United States Nuclear Regulatory Commission - Protecting People and the Environment

Frequently Asked Questions About State-of-the-Art Reactor Consequence Analyses (SOARCA)

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What is the State-of-the-Art Reactor Consequence Analyses (SOARCA) project?

As its name implies, the SOARCA research project is designed to develop realistic estimates of the potential public health effects, which might result from a nuclear power plant accident, in the event of very unlikely scenarios that could release radioactive material into the environment. Toward that end, this project is also designed to evaluate and improve, as appropriate, methods and models for realistically evaluating both the plant response during such severe accidents, including protective actions for the public (such as evacuation and sheltering), and the potential public health risk.

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Why is the U.S. Nuclear Regulatory Commission (NRC) conducting this study?

The NRC is conducting this study to develop the most realistic evaluations possible for the potential consequences of severe nuclear accidents. Over the years, the NRC, industry, and international nuclear safety organizations have completed substantial research on plant response to hypothetical accidents that could damage the core and containment. The results have significantly improved the NRC's ability to analyze and predict how nuclear plant systems and operators would respond to severe accidents. Also, plant owners have enhanced their plant designs, emergency procedures, inspection and maintenance programs, and operator training, all of which have improved plant safety. Plant owners and local governments have also refined and improved their emergency preparedness measures to further protect the public in the highly unlikely event of a severe accident. By combining the new analysis methods and plant-specific data, SOARCA will improve the realism of accident consequence evaluations.

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Why is the study of severe accidents important?

image of TMI-2 Core End-State Configuration

The NRC studies severe accidents because it is our mission to protect the safety of the public and the environment. One way we study accident progression is to understand historical events.

In 1979, the only U.S. commercial power reactor core melt accident occurred at Three Mile Island (TMI) Unit-2, and resulted in extensive fuel damage (see the illustration to the left). As a result, radioactive gases and contaminated cooling water were released to the containment. In addition, a small amount of radioactive material was released to the atmosphere through an indirect route. However, the containment itself performed as designed, and kept the vast majority of radioactive material safely inside. The actual release had negligible effects on the physical health of individuals or the environment, according to comprehensive investigations and assessments by several well respected organizations. (For additional detail, see the Backgrounder on the Three Mile Island Accident.)

A much more serious accident occurred at Chernobyl (in the former Soviet Union) in 1986. The Chernobyl reactor design (known as RBMK) is very different from the BWR and PWR designs used in the United States. (For additional detail, see the Backgrounder on Chernobyl Nuclear Power Plant Accident). That accident severely damaged the reactor core, and released large quantities of radioactive material to the environment. It also deposited radioactive material in nearby countries, and radioactive material was even detectable at very low levels in the United States.

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What is the technical basis for the state-of-the-art analyses?

Over the past 25 years, the NRC, industry, and international nuclear safety organizations have completed substantial research on plant response to hypothetical accidents that could damage the core and containment. That research has significantly improved the NRC's ability to analyze and predict how nuclear plant systems and operators will respond to severe accidents. During that same time, plant owners have enhanced their plant designs, emergency procedures, inspection and maintenance programs, and operator training, all of which have improved plant safety. Plant owners and local governments have also refined and improved their emergency preparedness measures to further protect the public in the highly unlikely event of a severe accident. The SOARCA team applies this accumulated research and incorporates plant enhancements to achieve a more realistic evaluation of the consequences from severe nuclear accidents. For additional detail, see the SOARCA project update from the NRC's 2009 Regulatory Information Conference, which includes the technical background for the MELCOR Integral Severe Accident Analysis Code.

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How will this study be different from earlier studies?

The SOARCA project will differ from earlier studies by achieving the following objectives:

  • Use an improved understanding of source terms and severe accident phenomenology.
  • Credit the use of severe accident mitigation strategies and procedures.
  • Use updated emergency preparedness modeling.
  • Account for plant improvements.
  • Use modern computer resources and advanced software to yield more accurate results.

In addition, the SOARCA project is designed to yield a more realistic estimate. Some of the earlier studies also were designed to be best estimates; however, because they were limited by the available knowledge of accident phenomenology, those older studies were conservative (particularly for the very improbable severe accidents). The SOARCA project will provide the latest basis from which the public and decisionmakers can assess the consequences of severe reactor accidents.

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What are the potential uses of the SOARCA study?

The overarching purpose of this study is to provide the public and other stakeholders (including Federal, State, and local authorities) with more realistic information about potential consequences, which might result from a nuclear power plant accident, in the event of very unlikely scenarios that could release radioactive material into the environment. This study will also increase understanding of the extent and value of defense-in-depth features of plant design and operation, as well as mitigation strategies.

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Which plants are participating in the SOARCA project?

The first phase of the SOARCA project involves analyzing examples of two major types of nuclear reactors in the United States, namely a boiling-water reactor (BWR) and a pressurized-water reactor (PWR). Toward that end, the project staff solicited volunteers from the nuclear industry to participate in the project. Peach Bottom Atomic Power Station (a BWR in Pennsylvania) and Surry Power Station (a PWR in Virginia) were the first two sites to volunteer. After the first phase has been completed, the NRC will consider whether analyses are needed for other reactor types and/or sites.

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Does this study consider new reactors that may be built?

No. The project analyzes existing reactors, and does not include new reactor designs and containments.

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Are terrorist acts, such as aircraft impacts, being analyzed as part of SOARCA?

No. This study focuses on accident scenarios—not terrorist-related ones—that could potentially lead to a radiological release into the environment. The NRC addresses security-related events in a separate, non-public analysis.

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Are accidents at spent fuel pools considered in this study?

No. The project focuses on evaluating the very unlikely severe accident scenarios that may occur at operating power reactors and, as such, it does not consider spent fuel pools.

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Why is it appropriate to use the criterion of a “one-in-a-million chance” per year to select accidents for analysis?

Using such a criterion allows the NRC to concentrate its resources and detailed analyses on those events that, while only remotely probable, are more likely to realistically contribute to public risk. Realistic and risk-informed regulatory decisionmaking focuses on the value of preventive and mitigative features for the more likely, albeit remotely probable, scenarios. To be analyzed in SOARCA, an accident scenario had to have a probability of occurring more than once in a million reactor years (or more than once in ten million reactor years for accidents that may bypass containment features). In addition, the staff considered scenarios that may have lower probabilities than the selection criteria, but potentially higher consequences.

Scientific knowledge, combined with theoretical projections, allows probabilities to be assigned to extremely unlikely events. However, the estimated probabilities of such events are highly suspect because there is no human experience from which to judge their accuracy. Thus, the study of unrealistically extreme events with incredibly low probabilities sheds little useful information on the safety of nuclear reactors. As a result, there is far greater value in focusing on more credible events. The particular threshold value selected for screening individual scenarios in SOARCA is quite low and represents a risk that is a hundred times smaller than the NRC's safety goal.

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What consequence measures are being estimated?

This study assesses the health effects of a potential radiation release to the general public. State-of-the-art analytical models estimate the individual risks of prompt fatality and latent cancer fatality that could occur in the highly unlikely event of a severe reactor accident. Prompt fatalities are those resulting from exposure to very high doses of radiation as the result of a release. These fatalities occur days to months after exposure. By contrast, latent cancer fatalities are those resulting from the long-term effect of radiation exposure. Unlike some past studies (which used generic assumptions), the estimates of public health effects in this new study realistically account for the emergency planning measures in place at each reactor site.

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Who is participating in the SOARCA project?

The NRC is conducting the SOARCA project with assistance from Sandia National Laboratories (SNL). SNL is the principal NRC contractor for severe reactor accident research and has developed much of the computer modeling to be used in this study. At the NRC, the study is a joint effort among the Offices of Nuclear Regulatory Research, Nuclear Security and Incident Response, and Nuclear Reactor Regulation.

Conducting the SOARCA project requires a wide array of disciplines. Staff working on the project include experts in reactor accident probabilistic assessment, system engineering, severe core damage accident phenomena and modeling, emergency planning, and offsite consequences. In addition, information will be required from the participating operating power plants selected for the study to obtain realistic input for the calculations.

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Is this study being reviewed by outside experts?

Yes. In addition to the peer review afforded by NRC's Advisory Committee on Reactor Safeguards, an independent external peer review of scientific and technical experts will assess the methodological approach, underlying assumptions, and results obtained for Peach Bottom and Surry to ensure that they are defensible and state-of-the-art. This peer review is a common practice in research and will show both the strengths and weaknesses of the research project. The NRC will continue to use the methods shown to be strengths of the research project, and the experts' comments on the weaknesses will help improve future research projects.

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Page Last Reviewed/Updated Wednesday, July 17, 2013