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Frequently Asked Questions About SOARCA
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What is the State-of-the-Art Reactor consequences Analyses (SOARCA) project?
The SOARCA is a project that will be used to develop a realistic estimate of the
potential effects on the public from a nuclear power plant accident, where low-likelihood
scenarios could release radioactive material into the environment and potentially cause
offsite consequences. The project will also evaluate and improve, as appropriate,
methods and models for realistically evaluating both the plant response during such
severe accidents, including evacuation and sheltering and the potential public risk.
Why is the U.S. Nuclear Regulatory Commission (NRC) performing this study?
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, resulting in the potential for offsite effects from
these accidents. That research has resulted in significant improvement in our ability to
analyze and predict how nuclear plant systems and operators would respond to severe
accidents. During that same time, plant owners have improved the plant design,
emergency procedures, inspection programs, and operator training, all of which have
improved plant safety. Emergency preparedness measures have also been refined and
improved to further protect the public in the highly unlikely event of a severe accident.
Applying this research, and taking into account the enhancements to plant safety and
emergency preparedness, will result in an improved and more realistic evaluation of
consequences from severe nuclear accidents.
How will this study improve upon earlier studies?
This updated realistic analysis, which incorporates all the insights we have gained
through research, can provide a better basis from which the public and decisionmakers
can assess the safety of nuclear power plants. Modern computer resources and
advanced software will reduce the need for conservative assumptions and will yield
more realistic results. In the past, studies of plant response and offsite consequences
were extremely conservative, to the point that the predictions were not useful for
characterizing results or guiding public policy. These results were often caused by
either conservative assumptions or simple bounding analyses. It is our belief that
communication of consequences from reactor accidents must be based on realistic
analyses.
What are the potential uses of the SOARCA study?
The overarching purpose of this study is to provide more realistic information on
potential nuclear power plant consequences to the public and other stakeholders,
including federal, state and local authorities. This study will also increase understanding
of the value of defense-in-depth features of plant design and operation, including
mitigative strategies.
Why is the initial scope of this study limited to not more than eight plants?
The initial scope will allow for an analysis of the different reactor and containment
design combinations in the U.S. fleet, and on a range of different population densities.
This will allow the NRC to complete the development of plant models and obtain early
insights and experience with the analytical models, methods, and assumptions used in
the analyses.
How will the plants be selected for the initial scope of this study?
Because this study is not required by any regulation, it is being performed on a voluntary
basis. Therefore, the NRC has requested volunteers to participate in this effort based
on three major considerations. The first consideration was reactor and containment
design1. The NRC staff requested one volunteer from each of the different reactor and
containment design combinations in the U.S. fleet of operating nuclear power generating
facilities. Secondly, the NRC requested volunteers based on existing computer
(MELCOR/MACCS) models. Building site-specific computer models typically requires a
one-year effort of a highly skilled individual. (Having well-established models will allow
the staff to provide the Commission with initial results in a more timely manner.) Finally,
the NRC is considering population density to ensure a cross section of population
densities in the initial analyses.
The first two sites that will be analyzed are the Peach Bottom Atomic Power Station and Surry Power Station. They represent two of the basic reactor containment designs, have very well-developed models, and represent different population densities. Three additional plants representing three of the remaining reactor containment designs; well or partially developed models; and different population densities have preliminary agreed to participate in SOARCA. The NRC is still looking for additional volunteers to capture all the different reactor and containment designs.
1 Westinghouse two and three loop pressurized water reactors (PWRs) with dry ambient
containments, Westinghouse four loop PWRs with subatmospheric and large dry containments,Westinghouse four loop PWRs with ice condensers containments, Combustion Engineering PWRs with dry ambient containments, Babcox & Wilcox PWRs with dry ambient containments,and General Electric boiling water reactors (BWRs) with Mark I, II, or III containments.
How will the SOARCA project be conducted?
Enhanced Standardized Plant Analysis Risk (SPAR) models will be used to identify
those accidents which have a probability of at least a “one-in-a-million” chance of
resulting in core damage and potentially releasing radioactivity into the environment.
This screening process is needed to avoid scenarios which have vanishingly small
probabilities. While it is mathematically possible to assign probabilities to such events,
these events are so remote that any consequences associated with such events are not
meaningful. After identification of the dominant severe accident scenarios, and the
response of plant operators to those scenarios, detailed analyses will be performed with
state-of-the-art analytical models to predict the plant response and offsite
consequences.
What is the basis for the state-of-the-art analyses?
The insights on severe accident phenomena accumulated through worldwide extensive
experimental and analytical research over the last 25-plus years have been incorporated
into the MELCOR computer code. The MELCOR code contains models for both active
and passive plant features and the important physical processes associated with severe
reactor accidents. MELCOR analyses will be used to predict, in a consistent integrated
fashion, the timing and progression of potential severe accidents (e.g., timing of reactor
core melt, timing of containment failure, and the magnitude of the radioactivity release to
the environment). Similar improvements have been made on the MACCS code to better
model radioactive releases and the potential consequences from those releases.
How is this state-of-the-art analysis different from past analysis and evaluations?
Earlier studies, which did not have integrated analytical methods such as MELCOR
available to them, often resorted to conservative assumptions (in some cases unrealistic
and extremely conservative ones) regarding the timing and magnitude of radioactivity
releases. Using the more realistic MELCOR prediction of radiation release, the study
will then use the MACCS2 code to predict the offsite consequences of the radioactivity
release. The MACCS2 code contains models for dispersion of radioactive plumes,
emergency planning responses (evacuation and sheltering of the public) and uptake of
radioactive material by the public and related health effects. Unlike some earlier studies
which assumed a generic, emergency planning response, this new study will incorporate
updated site-specific emergency planning in its prediction of potential offsite
consequences. The study will also assess the effectiveness of evacuation and
sheltering for selected sites.
Why is it appropriate to use the criteria of a “one-in-a-million chance” per year for
selection of accidents for analysis?
Realistic and risk-informed regulatory decision-making focuses on the value of
preventive and mitigative features for the more likely, albeit remote, scenarios. We will
therefore conduct consequence analyses only for scenarios that have a core damage
frequency greater than or equal to one-in-a-million chance per year of reactor operation.The threshold value selected for screening individual scenarios represents a risk which
is about 10 times smaller than the NRC’s safety goal.
Using such a criteria allows us to concentrate our resources and detailed analyses on
those events that, while remote, are more likely to realistically contribute to public risk.
Scientific knowledge, combined with theoretical projections, allows us to assign
probabilities to extremely remote events (e.g., massive destruction from meteors). But
the estimate of the probability of such events is highly suspect since there is no human
experience from which to judge the accuracy of such estimated probabilities. The study
of unrealistically extreme events with incredibly low probabilities sheds no useful
information on the safety of nuclear reactors. There is far greater value in focusing on
events which bear some credibility. The particular probabilistic threshold value chosen
for SOARCA is quite low and represents a realistic risk to the public due to the unlikely
severe accident scenarios.
What consequence measures are being estimated?
This study assesses the health effects of a potential radiation release on the general
public. State-of-the-art analytical models will be used to estimate the number of prompt
fatalities and the number of latent cancer fatalities that could occur in the remote event
that a severe reactor accident occurs. Prompt fatalities are those resulting from
exposure to very high doses of radiation as the result of a release. These fatalities
occur soon after exposure (days to months). Latent cancer fatalities are those resulting
from the long-term effect of radiation exposure. The estimates of public health effects in
this new study will realistically account for the emergency planning measures in place at
each reactor site, unlike some of the past studies that used overly conservative generic
assumptions that did not account for site-specific planning.
Who is participating in the SOARCA project?
The SOARCA project is being performed by the NRC 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.
Performing 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. Information will be required from the participating
operating power plants selected for the study to obtain realistic input for the calculations.
Are terrorist acts, such as aircraft impacts, being analyzed as part of SOARCA?
No. The focus of this study is on operational scenarios that could potentially lead to a
radiological release into the environment, but are not terrorist-related. If there are
important security-related events that are not captured by the spectrum of scenarios
identified from safety analyses, they will be addressed separately from this analysis.
Are spent fuel pools considered in this study?
No, spent fuel pools are not considered in this study. The project is focused on
evaluating the severe, and very unlikely, accidents that may occur at operating power
reactors. Accidents that may occur as a result of the spent fuel pool occur much more
slowly because there are much lower levels of energy (decay heat) involved. Such
accidents therefore take longer to evolve and allow ample time for response by
personnel to prevent any radiological release.
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