful to inform but not determine 

decisions.

• About half of the experts said another key challenge is the limited 

number of experts qualified to develop PRAs for natural hazards. One 

expert noted the number of practitioners that can do this type of risk 

assessment is small and that it is important that more people be 

trained to conduct them. Another expert said that finding people to do 

these analyses, more so than the cost involved in conducting them, 

represents the primary challenge to expanding the use of PRA. He 

further noted that this is even more the case now given the actions 

NRC and industry are taking in response to the Fukushima Daiichi 

disaster. In June 2008, we noted a similar challenge to NRC’s efforts 

to transition to a risk-informed approach to fire protection.45 At that 

time, numerous NRC, industry, and academic officials we spoke with 

expressed concern regarding the limited number of personnel with the 

necessary skills and training to design, review, and inspect against 

PRAs. Those officials told us that the shortage of skilled personnel 

was due in part to an increased demand for individuals with critical 

skills under NRC’s risk-informed approach. We have reported on this 

challenge of the limited number of PRA practitioners as far back as 

1985.46

• A few experts identified other challenges to expanding the use of PRA 

for natural hazards, including the cost of developing those risk 

assessments, the regulatory hurdles to NRC requiring licensees of 

operating reactors to use PRA, and the care that must be taken when 

deciding which natural hazards to include in a PRA and which to 

screen out of the analysis.

NRC and industry officials we interviewed agreed that the 

limited number of experts qualified in PRA remains a challenge today.

Some of the 15 experts we interviewed suggested changes to NRC 

processes for assessing the threats posed by natural hazards to 

operating U.S. nuclear power reactors that were unrelated to expanding 

 

45GAO-08-747. 

46GAO/RCED-85-11. 

Some Experts Suggested 

Other Changes to NRC 

Assessment Processes
 

Page 27 GAO-12-465 Nuclear Regulatory Commission

the use of PRA. Five such changes were suggested by at least two 

experts as follows:

• Several experts suggested that NRC should have licensees revisit 

their existing natural hazard assessments to incorporate current 

knowledge on natural hazards or ensure that the assumptions used in 

the assessments have not changed. Some of these experts noted that 

the state of knowledge on natural hazards such as earthquakes, 

floods, and high-wind events had changed since licensees completed 

their assessments. More than half of the experts we interviewed said 

the NRC Fukushima Daiichi task force’s recommendations that NRC 

require licensees to reevaluate their seismic and flooding hazards and 

confirm them on a periodic basis in the future were useful, and some 

of those experts suggested that the reevaluation should include all 

natural hazards, as it will, according to NRC officials.47

• A few experts suggested that NRC should examine the criteria that 

licensees use to decide whether to assess a natural hazard or screen 

it out of their assessment. One expert suggested that NRC create a 

special category of assessments to consider low probability, highconsequence natural hazards that otherwise might be screened out.

One expert 

said it would be better if NRC required licensees to perform a 

comprehensive assessment of natural hazards rather than a 

reevaluation of existing assessments against current requirements. A 

few experts said the reevaluations proposed by the task force would 

be of only limited utility. Two of those experts indicated that resources 

would be better devoted to having licensees enhance their reactors’ 

abilities to mitigate natural hazards and to reevaluate natural hazards 

as significant new information becomes available rather than on a set 

periodic basis.

• Two experts suggested that NRC should devote additional resources 

to researching natural hazards in order to inform agency assessments 

and guidance relating to those hazards.

 

47In October 2011, NRC’s Advisory Committee on Reactor Safeguards recommended that 

agency staff consider additional external hazards beyond seismic and flooding hazards as 

part of the lessons learned from the Fukushima Daiichi disaster. After we completed our 

interviews with experts, the 2012 Consolidated Appropriation Act was enacted with a 

provision that, among other things, directed NRC to require licensees to reevaluate the 

seismic, tsunami, flooding, and other external hazards at their sites against current 

requirements and guidance as expeditiously as possible, and thereafter when appropriate, 

as determined by the NRC.
 

Page 28 GAO-12-465 Nuclear Regulatory Commission

• Two experts suggested that NRC should do more work to assess how 

natural hazards might affect sites with multiple reactors. One expert 

expressed the view that there has not been a systematic enough 

examination of the potential consequences of a natural hazard that 

might affect multiple reactors.

• Two experts suggested that NRC should consider how potential future 

changes in natural hazards are addressed in assessments. One 

expert noted, for example, that climate change could potentially lead 

to changes in natural hazards over reactors’ operating lives.

In addition, two experts said that they do not believe NRC needs to make 

changes to its processes for assessing the threats posed by natural 

hazards beyond what NRC is already undertaking in response to the 

Fukushima Daiichi disaster. One expert said that NRC and licensees 

should focus on enhancing reactors’ ability to cope with and respond to 

natural hazards rather than devote those resources to further assessing 

natural hazard threats.

Operating a nuclear power reactor is never entirely free of risk, but 

comprehensive assessments of that risk and various contributing risk 

factors—including natural hazards—can help identify vulnerabilities that 

might otherwise be overlooked. Actions taken to mitigate such risks could 

decrease the chance of an accident that could have serious public health, 

environmental, and economic consequences. NRC documents and most 

experts we spoke with indicate that PRA offers NRC and licensees a 

number of advantages that, as a complement to traditional deterministic 

assessments, can provide a more robust approach to assessing natural 

hazards.

Because NRC and licensees of operating reactors continue to rely 

primarily on deterministic assessments of natural hazards, they are 

realizing PRA’s advantages to a limited extent. We recognize that NRC 

must undertake a thorough review to require licensees of operating 

reactors to develop and maintain PRAs that address natural hazards. In 

particular, we acknowledge the difficulty that NRC officials anticipate in 

reaching a determination that a PRA requirement is warranted for 

operating reactors under existing regulations. Further, we understand 

there are a limited number of experts qualified to develop PRAs for 

natural hazards, and that there is a cost to developing PRAs. However, 

we also note NRC’s overall endorsement of the use of PRA to enhance 

and extend traditional deterministic assessments, as well as the 

Conclusions 
 

Page 29 GAO-12-465 Nuclear Regulatory Commission

significant advantages cited by the experts we interviewed who 

suggested expanding the use of PRA for assessing natural hazards at 

operating nuclear power reactors. Moreover, we note that a team from the 

International Atomic Energy Agency has identified a need to increase 

efforts to develop PRAs for external events as a lesson learned from the 

Fukushima Daiichi disaster. While NRC suggested PRA in the 1990s as 

one potential option for licensees to use to assess natural hazards, most 

licensees opted not to do so, and agency officials and industry 

representatives told us they believe that licensees today have only 

applied PRA to natural hazards to a limited extent. Given all this, PRAs’ 

potential benefits in the natural hazard context may not be realized 

without efforts by NRC to promote its use. Without conducting the 

analyses necessary to determine whether a PRA requirement is 

warranted for operating reactors, however, NRC does not have a strong 

basis to judge whether or not such a requirement should be implemented.

To improve safety decision making by NRC in carrying out its 

responsibility under the Atomic Energy Act to ensure that the operation of 

nuclear power plants is consistent with the common defense and security 

and public health and safety, we recommend that the NRC 

Commissioners direct agency staff to conduct and document any needed 

analyses to determine whether the agency should require licensees of 

operating reactors to develop and maintain PRAs that address natural 

hazards.

We provided a draft of this report to the NRC Commissioners and the 

Secretaries of Commerce, Defense, and the Interior for their review and 

comment. The Secretaries of Commerce, Defense, and the Interior had 

no comments. NRC provided written comments on April 13, 2012, which 

are reproduced in appendix VI, and technical comments, which we 

incorporated into the report as appropriate.

NRC agreed with the report recommendation. The agency stated it would 

conduct and document the analysis recommended by GAO in the context 

of follow-on actions for related ongoing agency initiatives after sufficient 

information has been gathered from those activities to better inform the 

analysis. In addition, NRC asserted that it will continue to evaluate its 

processes and policies regarding the enhanced use of PRA and risk 

insights as part of its overall regulatory framework. As we noted in our 

report, NRC documents and most experts we spoke with indicate that 

PRA offers NRC and licensees a number of advantages that, as a 

Recommendation for 

Executive Action

Agency Comments 

and Our Evaluation
 

Page 30 GAO-12-465 Nuclear Regulatory Commission

complement to traditional deterministic assessments, can provide a more 

robust approach to assessing natural hazards. We also noted that 

officials we interviewed and data we obtained indicate that NRC and 

licensees apply PRA to natural hazards at operating nuclear power 

reactors to a limited extent. We continue to believe that until NRC 

completes the analysis necessary to determine whether a PRA 

requirement is warranted for operating reactors, NRC will not have a 

strong basis to judge whether or not such a requirement should be 

implemented, and PRAs’ potential benefits in the natural hazard context 

may not be realized.

As agreed with your offices, unless you publicly announce the contents of 

this report earlier, we plan no further distribution until 30 days from the 

report date. At that time, we will send copies to the Chairman of the NRC, 

the Secretaries of Commerce, Defense, and the Interior, the appropriate 

congressional committees, and other interested parties. In addition, this 

report will be available at no charge on the GAO website at 

http://www.gao.gov. 

If you or your staff members have any questions about this report, please 

contact me at (202) 512-3841 or ruscof@gao.gov. Contact points for our 

Offices of Congressional Relations and Public Affairs may be found on 

the last page of this report. Key contributors to this report are listed in 

appendix VII.

Frank Rusco

Director, Natural Resources and Environment
Appendix I: Objectives, Scope, and 

Methodology

Page 31 GAO-12-465 Nuclear Regulatory Commission

Our review provides information on: (1) the extent to which probabilistic 

risk assessment (PRA) is applied to natural hazards at operating U.S. 

nuclear power reactors and (2) expert views on and suggested changes, 

if any, to Nuclear Regulatory Commission (NRC) processes for assessing 

natural hazards at operating U.S. nuclear power reactors. To address 

both of these objectives, we reviewed relevant literature and NRC 

documents and met with officials from NRC, the nuclear power industry, 

public interest groups, and others to understand NRC processes for 

assessing natural hazards at nuclear power reactors.

Specifically, to address the first objective, we reviewed relevant 

documents and data obtained from NRC and selected licensees on the 

extent to which PRA has been applied to natural hazards. We interviewed 

knowledgeable officials about the data and found the data to be 

sufficiently reliable for the purposes of our report. We reviewed NRC 

policies and procedures, as well as NRC initiatives related to the 

assessment of natural hazards. We also reviewed the NRC near-term 

task force’s report on insights gained from the Fukushima Daiichi disaster 

and documented the actions taken by NRC on recommendations related 

to the assessment of natural hazards. In addition, we visited a 

nonprobability sample of five nuclear power plants to interview licensees 

about the actions they have taken to assess natural hazards.1

To address the second objective, we summarized the results of 

semistructured interviews with experts in assessing natural hazards, risks 

to nuclear power reactors, or both. We first reviewed NRC documentation 

about processes used to assess natural hazards at nuclear power 

reactors and confirmed our understanding of those processes with NRC. 

Then, we identified 43 experts from our review of the literature, prior GAO 

reports, congressional and NRC hearings, and recommendations from 

The five 

nuclear power plants we visited were the Browns Ferry Nuclear Plant in 

Alabama, the Diablo Canyon Power Plant in California, the North Anna 

Power Station in Virginia, the San Onofre Nuclear Generating Station in 

California, and the Turkey Point Nuclear Plant in Florida. We selected 

these sites to capture a variety of characteristics, including reactor and 

containment vessel type, operating license issuance date, license 

renewal status, and natural hazard activity level.

 

1

Because this was a nonprobability sample, the information we gathered from these site 

visits is not generalizeable to all 65 operating nuclear power plants but provides important 

illustrative information.

Appendix I: Objectives, Scope, and 

Methodology 
Appendix I: Objectives, Scope, and 

Methodology

Page 32 GAO-12-465 Nuclear Regulatory Commission

NRC, the Nuclear Energy Institute, the Union of Concerned Scientists, 

and experts in assessing risks to nuclear power reactors. From this list, 

we selected 15 experts based on (1) the relevance of their expertise as 

reflected in publications, testimonies, and their biographies; (2) the 

relevance and extent of their publications; (3) the relevance and extent of 

their testimonies before Congress and NRC; and (4) recommendations 

from others in the field. Finally, to ensure coverage and a range of 

perspectives, we selected experts who came from academia, 

government, industry, public interest groups, and other affiliations; 

provided perspectives from both those inside NRC or the nuclear power 

industry and from those outside of those two groups; and provided 

perspectives from those publicly supportive, publicly critical, and those 

without a public position on nuclear power. Appendix II lists the experts 

we interviewed. We conducted a content analysis to assess experts’ 

responses to a standard set of questions and grouped responses into 

overall themes. The views expressed by experts do not necessarily 

represent the views of GAO. Not all of the experts provided their views on 

all issues. We did not independently evaluate the quality of NRC 

processes for assessing natural hazards and the threats they pose to 

nuclear power reactors.

During the course of our review, we interviewed NRC officials from the 

Office of General Counsel, Office of New Reactors, Office of Nuclear 

Reactor Regulation (including the Japan Lessons Learned Project 

Directorate), Office of Nuclear Regulatory Research, and Risk 

Management Task Force in headquarters, as well as NRC officials in 

Region II and Region IV and the resident inspectors at the five nuclear 

power plants we visited. We also interviewed officials and reviewed 

documents from the National Oceanic and Atmospheric Administration, 

the U.S. Army Corps of Engineers, and the U.S. Geological Survey on the 

current state of knowledge on natural hazards, how that knowledge has 

changed over time, and the related uncertainty. Further, we interviewed 

representatives from American Nuclear Insurers, the Nuclear Energy 

Institute, and the Union of Concerned Scientists to discuss their views on 

NRC processes for assessing natural hazards and the threats those 

hazards pose to nuclear power reactors.

We conducted this performance audit from May 2011 to April 2012 in 

accordance with generally accepted government auditing standards. 

Those standards require that we plan and perform the audit to obtain 

sufficient, appropriate evidence to provide a reasonable basis for our 

findings and conclusions based on our audit objectives. We believe that 
Appendix I: Objectives, Scope, and 

Methodology

Page 33 GAO-12-465 Nuclear Regulatory Commission

the evidence obtained provides a reasonable basis for our findings and 

conclusions based on our audit objectives.
Appendix II: List of Experts

Page 34 GAO-12-465 Nuclear Regulatory Commission

• Dennis C. Bley, Buttonwood Consulting, Inc., Member of NRC’s 

Advisory Committee on Reactor Safeguards

• Biff Bradley, Nuclear Energy Institute

• Robert J. Budnitz, Lawrence Berkeley National Laboratory

• Thomas B. Cochran, Natural Resources Defense Council

• Karl N. Fleming, KNF Consulting Services LLC

• B. John Garrick, U.S. Nuclear Waste Technical Review Board

• Michael W. Golay, Massachusetts Institute of Technology

• N. Prasad Kadambi, Consultant

• William Leith, U.S. Geological Survey

• William Levis, PSEG Power LLC

• Stuart Lewis, Electric Power Research Institute

• David A. Lochbaum, Union of Concerned Scientists

• Mohammad Modarres, University of Maryland

• Ali Mosleh, University of Maryland

• Doug True, ERIN Engineering & Research, Inc.

Appendix II: List of Experts
Appendix III: Key Developments in NRC’s 

Approach to Probabilistic Risk Assessment

Page 35 GAO-12-465 Nuclear Regulatory Commission

According to NRC documents, NRC is moving toward a risk-informed, 

performance-based approach to decision making that is being 

implemented in phases. This approach extends the traditional 

deterministic approach in part by incorporating PRA—a systematic 

method for assessing what can go wrong, its likelihood, and its potential 

consequences to determine quantitative estimates of risk. The timeline 

below describes some of the key developments in NRC’s approach to

PRA, including its application to natural hazards.

NRC publishes the first PRA of nuclear power reactors.1 According to an 

NRC document, this study marks the first U.S. attempt to systematically 

evaluate a large spectrum of accidents and to use quantitative techniques 

to evaluate severe accident probabilities and off-site radiological 

consequences in an integrated manner to obtain a more realistic estimate 

of public risk. The study challenged the concept that conservative safety 

analysis of design-basis accidents could establish an upper limit of public 

risk. 

An accident occurs at the Three Mile Island nuclear power plant, resulting 

in a partial meltdown of a reactor core. According to an NRC document, 

the accident substantiated risk insights generated through the 1975 study 

and led to the increased use of PRA to identify vulnerabilities in the 

nuclear industry.

NRC issues its Safety Goal Policy Statement in which it broadly defines 

an acceptable level of radiological risk that might be imposed on the 

public as a result of nuclear power reactor operations and calls for the 

use of PRA to measure achievement of the agency’s individual and 

societal safety goals.2

 

1

NRC, Reactor Safety Study: An Assessment of Accident Risks in U.S. Commercial 

Nuclear Power Plants, WASH-1400(NUREG 75/014) (Washington, D.C.: October 1975). 

2

NRC, Safety Goals for the Operations of Nuclear Power Plants; Policy Statement, 51 

Fed. Reg. 30,028 (Aug. 21, 1986).

Appendix III: Key Developments in NRC’s 

Approach to Probabilistic Risk Assessment

1975

1979

1986
Appendix III: Key Developments in NRC’s 

Approach to Probabilistic Risk Assessment

Page 36 GAO-12-465 Nuclear Regulatory Commission

NRC initiates the Individual Plant Examination program and requests that 

each licensee perform an examination to identify plant-specific 

vulnerabilities to severe accidents that could be fixed with low-cost 

improvements.3 One of several program objectives is for each licensee to 

gain a more quantitative understanding of the overall probability of core 

damage and radioactive material releases. NRC identifies PRA as one 

acceptable approach for licensees to use in conducting their examination 

but limits the scope of the effort to internal events.

NRC publishes a follow-on study to its original 1975 study that advances 

the state of the art in PRA, particularly the analysis of uncertainty.4

NRC initiates the Individual Plant Examination of External Events 

program and requests that each licensee identify and report to the agency 

all plant-specific vulnerabilities to severe accidents caused by external 

events, including natural hazards such as seismic events, floods, and 

high-wind events.5 NRC suggests PRA as one possible method for 

licensees to use in responding to NRC’s request.

NRC develops a PRA implementation plan that aims to ensure PRA is 

implemented in a consistent and predictable manner in regulatory 

activities.6

 

3

NRC, Individual Plant Examination for Severe Accident Vulnerabilities, 10 CFR 50.54(f), 

Generic Letter No. 88-20 (Washington, D.C.: Nov. 23, 1988). 

4

NRC, Severe Accident Risks: An Assessment for Five U.S. Nuclear Power Plants, 

NUREG-1150 (Washington, D.C.: December 1990). 

5

NRC, Individual Plant Examination of External Events for Severe Accident 

Vulnerabilities—10 CFR 50.54(f), Generic Letter No. 88-20, Supplement 4 (Washington, 

D.C.: June 28, 1991).

6

NRC, Proposed Agency-Wide Implementation Plan for Probabilistic Risk Assessment, 

SECY-94-219 (Washington, D.C.: Aug. 19, 1994).

1988

1990

1991

1994
Appendix III: Key Developments in NRC’s 

Approach to Probabilistic Risk Assessment

Page 37 GAO-12-465 Nuclear Regulatory Commission

NRC issues a policy statement regarding the expanded use of PRA.7 In 

its approval of the policy statement, the NRC stated its expectation that 

“the use of PRA technology should be increased in all regulatory matters 

to the extent supported by state-of-the-art in PRA methods and data and 

in a manner that complements the NRC’s deterministic approach and 

supports the NRC’s traditional defense-in-depth philosophy.” 

NRC requires new nuclear power reactors licensed after January 1997 to 

be sited and designed with respect to geological and seismic 

determinations based on an appropriate analysis, such as a probabilistic 

seismic hazard analysis.

8

NRC establishes a structure for risk-informed decision making to improve 

consistency in regulatory decisions where PRA results are used to 

supplement traditional deterministic and defense-in-depth approaches.9

The structure consists of five key principles: proposed changes must (1) 

meet current regulations, (2) be consistent with the defense-in-depth 

philosophy, and (3) maintain sufficient safety margins; (4) when proposed 

changes result in an increase in risk, the increases should be small and 

consistent with the intent of NRC’s Safety Goal Policy Statement; and (5) 

the impact of the proposed changes should be monitored using 

performance management strategies. The structure acknowledges that 

assurance of adequate protection of public health and safety 

encompasses more than simply demonstrating an acceptable level of 

overall risk.

 

7

NRC, Use of Probabilistic Risk Assessment Methods in Nuclear Regulatory Activities; 

Final Policy Statement, 60 Fed. Reg. 42,622 (Aug. 16, 1995).

8

10 C.F.R. Part 100, subpart B. 

9

NRC, An Approach for Using Probabilistic Risk Assessment in Risk-Informed Decisions 

on Plant-Specific Changes to the Licensing Basis, Regulatory Guide 1.174 (Washington, 

D.C.: July 1998). 

1995

1996

1998
Appendix III: Key Developments in NRC’s 

Approach to Probabilistic Risk Assessment

Page 38 GAO-12-465 Nuclear Regulatory Commission

NRC develops a strategy10 for implementing risk-informed regulation in 

response to a GAO recommendation.11

NRC issues regulations requiring that applicants for combined licenses 

for new nuclear power reactors submit a description and the results of a 

reactor-specific PRA to NRC as part of their license application.12 NRC 

also requires licensees of new nuclear power reactors that will operate 

their reactors under a combined license to develop by the scheduled date 

of their initial fuel loading a Level 1 and Level 2 reactor-specific PRA that 

covers, among other things, initiating events for which there are NRCendorsed consensus quality standards 1 year prior to the scheduled date 

for initial fuel loading.13 Licensees are required to maintain their PRA and 

upgrade it every 4 years over the operating life of the reactor, as well as 

when the licensee submits a license renewal application.14 Also, NRC 

develops its risk-informed and performance-based implementation plan.15

The plan provides guidance and direction regarding activities the agency 

undertakes to integrate risk information and performance measures into 

its regulations, regulatory guidance, and oversight processes.

NRC issues a regulatory guide that provides guidance to licensees to use 

in determining the technical adequacy of a PRA for regulatory decision 

making and endorses certain American Society of Mechanical Engineers 

and American Nuclear Society national consensus standards for PRA 

 

10NRC, Risk-Informed Regulation Implementation Plan, SECY-00-0213 (Washington, 

D.C.: Oct. 26, 2000).

11GAO/RCED-99-95. GAO recommended that “the Commissioners of NRC direct the staff 

to develop a comprehensive strategy that includes but is not limited to objectives, goals, 

activities, and time frames for the transition to risk-informed regulation; specifies how the 

Commission expects to define the scope and implementation of risk-informed regulation; 

and identifies the manner in which it expects to continue the free exchange of operational 

information necessary to improve the quality and reliability of risk assessments.” 

1210 C.F.R. § 52.79(a)(46). 

1310 C.F.R. § 50.71(h)(1). 

1410 C.F.R. § 50.71(h)(2), (3). 

15NRC, Update on the Improvements to the Risk-Informed Regulation Implementation 

Plan, Enclosure 1 - Risk-Informed and Performance-Based Plan, SECY-07-0074 

(Washington, D.C.: Apr. 26, 2007).

2000

2007

2009
Appendix III: Key Developments in NRC’s 

Approach to Probabilistic Risk Assessment

Page 39 GAO-12-465 Nuclear Regulatory Commission

quality.16 Among other things, those standards address the application of 

PRA to natural hazards.17

The Chairman of the NRC charters a task force to develop a strategic 

vision and assess options for a more comprehensive and holistic riskinformed, performance-based regulatory approach, with one option being 

better incorporating risk management concepts into NRC’s regulatory 

programs.18 Later that year, in response to the Fukushima Daiichi 

disaster, NRC charters a staff task force to review its processes and 

regulations and to recommend whether NRC should make near-term 

improvements to its regulatory system.19 The task force makes several 

recommendations in its final report, including requiring that licensees 

reevaluate the seismic and flooding hazards at their sites against current 

NRC guidance and requirements.20 With policy direction from the NRC, 

the agency staff moves forward with actions to implement the task force 

recommendations.21

 

16NRC, An Approach for Determining the Technical Adequacy of Probabilistic Risk 

Assessment Results for Risk-Informed Activities, Regulatory Guide 1.200 (Washington, 

D.C.: March 2009). 

17American Society of Mechanical Engineers and American Nuclear Society, Addenda to 

ASME/ANS RA-S-2008: Standard for Level 1/Large Early Release Frequency Probabilistic 

Risk Assessment Results for Nuclear Power Plant Applications, ASME/ANS RA-Sa-2009 

(ASME, New York, NY; ANS, La Grange Park, IL: February 2009). 

18NRC, Charter for Task Force for Assessment of Options for More Holistic Risk-Informed, 

Performance-Based Regulatory Approach (Washington, D.C.: Feb. 11, 2011). 

19NRC, Charter for the Nuclear Regulatory Commission Task Force to Conduct a NearTerm Evaluation of the Need for Agency Actions Following the Events in Japan

(Washington, D.C.: Mar. 30, 2011).

20NRC, Recommendations for Enhancing Reactor Safety in the 21st Century: The NearTerm Task Force Review of Insights From the Fukushima Dai-ichi Accident (Washington, 

D.C: July 12, 2011).

21NRC, Prioritization of Recommended Actions to Be Taken in Response to Fukushima 

Lessons Learned, SECY-11-0137 (Washington, D.C.: Oct. 3, 2011). 

2011
Appendix III: Key Developments in NRC’s 

Approach to Probabilistic Risk Assessment

Page 40 GAO-12-465 Nuclear Regulatory Commission

NRC issues three orders and a request for information in response to 

lessons learned from the Fukushima Daiichi disaster.22

 

22NRC, Staff Requirements—SECY-12-0025—Proposed Orders and Requests for 

Information in Response to Lessons Learned From Japan’s March 11, 2011, Great 

Tohoku Earthquake and Tsunami (Washington, D.C.: Mar. 9, 2012). 

As part of the 

request for information, the staff asks licensees to reevaluate the seismic 

and flooding hazards at their sites and suggests PRA as one possible 

method to address the seismic hazard reevaluation.

2012
Appendix IV: Information on Developing a 

Probabilistic Risk Assessment

Page 41 GAO-12-465 Nuclear Regulatory Commission

Probabilistic risk assessments are used to evaluate several categories of 

initiating events––that is, events that can lead to a reactor accident—

including internal events, which start inside the nuclear power plant or the 

electric system serviced by the power plant (e.g., random hardware 

failures and operator actions), and external events, which typically start 

outside the nuclear power plant, such as natural hazards (e.g., 

earthquakes, external floods, external fires, and high-wind events). 

According to NRC documents, external events are of significant concern 

since they can both initiate core damage accidents and negate or 

compromise the safety systems or procedures used to prevent or mitigate 

such accidents.

According to NRC documents, PRA can estimate three different levels of 

risk—one focused on reactor core damage, one focused on containment 

release, and one focused on radiological consequences to the public. 

Specifically, Level 1 PRAs evaluate events that can lead to plant 

accidents and examine reactor systems and operators’ responses to 

calculate “core damage frequency”—the frequency of the combinations of 

initiating events, hardware failures, and human errors leading to the 

uncovering of the reactor core such that reflooding of the core is not 

imminent. A Level 1 PRA consists of six major steps: (1) identifying a 

comprehensive list of events that can lead to accidents and grouping 

those events into categories based on their potential impact on accident 

response systems; (2) establishing success criteria based on traditional 

engineering analyses; (3) modeling accident sequences; (4) estimating 

parameters for the analysis such as initiating event frequencies and the 

failure rates of particular components; (5) quantifying accident 

sequences; and (6) documenting and evaluating the results. To 

accomplish these steps, a PRA employs several specific techniques, 

including event trees, fault trees, human reliability analyses, and Monte 

Carlo methods (see table 2). Analysts use these methods to estimate the 

core damage frequency for each accident sequence. Those frequencies 

are then totaled to calculate the total core damage frequency for a 

reactor.

Appendix IV: Information on Developing a 

Probabilistic Risk Assessment 
Appendix IV: Information on Developing a 

Probabilistic Risk Assessment

Page 42 GAO-12-465 Nuclear Regulatory Commission

Table 2: Selected Techniques Used in PRA

Technique Description

Accident sequence analysis (event 

trees)

Model responses to each event that can initiate an accident. For any given initiating event, 

there are various accident sequences resulting from whether systems operate properly or fail 

and what actions operators take. Some accident sequences will result in a safe recovery, and 

some will result in reactor core damage. Event trees graphically represent accident 

sequences. 

System analysis (fault trees) Model systems in detail. Many events in an event tree represent a system that is needed to 

respond to the initiating event. An analysis is performed for each such event, which is 

graphically represented with a fault tree that identifies the combination of failures that cause 

the overall system to fail. The fault tree logic is then used to calculate overall failure probability.

Human reliability analysis Used to evaluate human errors that are important to the outcome of an event. Analysts assess 

the probability of a human error in light of factors like training, procedures, and expected 

conditions during an event.

Monte Carlo methods Used to compute risks. Allows analysts to consider variations in each analysis factor, imperfect 

knowledge, as well as many possible ways that factors can interact.

Source: NRC.

Level 2 PRAs start with Level 1 core damage accidents and assess the 

frequencies of various categories of containment releases. The results for 

operating reactors are typically reported in terms of “large early release 

frequency”—the frequency of those accidents leading to significant, 

unmitigated releases from the reactor’s containment in a time frame prior 

to effective evacuation of the nearby population such that there is a 

potential for early health effects. Level 3 PRAs start with Level 2 

radiological release accidents and assess the public health 

consequences of a radiological release in terms of injury to the public and 

damage to the environment.1

 

1

Because core damage and large early release estimates are easier to calculate than 

public health consequences, the results of Level 1 and Level 2 PRAs are often used as 

surrogates for the results of a Level 3 PRA. NRC has plans to perform a new Level 3 PRA 

for a single operating power plant that will include assessment of natural hazards and be 

completed within 4 years.

 
Appendix V: NRC Actions on Natural Hazard 

Assessment in Response to the Fukushima 

Daiichi Disaster

Page 43 GAO-12-465 Nuclear Regulatory Commission

NRC has taken a number of actions in response to the Fukushima Daiichi 

disaster. The timeline below describes actions that relate to natural 

hazard assessments.

A 9.0 magnitude earthquake and subsequent tsunami lead to a prolonged 

loss of alternating current electrical power at several reactors at Japan’s 

Fukushima Daiichi nuclear power plant. As a result of the loss of power, 

plant operators are unable to keep three of the reactors cool, which leads 

to fuel melting, hydrogen explosions, and the release of radioactive 

material into the environment. Among several actions, NRC charters a 

staff task force to review its processes and regulations in light of the 

Fukushima Daiichi disaster and to recommend whether NRC should 

make near-term improvements to its regulatory system.1

The NRC task force issues its final report.2

 

1

NRC, Charter for the Nuclear Regulatory Commission Task Force to Conduct a NearTerm Evaluation of the Need for Agency Actions Following the Events in Japan

(Washington, D.C.: Mar. 30, 2011).

The task force finds that NRC 

regulations and guidance provide a robust approach for assessing natural 

hazards but notes that NRC’s processes for assessing natural hazards 

have evolved over time. As a result, the licensing bases, design, and level 

of protection from natural hazards differ among operating reactors 

depending on when the reactors were built and when they were licensed 

for operation. Based on these findings, the task force recommends 

actions to, among other things, clarify and strengthen NRC’s regulatory 

framework for protection against natural hazards. Three of these 

recommended actions relate to the assessment of natural hazards. 

Specifically, the task force recommends that (1) NRC order licensees to 

reevaluate the seismic and flooding hazards at their sites against current 

NRC requirements and guidance and, if necessary, update their reactor 

design bases; (2) NRC initiate rulemaking to require licensees to confirm 

their seismic and flooding hazards every 10 years, address any new and 

significant information and, if necessary, update their reactor design 

bases; and (3) NRC order licensees to check their seismic and flood 

protection features to identify and address any plant-specific 

2

NRC, Recommendations for Enhancing Reactor Safety in the 21st Century: The NearTerm Task Force Review of Insights from the Fukushima Dai-ichi Accident (Washington, 

D.C.: July 12, 2011). 

Appendix V: NRC Actions on Natural Hazard 

Assessment in Response to the Fukushima 

Daiichi Disaster

March 2011

July 2011
Appendix V: NRC Actions on Natural Hazard 

Assessment in Response to the Fukushima 

Daiichi Disaster

Page 44 GAO-12-465 Nuclear Regulatory Commission

vulnerabilities and verify the adequacy of monitoring and maintenance for 

protection features such as watertight barriers and seals.

The NRC directs agency staff, among other things, to engage 

stakeholders on the task force recommendations and to identify and 

prioritize the recommendations that in the staff’s view can and should be 

implemented without unnecessary delay.3

Agency staff report to the NRC Commissioners on the task force 

recommendations that in their view can and should be implemented 

without unnecessary delay.4 Among other things, the staff recommends 

that the NRC move forward on requiring licensees to reevaluate the 

seismic and flooding hazards at their sites against current NRC 

requirements and guidance. The staff also recommends that the NRC 

move forward on requiring licensees to check their seismic and flood 

protection features.

Agency staff propose a prioritization of the task force recommendations to 

the NRC Commissioners and describe the actions necessary to 

implement the recommendations.5

 

3

NRC, Staff Requirements—SECY-11-0093—Near-Term Report and Recommendations 

for Agency Actions Following the Events in Japan (Washington, D.C.: Aug. 19, 2011). 

The staff suggests deferring the task 

force recommendation that NRC initiate rulemaking to require licensees 

to confirm their seismic and flooding hazards every 10 years until the 

related recommendation for licensees to reevaluate seismic and flooding 

hazards is implemented. Later in the month, the NRC approves the staff’s 

proposal on the task force recommendations that can and should be 

implemented without unnecessary delay and says that the agency should 

4

NRC, Recommended Actions to be Taken Without Delay From the Near-Term Task 

Force Report, SECY-11-0124 (Washington, D.C.: Sept. 9, 2011). 

5

NRC, Prioritization of Recommended Actions to Be Taken in Response to Fukushima 

Lessons Learned, SECY-11-0137 (Washington, D.C.: Oct. 3, 2011). 

August 2011

September 2011

October 2011
Appendix V: NRC Actions on Natural Hazard 

Assessment in Response to the Fukushima 

Daiichi Disaster

Page 45 GAO-12-465 Nuclear Regulatory Commission

strive to complete and implement the lessons learned from the 

Fukushima Daiichi disaster by 2016.6

The NRC approves the agency staff’s proposals on implementing task 

force recommendations and supports actions on several, including the 

recommendations that licensees reevaluate their seismic and flooding 

hazards against current NRC requirements and guidance and check their 

seismic and flood protection features.7 Later in the month, Congress 

passes and the President signs the 2012 Consolidated Appropriations 

Act, which includes a provision directing NRC to require that licensees 

reevaluate external hazards at their sites, including seismic and flooding 

hazards, as expeditiously as possible.8

NRC announces that it is accelerating its plans for implementing the task 

force recommendations. According to NRC officials, agency staff will 

propose expanding licensee reevaluations beyond seismic and flooding 

hazards to include all natural hazards that are relevant to a site, but they 

expect to recommend that the reevaluation of seismic and flooding 

hazards take priority over other natural hazards.

The NRC staff proposes issuing three orders and a request for 

information in response to lessons learned from the Fukushima Daiichi 

disaster.9

 

6

NRC, Staff Requirements—SECY-11-0124—Recommended Actions to be Taken 

Without Delay From the Near-Term Task Force Report (Washington, D.C.: Oct. 18, 2011). 

The staff’s stated goal is for the orders and request for 

information to be issued before the first anniversary of the earthquake 

and tsunami that led to the Fukushima Daiichi disaster. As part of the 

proposed request for information, the staff plans to ask licensees to 

7

NRC, Staff Requirements—SECY-11-0137—Prioritization of Recommended Actions to 

Be Taken in Response to Fukushima Lessons Learned (Washington, D.C.: Dec. 15, 

2011). 

8

Consolidated Appropriations Act, 2012, Pub. L. No. 112-74, Div. B, § 402 (Dec. 23, 

2011). 

9

NRC, Proposed Orders and Requests for Information in Response to Lessons Learned 

from Japan’s March 11, 2011, Great Tohoku Earthquake and Tsunami, SECY-12-0025 

(Washington, D.C.: Feb. 17, 2012).

December 2011

January 2012

February 2012
Appendix V: NRC Actions on Natural Hazard 

Assessment in Response to the Fukushima 

Daiichi Disaster

Page 46 GAO-12-465 Nuclear Regulatory Commission

reevaluate the seismic and flooding hazards at their sites. NRC staff 

propose addressing other natural hazards, such as wind and missile 

loads from tornados and hurricanes, at a later date once sufficient 

resources are available for the reevaluations. The staff also plans to ask 

licensees to check their seismic and flood protection features.

NRC issues the three orders and the request for information proposed by 

agency staff in February.10

 

10NRC, Staff Requirements—SECY-12-0025—Proposed Orders and Requests for 

Information in Response to Lessons Learned From Japan’s March 11, 2011, Great 

Tohoku Earthquake and Tsunami (Washington, D.C.: Mar. 9, 2012). 

March 2012
Appendix VI: Comments from the Nuclear 

Regulatory Commission

Page 47 GAO-12-465 Nuclear Regulatory Commission

Appendix VI: Comments from the Nuclear 

Regulatory Commission 
Appendix VI: Comments from the Nuclear 

Regulatory Commission

Page 48 GAO-12-465 Nuclear Regulatory Commission
Appendix VII: GAO Contact and Staff 

Acknowledgments

Page 49 GAO-12-465 Nuclear Regulatory Commission

Frank Rusco, (202) 512-3841, or ruscof@gao.gov

In addition to the individual named above, Kimberly Gianopoulos, 

Assistant Director; David Marroni; Emmy Rhine; and Ashley Vaughan 

made key contributions to this report. Important contributions were also 

made by R. Scott Fletcher, Cindy Gilbert, Jonathan Kucskar, Alison 

O’Neill, Timothy Persons, Kiki Theodoropoulos, and Jack Wang.

Appendix VII: GAO Contact and Staff 

Acknowledgments 

GAO Contact 

Staff 

Acknowledgments 

(361295)
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