Lessons Learned from the Fukushima Dai-ichi Accident
and Responses in Regulatory Requirements
Toyoshi Fuketa
Nuclear Regulation Authority

27th Annual Regulatory Information Conference
Bethesda North Marriott Hotel & Conference Center
March 11, 2015

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TEPCO’s Fukushima Dai-ichi accident revealed theweakness of the foregone regulatory requirements, e.g.
Insufficient design provisions against tsunami,
Unpractical management measures under severe
accident conditions, andInsufficient provision for accidents far-exceeding thepostulated design conditions.
We re-realized the importance of the Defense in Depth (DiD)
approach in design and preparations of countermeasuresagainst beyond design basis accidents (b-DBAs).
We learned from the accident that we must evaluate inadvance the potential and consequences of a wide spectrumof internal and external initiators, including earthquake,tsunami, volcanism, aircraft crash, fire, terrorist attack, etc.

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1. Prevention of SSCs failures

The Fukushima Dai-ichi accident revealed vulnerability ofstructures, systems and components (SSCs) against
extreme loads and conditions caused by some specificinternal/external initiators. The NRA, accordingly,
enhanced design requirements significantly.
Due considerations are required for all the significant
internal and external initiators.

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1. Prevention of SSCs failures(cont'd)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 veryrare events.
As for earthquakes, more stringent criteria are prepared for
active faults, more precise methods are provided fordesign-basis ground motions, etc.
As for tsunami, more comprehensive methods are requiredfor 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 suchas coastal levee and watertight doors are required.

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2. Measures to Prevent CCFs
The new requirements extend design-basis events andstrengthen protective measures against natural
phenomena and other initiators which may lead tocommon cause failures (CCFs).
They put a particular importance in due consideration toensure 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.

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2. Measures to Prevent CCFs
(cont'd)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.

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3. Prevention of Core Damage

In the new requirements by the NRA, the definitions ofsome 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 ofreactor depressurization.
The new regulation requires licensees to validate theeffectiveness of countermeasures against b-DBAs.

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4. Mitigation of Severe Accident
In the Fukushima Dai-ichi accident, many attempts toactivate the AM measures were unsuccessful due to theaggravated plant conditions, such as loss of power, loss ofcontrol 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 insevere accident conditions, e.g., filtered venting system,shall be installed to prevent the containment failure due toover-pressurization and to minimize the radioactive
consequences.

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5. Emergency Preparedness
The guidelines existed before the accident primarily andexcessively relied on code predictions on source terms
and radionuclide diffusion. Projected dose and dose thathas been received are not measurable quantities andcannot be used as a basis for quick actions in anemergency. The new guidelines by the NRA accordingly
introduce operational criteria (values of measurable
default quantities or observables, such as the emergencyaction 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 andfunctions of off-site emergency response centers,execution of nuclear emergency drills, etc.

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6. Continuous Improvement of Safety

The amended “Reactor Regulation Act” stipulates
licensees’ responsibility for “safety improvement” andrequires licensees to conduct “self-assessment for safetyimprovement” 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.

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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 andmaintenance activities.
In addition, it is required to conduct level 1 and 2
probabilistic risk assessments (PRAs) periodically forboth internal and external initiators including hazard
re-evaluation to demonstrate the effectiveness of theplant modifications.

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Closing Remarks

In the light of the Fukushima Dai-ichi accident, the NRAdeveloped the new design requirements and establishedthe new regulatory framework to ensure the nuclear power
plant safety.
The NRA continues to address the lessons learned fromthe Fukushima Dai-ichi accident, keeps updating
regulatory requirements where appropriate, and neverbecomes complacent.

With thanks toKiyoharu AbeMasahiro Aoki
Tomoho Yamada
Hiroshi Yamagata
NRA
Norio Watanabe
NSRC, JAEA