Nuclear Regulatory Commission: Natural Hazard Assessments Could Be More Risk-Informed
Published: 2012-04-26T00:00:00

Fast Facts:

Highlights:
section_title: Why GAO Did This Study
paragraphs: 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.
section_title: What GAO Found
paragraphs: 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.
section_title: What GAO Recommends
paragraphs: 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.

section_title: Letter
paragraphs: 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. 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  further highlighted by natural hazards that affected the sites of several  U.S. commercial nuclear power plants and their reactors in 2011,  including flooding near two power plants in Nebraska, severe storms at a  plant in Alabama, and an earthquake at a plant in Virginia.
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. 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.
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  considering quantitative estimates of risk. 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. The agency has been moving since the 1990s toward a risk- informed, performance-based approach to regulatory decision making  that is being implemented in phases according to NRC documents. 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.
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  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  actions they have taken to assess natural hazards. 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,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.
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.
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  the evidence obtained provides a reasonable basis for our findings and  conclusions based on our audit objectives.
subsections: section_title: Background
paragraphs: 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.
subsections: section_title: Commercial Nuclear Power Reactors in the United States
paragraphs: 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. 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.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.
section_title: Actions NRC Has Taken on Natural Hazard Assessments Since the Fukushima Daiichi Disaster
paragraphs: 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   The task force issued its final report in July 2011.  regulatory system.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 plant- 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. 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. 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,  NRC issued the request for information proposed by agency staff in  February.
section_title: PRA Is Applied to Natural Hazards to a Limited Extent
paragraphs: 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.
subsections: section_title: Licensees and NRC Apply PRA to Natural Hazards to a Limited Extent
paragraphs: 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  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  are a significant contributor to risk and provide that information to NRC.  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.
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). 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  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.
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.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   These models are available to NRC  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. 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. 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, 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.
section_title: NRC Has Not Analyzed Whether It Should Require PRAs That Address Natural Hazards
paragraphs: 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  a public health and safety standpoint. 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.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   NRC officials we spoke with were unaware of any formal  requirement from the backfit rule. 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.
section_title: Experts Offered a Range of Views on NRC Assessment Processes
paragraphs: 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.
subsections: section_title: Experts Noted Strengths and Limitations of NRC Assessment Processes
paragraphs: 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  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-in- depth 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.
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 up- to-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.
section_title: More Than Half of Experts Suggested Expanded Use of PRA
paragraphs: 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.the experts who suggested expanding the use of PRA, more than half   Of  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- 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. 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.limited number of experts qualified in PRA remains a challenge today.
NRC and industry officials we interviewed agreed that the
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.
section_title: Some Experts Suggested Other Changes to NRC Assessment Processes
paragraphs: 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  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. 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.
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, high- consequence natural hazards that otherwise might be screened out.
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.
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.
section_title: Conclusions
paragraphs: 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  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.
section_title: Recommendation for Executive Action
paragraphs: 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.
section_title: Agency Comments and Our Evaluation
paragraphs: 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  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.
section_title: Appendix I: Objectives, Scope, and Methodology
paragraphs: 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. 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.
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  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  the evidence obtained provides a reasonable basis for our findings and  conclusions based on our audit objectives.
section_title: Appendix II: List of Experts
paragraphs: 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.
section_title: Appendix III: Key Developments in NRC’s Approach to Probabilistic Risk Assessment
paragraphs: 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. 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.
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. 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.
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. 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.
NRC issues a policy statement regarding the expanded use of PRA. 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.
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.
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.
NRC develops a strategy for implementing risk-informed regulation in  response to a GAO recommendation.
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.  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 NRC- endorsed consensus quality standards 1 year prior to the scheduled date  for initial fuel loading. 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. Also, NRC  develops its risk-informed and performance-based implementation plan.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  quality. Among other things, those standards address the application of  PRA to natural hazards.
The Chairman of the NRC charters a task force to develop a strategic  vision and assess options for a more comprehensive and holistic risk- informed, performance-based regulatory approach, with one option being  better incorporating risk management concepts into NRC’s regulatory  programs. 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. 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. With policy direction from the NRC,  the agency staff moves forward with actions to implement the task force  recommendations.
NRC issues three orders and a request for information in response to  lessons learned from the Fukushima Daiichi disaster. 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.
section_title: Appendix IV: Information on Developing a Probabilistic Risk Assessment
paragraphs: 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.
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.
section_title: Appendix V: NRC Actions on Natural Hazard Assessment in Response to the Fukushima Daiichi Disaster
paragraphs: 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.
subsections: section_title: March 2011
paragraphs: 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.
section_title: July 2011
paragraphs: The NRC task force issues its final report.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   The task force finds that NRC  vulnerabilities and verify the adequacy of monitoring and maintenance for  protection features such as watertight barriers and seals.
section_title: August 2011
paragraphs: 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.
section_title: September 2011
paragraphs: Agency staff report to the NRC Commissioners on the task force  recommendations that in their view can and should be implemented  without unnecessary delay.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.
section_title: October 2011
paragraphs: Agency staff propose a prioritization of the task force recommendations to  the NRC Commissioners and describe the actions necessary to   The staff suggests deferring the task  implement the recommendations.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  strive to complete and implement the lessons learned from the  Fukushima Daiichi disaster by 2016.
section_title: December 2011
paragraphs: 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. 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.
section_title: January 2012
paragraphs: 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.
section_title: February 2012
paragraphs: The NRC staff proposes issuing three orders and a request for  information in response to lessons learned from the Fukushima Daiichi  disaster. 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  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.
section_title: March 2012
paragraphs: NRC issues the three orders and the request for information proposed by  agency staff in February.
section_title: Appendix VI: Comments from the Nuclear Regulatory Commission
section_title: Appendix VII: GAO Contact and Staff Acknowledgments
subsections: section_title: GAO Contact
section_title: Staff Acknowledgments
paragraphs: 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.