Toyoshi Fuketa
Nuclear Regulation Authority
Lessons Learned from the Fukushima Dai-ichi Accident
and Responses in Regulatory Requirements
27th Annual Regulatory Information Conference
Bethesda North Marriott Hotel & Conference Center
March 11, 20151NRA
TEPCO’s Fukushima Dai-ichi accident revealed the
weakness of the foregone regulatory requirements, e.g.
Insufficient design provisions against tsunami,
Unpractical management measures under severe
accident conditions, and
Insufficient provision for accidents far-exceeding the
postulated design conditions.
We re-realized the importance of the Defense in Depth (DiD)
approach in design and preparations of countermeasures
against beyond design basis accidents (b-DBAs).
We learned from the accident that we must evaluate in
advance the potential and consequences of a wide spectrum
of internal and external initiators, including earthquake,
tsunami, volcanism, aircraft crash, fire, terrorist attack, etc.
Introduction2NRA
The Fukushima Dai-ichi accident revealed vulnerability of
structures, systems and components (SSCs) against
extreme loads and conditions caused by some specific
internal/external initiators. The NRA, accordingly,
enhanced design requirements significantly.
Due considerations are required for all the significant
internal and external initiators.
1. Prevention of SSCs failures3NRA
Re-evaluation of external hazards is also requested,
particularly for natural phenomena, based not only on
historical records but also on expert judgment to cover very
rare events.
As for earthquakes, more stringent criteria are prepared for
active faults, more precise methods are provided for
design-basis ground motions, etc.
As for tsunami, more comprehensive methods are required
for defining design-basis tsunami, covering possible
earthquakes or other natural hazards, e.g., landslides in
the ocean bed, as causes of tsunami based on national
and international experiences, and countermeasures such
as coastal levee and watertight doors are required.
1. Prevention of SSCs failures
(cont'd)4NRA 2. Measures to Prevent CCFs
The new requirements extend design-basis events and
strengthen protective measures against natural
phenomena and other initiators which may lead to
common cause failures (CCFs).
They put a particular importance in due consideration to
ensure diversity and independence, i.e., shift of emphasis
from "redundancy centered".
Diversity of operating mechanisms, e.g., diesel and gas
turbine generators, motor-driven and diesel-driven pumps,
is important as well as physical separation.5NRA
Safety-related system trains shall be
• located at different elevations and/or different areas,
• compartmentalized by installing bulkhead, or
• distanced enough from each other.
Mobile equipment shall be
• stored in different locations, which are not easily
affected by external initiators including intentional
aircraft crash, and
• easily and surely connectable to the target system by
preparing spatially-dispersed multiple connecting ports.
2. Measures to Prevent CCFs
(cont'd)6NRA 3. Prevention of Core Damage
In the new requirements by the NRA, the definitions of
some DBAs are changed. Design provision is now
required against prolonged SBO and LUHS.
Also required is provision against some b-DBAs
involving multiple failures, including anticipated transient
without scram (ATWS), loss of core cooling, and loss of
reactor depressurization.
The new regulation requires licensees to validate the
effectiveness of countermeasures against b-DBAs.7NRA 4. Mitigation of Severe Accident
In the Fukushima Dai-ichi accident, many attempts to
activate the AM measures were unsuccessful due to the
aggravated plant conditions, such as loss of power, loss of
control air, aftershocks, and high radiation. The feasibility
and effectiveness of AM measures are now strictly
examined in licensing processes.
Containment cooling/depressurization system to be used in
severe accident conditions, e.g., filtered venting system,
shall be installed to prevent the containment failure due to
over-pressurization and to minimize the radioactive
consequences.8NRA
The guidelines existed before the accident primarily and
excessively relied on code predictions on source terms
and radionuclide diffusion. Projected dose and dose that
has been received are not measurable quantities and
cannot be used as a basis for quick actions in an
emergency. The new guidelines by the NRA accordingly
introduce operational criteria (values of measurable
default quantities or observables, such as the emergency
action level, EAL, and the operational intervention level,
OIL) as a surrogate for the generic criteria for undertaking
different protective actions and other response actions.
The new guidelines also define requirements on roles and
functions of off-site emergency response centers,
execution of nuclear emergency drills, etc.
5. Emergency Preparedness9NRA
The amended "Reactor Regulation Act" stipulates
licensees’ responsibility for "safety improvement" and
requires licensees to conduct "self-assessment for safety
improvement" periodically.
This framework strongly encourages licensees’ initiatives
towards continuous improvement of safety by requesting
licensees to prepare the final safety analysis report which
provides "as-built" or "as-is" plant description and to update
it when major design modifications or procedural changes
take place.
6. Continuous Improvement of Safety10NRA 6. Continuous Improvement of Safety
(cont'd)
Licensees are also requested to carry out the periodic
safety review (PSR) to incorporate the state-of-the-art
knowledge into the plant design, operation and
maintenance activities.
In addition, it is required to conduct level 1 and 2
probabilistic risk assessments (PRAs) periodically for
both internal and external initiators including hazard
re-evaluation to demonstrate the effectiveness of the
plant modifications.11NRA Closing Remarks
In the light of the Fukushima Dai-ichi accident, the NRA
developed the new design requirements and established
the new regulatory framework to ensure the nuclear power
plant safety.
The NRA continues to address the lessons learned from
the Fukushima Dai-ichi accident, keeps updating
regulatory requirements where appropriate, and never
becomes complacent.
With thanks to
Kiyoharu Abe
Masahiro Aoki
Tomoho Yamada
Hiroshi YamagataNRANorio Watanabe
NSRC, JAEA