PRA Is Applied to Natural Hazards to a Limited Extent

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. 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. 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. NRC Has Not Analyzed Whether It Should Require PRAs That Address Natural Hazards

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. Experts Offered a Range of Views on NRC Assessment Processes

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 Noted Strengths and Limitations of NRC Assessment Processes

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. More Than Half of Experts Suggested Expanded Use of PRA

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. PRA can help identify design vulnerabilities that might otherwise be  overlooked by relying on traditional deterministic assessments alone. 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. We recognize that NRC  must undertake a thorough review to require licensees of operating  reactors to develop and maintain PRAs that address natural hazards. 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. NRC agreed with the report recommendation. Appendix I: Objectives, Scope, and Methodology

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. 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.
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 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.

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.