October 2013
U.S. Government and Nuclear
Energy Industry Response to
the Fukushima Accident
Summary
The 2011 accident at Japan’s Fukushima Daiichi nuclear
power station had a profound impact on nuclear power
operators worldwide. The policy discussion on nuclear
energy changed from the benefits of this clean energy
technology to safety and security—issues that, informed
by decades of safe operation, had been largely resolved
in the minds of the public.
Within one week of the Fukushima accident, operators at
U.S. reactors were reviewing systems and components to
ensure that the designed features and measures could
mitigate the effects of a seismic event, flood or complete
loss of AC power.
The U.S. nuclear industry has pooled resources to ensure
the lessons from Japan are systematically gathered, analyzed and implemented. This process has already identified near-term enhancements that will add further to the
margin of safety.
1201 F Street NW  Suite 1100  Washington DC 20004
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The industry will also take action to satisfy additional
requirements that the U.S. Nuclear Regulatory Commission (NRC) imposes. The NRC is one of the world’s largest independent regulators with approximately 4,000
employees and an annual budget of $1 billion. On March
23, 2011, while the accident in Japan was still unfolding,
the NRC established a senior-level task force to review
the NRC’s processes and regulations to determine whether the agency should change its regulations governing
nuclear power in the United States, and to make policy
recommendations to the Commission. On July 12, 2011,
2
the task force, called the “Near-Term Task Force,” published its recommendations. The report also provided the basis for NRC development of regulatory
actions to enhance safety.
Since then, the NRC has repeatedly affirmed one of the Task Force’s conclusions: “continued operation and continued licensing activities [at nuclear power
plants] do not impose an imminent risk to the public health and safety and are
not inimical to the common defense and security.”1
“Continued
operation and
continued licensing
activities do not
impose an
imminent risk to
the public health
and safety and are
In the absence of any immediate threat to public health and safety, plant operators will incorporate enhancements to equipment and emergency plans in an
organized, sequential and structured way. Plant operators are integrating new
NRC requirements with existing regulatory and licensing activities consistent
with their safety benefit and significance of the issue being addressed.
The robust design of U.S. reactors, and their ability to manage severe natural
events, was demonstrated in 2011 and 2012. U.S. plants maintained safety
through severe tornadoes, historic floods, an historic earthquake and two hurricanes that wreaked havoc along the east coast. The reactor safety systems
performed well during all of these events, while nearby infrastructure experienced billions of dollars in damage.
not inimical to the
common defense
and security.”
— Conclusion of
Nuclear Regulatory
Commission Task
Force that reviewed
the implications of
the Fukushima
accident.
NRC Study Confirms Nuclear Plants Are Safe
A multi-year, multi-million-dollar severe accident analysis (State-of-the-Art Reactor Consequence Analyses (SOARCA)) conducted by the Nuclear Regulatory
Commission and released on February 1, 2012, found that in the event of a
total loss of electrical power and the failure of all permanent safety systems
(similar to the accident in Japan), the public would be protected effectively with
no significant near-term or long-term health effects. The report concluded that
in all scenarios studied, even the unmitigated ones, events progress more slowly and release much less radioactive material than an extremely conservative
1982 siting study had found. Among its conclusions: “the calculated risks of
public health consequences from severe accidents ... are very small.” 2
Federal Government and Industry Actions to Enhance Safety
Understanding fully the progression of events and the extent of the damage to
the reactor fuel and major systems, structures and components at the Fukushima Daiichi reactors will take more than a decade. The industry has already
identified issues that deserve attention, based on what is now understood
about the events and their consequences. These issues include availability of
1
Recommendations for Enhancing Reactor Safety in the 21st Century: The Near-Term
Task Force Review of Insights from the Fukushima Daiichi Accident , U.S. Nuclear Regulatory Commission, July 12, 2011, at 18.
2
U.S. Nuclear Regulatory Commission, State-of-the-Art Reactor Consequence Analyses
(SOARCA) Report (Draft Report for Comment), NUREG-1935, January 2012, at xxiii.
http://pbadupws.nrc.gov/docs/ML1202/ML120250406.pdf
U.S. Government and Nuclear Energy Industry Response to the Fukushima Accident
October 2013
back-up power sources and protection of primary and back-up components
from flooding or seismic risks.
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The industry and
the NRC are in
general agreement
on the topics to be
considered in the
near term,
including
assessment of
earthquake and
flooding hazards
and an assessment
of plants’ ability to
The Nuclear Regulatory Commission has identified a number of new requirements in response to the Fukushima accident. These requirements are organized into three tiers. Those in Tier 1 will be implemented “without unnecessary
delay” and have the highest priority. Tier 2 actions depend on the resolution of
Tier 1 issues, technical resources or further technical assessment. Tier 3 recommendations will be evaluated later based on the availability of additional information from Japan, the types of regulatory actions required, and the outcome
of Tier 1 activities. (See page 9 for the tiers and their recommendations.)
There are eight Tier 1 recommendations that are considered to have the highest priority and the greatest safety benefit. The Commission has directed the
NRC staff to engage stakeholders—communities, states, and nuclear plant
operators—to develop the scope, approach and acceptance criteria for these
items. Given the technical complexity of some of the issues, some of the Tier 1
recommendations will take several years to complete.
NRC Response to Fukushima
On March 12, 2012, the NRC issued the first regulatory requirements for the
nation’s operating reactors on the eight recommended actions in Tier 1. The
NRC issued three orders requiring safety enhancements related to:

cope with the
mitigation strategies to respond to extreme natural events resulting in the
loss of AC power
extended loss of

ensuring reliable hardened containment vents
electric power.

enhancing used fuel pool instrumentation.
The NRC has required that plants begin implementation of these safety enhancements immediately and complete implementation within two refueling
outages or by December 31, 2016, whichever comes first.
In addition, the NRC issued formal Requests for Information to plant operators
asking for their specific plans for how they will:
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re-evaluate their earthquake and flooding hazards using present-day methods and information
conduct walk-downs (inspections) of facilities to ensure protection against
the hazards in their current design basis, and
re-evaluate their emergency communications systems and staffing levels.
Companies submitted plant-specific plans to NRC in February 2013 describing
how they intend to comply with the NRC orders for responding to beyonddesign-basis events like those experienced at Fukushima. (Those orders covered mitigation strategies, reliable hardened vents and spent fuel pool instrumentation.)
Seismic Hazards. The NRC has endorsed the industry’s proposed approach
to re-evaluate the seismic hazard at nuclear plants. The industry approach in-
U.S. Government and Nuclear Energy Industry Response to the Fukushima Accident
October 2013
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Known as FLEX, for
diverse and flexible
mitigation
capability, the
industry will
position emergency
equipment at
diverse locations
onsite, supported
by reserve
equipment prestaged elsewhere.
cludes an expedited seismic evaluation to demonstrate sufficient seismic margin, implementing plant modifications where appropriate to increase seismic
margin, and (in the longer term) comprehensive seismic probabilistic risk analyses (PRAs) of the entire plant. Reactors in the central and eastern regions of
the U.S. must complete their expedited evaluations (and any modifications necessary) no later than two refueling outages after 2014, using an updated
ground motion model. Reactors in the West have until two refueling outages
after 2016.
Flooding Hazards. The first group of 22 nuclear power plants required to
conduct flooding re-evaluations either submitted their evaluations, or rescheduled their submittals, by the NRC’s March 12, 2013, deadline. Ten of the 22
sites found that their flooding re-evaluation results exceeded their design basis,
and they are required to perform integrated assessments of their responses to
various flooding scenarios identified by the NRC. These assessments could lead
to plant modifications—e.g., watertight doors for certain compartments, or
moving essential equipment to higher ground. In addition to this first group, 24
sites have flooding re-evaluations due in March 2014, and 20 sites have flooding re-evaluations due in March 2015.
FLEX: The Industry’s Initiative
The nuclear industry is implementing a response strategy called “FLEX” which
will provide the greatest safety benefits in the shortest period of time. FLEX will
mitigate those extreme, unexpected scenarios that are beyond the plants’ design parameters. Linked directly to the lessons learned from the Fukushima
accident, the strategy:
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addresses one of the major problems encountered in Japan: the loss of
power to maintain effective cooling
builds on existing safeguards to protect the fuel and reduce the
likelihood of severe-accident scenarios
maintains cooling of reactors and used fuel pools if normal systems fail,
by stationing additional pumps in multiple locations to provide cooling
water to the reactors
maintains continuous backup power after an extreme event by prepositioning emergency equipment—generators, chargers, pumps—in multiple locations around the plant site
maintains safety even after a catastrophic event by stationing
emergency equipment in secure regional locations for use if onsite and
offsite reserve equipment is insufficient, and
coordinates sharing of backup equipment among the 62 nuclear sites.
The FLEX approach adds equipment—pumps, generators and chargers—located
in diverse locations throughout the plant site. The equipment will be commercial-grade with engineered specifications, with requirements for equipment testing and maintenance subject to NRC oversight. The strategy is flexible: It does
not mandate installation of permanent equipment. Instead, it requires that the
plant sites obtain, prepare and maintain portable equipment that could be used
for any catastrophic event. The equipment will be able to connect to a variety
U.S. Government and Nuclear Energy Industry Response to the Fukushima Accident
October 2013
of locations for injecting coolant and providing a continuous supply of electricity.
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The objective of FLEX is to ensure plant-specific capability to cope for an indefinite period through a combination of installed plant capacity, portable on-site
equipment and off-site resources.
The FLEX concept was informed by the industry’s response to the September
11, 2001, terrorist attacks, in which additional security precautions—such as
portable generators, water pumps, hoses
and batteries—were put in place to mitigate the effects of large fires and explosions. The FLEX approach provides a further layer of mitigation for events that are
beyond what a plant is designed to withstand. Multiple means of obtaining power
and water fulfill the key safety functions of
reactor cooling, containment integrity and
used fuel pool cooling that would prevent
damage to nuclear fuel and protect the
public and the environment.
By September 2013, U.S. nuclear plants
had already procured more than 1,500
Workers at the Monticello nuclear plant test its new portable pumps.
pieces of equipment, including portable
generators, diesel-driven pumps, fire trucks
and portable ventilation units. The additional equipment needed to implement FLEX will vary from site to site. The
post-Fukushima safety enhancements are part of the industry’s continuing investment in the operational safety and efficiency of U.S. reactors.
FLEX Concepts Effective at Fukushima Daini
Six miles south of Fukushima Daiichi lies the four-reactor Fukushima Daini nuclear station, also owned by Tokyo Electric Power Company (TEPCO). On March
11, 2011, the Daini plant, which also sits on the Pacific shoreline, experienced
the same earthquake, tsunami, and flooding as its larger sister plant to the
north. Yet the outcome at Fukushima Daini was very different.
At Fukushima Daini, all four reactors shut down automatically as a result of the
earthquake, but water damage from the tsunami resulted in the loss of cooling
capability to remove decay heat in three of the reactors. The one reactor retaining core cooling capability was brought to cold shutdown in less than a day.
Core cooling capability was re-established for the other three reactors over the
subsequent three days, allowing operators to bring them to cold shutdown.
Conditions at Fukushima Daini were not drastically different than at its sister
plant six miles up the coast of Japan.
U.S. Government and Nuclear Energy Industry Response to the Fukushima Accident
October 2013
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In a period of 30 hours, plant staff replaced residual heat removal pump motors
and laid and energized 5.5 miles of heavy-duty electric cables. Core cooling was
restored at all reactors, and they were brought to cold shutdown.
TEPCO’s response to the events at Fukushima Daini resembles U.S. industry’s
diverse and flexible coping strategy (FLEX), with one significant difference: At
Fukushima Daini, there were no pre-planned procedures and training and no
portable or replacement equipment staged at the site. Nuclear stations in Japan
had not implemented measures similar to the capability U.S. plants added after
the 2001 terrorist attacks. Nonetheless, there are several elements of TEPCO’s
response at Fukushima Daini that
match the U.S. FLEX approach,
including steps taken by control
room operators, use of equipment
from other nearby sites or suppliers and the use of government
assets to assist with transportation
or other logistics.
Regional Response
Centers
The March 11, 2011, tsunami inundating the Fukushima Daini nuclear power
facility. The Daini station is six miles south of Fukushima Daiichi and managed
a different outcome. Photograph courtesy Tokyo Electric Power Company.
The nuclear energy industry is adding another layer of safety and
public protection by developing
regional centers for critical equipment that could be needed to
maintain safety in an extreme
event at a U.S. nuclear energy facility. All companies that operate
nuclear energy facilities have
joined to develop and operate two
regional response centers managed
by Pooled Equipment Inventory Co.
The regional response centers will be located near Memphis and Phoenix and
capable of delivering supplemental emergency equipment to any of America’s
nuclear energy facilities within 24 hours, enabling them to manage an extended
loss of electrical power and/or cooling water supply. The equipment and materials provided by the regional response centers supplement the additional portable equipment purchased at all 62 nuclear energy facilities, which can also be
utilized and shared during a site emergency.
The regional response centers will be capable of delivering another full set of
portable safety equipment, radiation protection equipment, electrical generators, pumps and other emergency response equipment to an affected site within the first 24 hours after an extreme event. The regional response centers near
Memphis and Phoenix are expected to be operational by August 2014.
U.S. Government and Nuclear Energy Industry Response to the Fukushima Accident
October 2013
Industry Performance Demonstrates Readiness
for Natural Disasters
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Nuclear stations across the country are challenged periodically by extreme
weather, and always manage these events using their emergency response
procedures and guidelines, protecting worker and public health and safety
throughout. The layers of protection at nuclear energy facilities were tested,
and proved the effectiveness of training, procedures and planning in the face of
major floods, tornadoes, earthquakes and hurricanes.
The layers of
protection at
nuclear energy
facilities were
tested, and proved
the effectiveness of
training,
procedures and
planning in the face
of major floods,
tornadoes,
earthquakes and
hurricanes.
Tornadoes
On April 16, 2011, an EF3-rated tornado touched down in the switchyard of the
Surry nuclear power station in Virginia, damaged equipment and severed offsite electric power. The station shut down automatically and back-up diesel
generators started immediately to provide electricity to maintain both units in a
stable condition.
Also in 2011, three waves of storms in the Southeast U.S. on April 26 and 27
generated an unprecedented 226 tornadoes in a 24-hour period. These tornadoes killed approximately 330 people, obliterated entire towns and devastated
the Tennessee Valley Authority (TVA) transmission system in this region. TVA’s
Browns Ferry reactors in Alabama suffered a loss of off-site power when the
storm damaged the station’s transmission lines. The layers of protection for
nuclear units ensure that even though Browns Ferry had diverse off-site power
feeds, the next layer of defense, the emergency diesel generators, were available when the unlikely combination of storms occurred. Due to the extensive
damage to the transmission system (337 damaged structures, 96 lines out of
service) off-site power was not restored for five days. All three reactors shut
down safely and were maintained in safe shutdown condition during this period.
Major Floods
Historic levels of flooding in the Midwest along the Missouri River occurred in
the summer of 2011. The flood devastated homes, businesses and many
square miles of farmland in Montana, North Dakota, South Dakota, Iowa, Nebraska, Missouri and Kansas. The Fort Calhoun and Cooper nuclear stations are
located in the flooded region. Both plants took precautions to implement and
bolster flood protection measures and ensure safety would be maintained as
water levels rose.
Fort Calhoun had shut down for a scheduled refueling outage before the flood.
It remains in a safe shutdown condition pending NRC permission to restart.
Flood protection measures included sandbags, aqua berms and additional testing of emergency diesel generators and loss of off-site power procedures.
Cooper station used more than 5,000 tons of sand to create additional flood
barriers. The plant monitored the flood waters continuously and coordinated
with the Army Corps of Engineers but was not required to shut down since
flood waters did not exceed technical specifications.
U.S. Government and Nuclear Energy Industry Response to the Fukushima Accident
October 2013
Earthquakes
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On August 23, 2011, a 5.8-magnitude earthquake in central Virginia was felt
from Georgia to Maine. The two nuclear reactors at the North Anna Power Station in Mineral, Virginia, automatically shut down as a result. The epicenter was
approximately 11 miles west-southwest of the station. There was only minor
cosmetic damage to the plant and the reactor and safety systems were not
damaged even though the earthquake exceeded the design specifications.
The North Anna staff "responded to the event in a manner that protected the
public health and safety," an inspection team from the U.S. Nuclear Regulatory
Commission (NRC) reported on October 3. Although the ground motion at the
site during the earthquake exceeded levels for which the plant was originally
licensed, the NRC team report said "safety system functions were maintained"
and "reviews of the plant equipment, systems and structures did not reveal
significant damage." The condition of the reactors following the event demonstrates the robust seismic design and construction
of nuclear facilities. The NRC authorized the restart
of both reactors in November 2011 after extensive
analyses and inspections.
Hurricanes
Storm damage to TVA high-voltage transmission tower,
Stevenson, Alabama, April 27, 2011. Photograph courtesy
Tennessee Valley Authority.
A few days after the earthquake, on August 27,
Hurricane Irene made landfall in the Outer Banks of
North Carolina as a category 1 hurricane. Irene
pounded the east coast with high winds and rainfall
and caused particularly severe floods in upstate
New York and Vermont with total damage estimates
for the U.S. as high as $7 billion. Twenty-four nuclear plants in affected states monitored the storm and
all were maintained in a safe condition.
On October 29, 2012, Hurricane Sandy hit the East
Coast, destroying property and knocking out electricity to seven million customers in 13 states. Of the 34 nuclear facilities from South Carolina to Vermont in
Hurricane Sandy’s path, 24 continued to operate safely and generate electricity
throughout the event. Seven were already shut down for refueling or inspection, and three in New Jersey and New York shut down safely, as designed.
Indian Point 3 and Nine Mile Point 1 in New York shut down October 29 due to
grid disturbances and Salem 1 in New Jersey shut down October 30 because of
storm conditions affecting circulating water pumps. The three units returned to
operation on November 2.
These events demonstrate the robustness of U.S. commercial nuclear plant
designs and the effectiveness of operator and emergency response training
programs and plant knowledge. These attributes will be strengthened with the
additional measures being taken in response to the accidents at Fukushima.
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U.S. Government and Nuclear Energy Industry Response to the Fukushima Accident
October 2013
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Tier
Tier 1
Safety Enhancements Recommended by the NRC’s
Fukushima Task Force
Mitigating strategies for beyond-design-basis events
Order
Reliable hardened vents for Mark I and Mark II containments
Order
Spent fuel pool instrumentation
Request for Information
Seismic and flood walk-downs
Request for Information
Emergency preparedness regulatory actions (staffing and communications)
Request for Information
Advanced Notice of
Proposed Rulemaking
Advanced Notice of
Proposed Rulemaking
Strengthening and integration of emergency operating procedures, severe accident
management guidelines, and extensive damage mitigation guidelines
Spent fuel pool makeup capability (dependent on Tier 1 spent fuel pool issues)
Emergency preparedness regulatory actions
Re-evaluation of other external hazards (e.g., tornadoes, hurricanes, drought)
Tier 3
Order
Seismic and flood hazard re-evaluations
Station blackout (SBO) regulatory actions
Tier 2
NRC Action
Rulemaking in 2013
Staff plan to subsume into
Tier 3 but was denied
by Commission due to
more information needed
Request for Information (as
resources are available)
Ten-year confirmation of seismic and flooding hazards
Potential enhancements to the capability to prevent or mitigate seismically-induced fires
and floods (long-term evaluation)
Reliable hardened vents for other containment designs (long-term evaluation)
Hydrogen control and mitigation inside containment or in other buildings (long-term
evaluation)
Emergency preparedness enhancements for prolonged station blackout and multiunit
events (dependent on availability of critical skill sets)
Emergency Response Data System capability (long-term evaluation)
Additional emergency preparedness topics for prolonged station blackout and multiunit
events (long-term evaluation)
Emergency preparedness topics for decision-making, radiation monitoring, and public
education (long-term evaluation)
Reactor Oversight Process modifications to reflect the recommended defense-in-depth
framework (dependent on recommendations for establishing a new regulatory framework)
Program plans based on
resources required,
information needed, and
resolution of Tier 1 and 2
issues.
NRC staff training on severe accidents and resident inspector training on severe
accident management guidelines (dependent on recommendations for strengthening
and integrating onsite emergency response capabilities)
Basis of emergency planning zone size
Pre-staging of potassium iodide beyond 10 miles
Transfer of spent fuel to dry cask storage
U.S. Government and Nuclear Energy Industry Response to the Fukushima Accident
October 2013
The ABCs of Filtered Containment Venting
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Why Venting is Important
The accident at Fukushima Daichi focused attention on the issue of containment venting, and how best to ensure that containment pressures do not
exceed safe levels. At Fukushima, three of the six boiling water reactors
(BWR) were operating at the time of the March 11, 2011, earthquake and
tsunami. The tsunami flooded important electrical gear, causing a loss of AC
power (sometimes called “station black out”). This power loss meant the
loss of instrumentation and the ability to cool the reactors. Without adequate cooling, the fuel in the three reactors that were operating overheated
and melted, releasing steam, radionuclides and other gases, notably hydrogen.
The steam and gases built up pressure in the containments. The power failure prevented the operators from opening valves and reading instruments
so venting to relieve pressure was delayed. The operators also delayed
venting the containments out of concern for the local population who had
not yet evacuated.
Although attempts at venting were underway, on the second day hydrogen
that had collected in one building exploded. This was followed by hydrogen
explosions in two other units on succeeding days. These explosions resulted
in uncontrolled releases of radionuclides to the surrounding countryside.
That contamination has prevented tens of thousands of residents from returning to their homes.
If the operators at Fukushima had had access to equipment, systems and
procedures that allowed them to perform controlled venting of the containments at an early stage of the accident, they could have maintained the
containment pressures at acceptable levels and also vented the hydrogen.
This could have prevented the explosions and the uncontrolled release of
radionuclides.
NRC’s Path Forward
NRC has decided (and the industry agrees) that control of radioactive materials produced during a severe accident is imperative. With input from the
industry and other stakeholders, the NRC has analyzed the options and developed a two-track strategy for enhancing venting and filtering in BWRs
with Mark I and Mark II containments:

In mid-2013, the NRC ordered operators of Mark I and II-type boiling
water reactors to install enhanced, hardened vents capable of operating
even in the event of a severe accident, with the elevated temperatures,
pressures and radiation levels associated with a damaged core. This will
ensure that plant operators have the ability to perform controlled vent-
U.S. Government and Nuclear Energy Industry Response to the Fukushima Accident
October 2013
ing to relieve high pressure in containment and reduce potentially explosive hydrogen.
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Second, the NRC commissioners decided to examine (through its rulemaking process) the appropriate strategy for mitigating radioactive releases during venting. NRC staff had recommended installation of vent
filters to reduce the radioactive particles released to the environment
during an accident that requires venting the containment. The NRC
commissioners decided instead to develop the technical bases and
technical acceptance criteria for operators to consider all options that
could be more effective, such as suppression pools, containment sprays
and separate filters.
The NRC will develop the technical bases for such a regulation by March
2014, the proposed rule in two years, and the final rule in four years (March
2017). The rulemaking process will include substantial involvement, review
and comment by all stakeholders, including the industry.
This approach is largely in line with industry recommendations for a performance-based approach to venting and filtration issues – i.e., define first the
results to be achieved, followed by plant-specific assessment of the best
approach to achieve that result. Extensive industry analysis has shown that
a “one-size-fits-all” approach (e.g., simply adding filters) would not be an
effective or efficient way to proceed. Other techniques – containment
sprays, for example – may be more effective.
In simple terms, containment venting filtration systems consist of large
tanks of water outside containment. The gases generated during an accident are routed through the vent and associated piping into the water,
which scrubs out the radionuclides – exactly what happens inside containment with flooding and water sprays. As a general principle, the industry
thinks it makes more sense to scrub out the radionuclides generated during
an accident inside containment rather than outside.
The Technical Basis for a Performance-Based Approach
In 2012, the Electric Power Research Institute (EPRI) conducted a study of
what happens inside BWR Mark I and II containments during severe accidents. EPRI found that substantial reductions of radioactive releases can be
achieved.
EPRI evaluated several approaches – including water injection by flooding
and sprays. Flooding removes heat from the debris (melted material), reducing the amount of airborne radionuclides created. Water sprays strip
radionuclides out of the containment atmosphere (like rain knocks dust out
of the air during a storm). Other scenarios evaluated included controlled,
unfiltered venting; filtered venting, and combinations of water injection and
venting. Insights from the analysis included:
U.S. Government and Nuclear Energy Industry Response to the Fukushima Accident
October 2013
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No single strategy is effective. Containment vent filters are ineffective
when water injection is not used. The most effective strategies involved
combinations of water injection and containment venting.
Active core debris cooling is required. If debris cooling is not provided
through water injection or spray, containment failure is likely.
Existing severe accident management guidelines (SAMGs) for plant operators provide substantial benefit. Flooding or spraying is addressed in
the SAMGs and is an essential element of a strategy to reduce radioactive releases.
Spraying the containment atmosphere is beneficial. The amount of fission products removed is higher when sprays are used.
Venting prevents uncontrolled releases and manages hydrogen. Flooding or sprays produce a large amount of steam, which increases pressures, possibly to the point of containment failure. Venting maintains
the containment pressure below the design pressure and removes hydrogen and other gases.
Control of the vent provides significant benefit. Opening and closing the
vent at the most opportune times (e.g., venting right after sprays have
reduced airborne contaminants) is essential.
Filters may further reduce radionuclide releases. Several of the combined water injection and venting strategies could reduce radiological
releases significantly. This could potentially be enhanced by adding a
vent filter, but additional research is needed to determine its effectiveness.
The research concluded that viable methods exist for reducing radiological
releases and the potential for land contamination. Combinations of water
injection approaches and various containment venting approaches appear
to provide the greater benefit. Plant-specific analyses are necessary to determine the most effective approach.
Position of the NRC’s Advisory Committee on Reactor Safeguards
NRC’s Advisory Committee on Reactor Safeguards (ACRS), a group of seasoned technical advisers, was asked to provide an independent evaluation
of the possible approaches. Based on its review of studies by both the NRC
and industry, the ACRS concluded:
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
Additional defense-in-depth measures should be added in response to
Fukushima lessons learned; a no-action response is not appropriate.
Severe accident-capable vents are an essential part of any controlled
venting strategy; they would allow both opening and closing vents during the accident.
For certain accident sequences, additional filtration systems could reduce the cesium or iodine release from containment by a meaningful
amount.
U.S. Government and Nuclear Energy Industry Response to the Fukushima Accident
October 2013
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For other sequences, however, the existing filtration systems of the
plant (flooding and sprays) operated efficiently and little additional radioactive material would be removed by an external filter system.
ACRS recommended a plant-by-plant, performance-based approach as the
most effective means to reduce radioactive material releases. At certain
plants, installation of external filtered vents could result from this performance-based approach.
Industry Position
The industry believes the decision by the NRC to examine the venting issue
in detail is the wise path. The approach to mitigating release of radionuclides in the unlikely event of a severe accident should be developed around
the findings of the EPRI study. This is consistent with the recommendation
of the ACRS and is based on the following:
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A filter-only option will not be effective in several scenarios, and therefore its benefits do not support the costs. Initial cost estimates are tens
of millions of dollars per reactor.
A performance-based approach to filtering focuses the response to a
severe accident on those actions best able to manage the accident and
mitigate potential radiation releases. It protects public safety and the
environment better than other options by providing greater defense-indepth for the containment function.
The performance-based approach is an integrated response that maximizes the retention of radionuclides in containment, reduces radionuclide release to a greater degree than external filtering systems alone,
and provides plant operators with additional tools to manage a severe
accident effectively.
A performance-based approach to filtering would take advantage of the
required addition of reliable vent valves to allow operators to control
the venting process.
Finally, using a performance-based approach allows licensees to tailor
filtering and venting strategies to their plants appropriately, taking into
account the unique circumstances of each facility.
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U.S. Government and Nuclear Energy Industry Response to the Fukushima Accident
October 2013