Commentary on
Basic Policy on Nuclear Energy
George Apostolakis
Director, Nuclear Risk Research Center
Former Commissioner USNRC
apostola@mit.edu
http://criepi.denken.or.jp/en/nrrc/index.html
Presented at
The 24th JAEC Regular Meeting
21 June 2022
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Items to be addressed•

Unremitting effort to improve safety on the basis ofno zero‐risk•

Rebuilding public trust

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Safety Improvement•

A major step forward: Establishing the independent
Nuclear Regulation Authority•“The new regulatory standards include unparalleledsafety requirements for existing reactors and arethus regarded as the world’s highest standards.”•“On the other hand, some are concerned that sucha high level of strictness may easily lead to a newsafety myth in which people may complacently
think that as long as a station satisfies thestandards, it is perfectly safe.”“White Paper on Nuclear Energy 2020,” page 13, JAEC, 2021.
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Are we creating another Myth?
• There is room for safety improvement even if theregulatory requirements are “the world’s higheststandards,” because these regulatory requirements
are based on judgments that are largely unquantified
and presumed to be conservative.
AND•

The system’s approach (including hardware andhumans) of Probabilistic Risk Assessment (PRA) isnot utilized.
What can go wrong? (accident scenarios)
How likely is it? (probabilities and frequencies)
What are the consequences, if it goes wrong?
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PRA Example: Tsunami Scenarios; Big Picture
Tsunami water level rise / water level fall / site inundationroute
Water level rise

Sea wall

Various wave sources
water level fall

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Tsunami Scenarios: Plant Configuration and Human Actions areimportant<Scenario>• R/B is inundated through thedoor on the third floor<Evaluation policy>• Treat a door as aninundation route to R/B<Scenario>• R/B is inundated from louver<Evaluation policy>• Treat a louver as an inundationroute to R/B

Louver ofHVAC

Door on the third floor
Watertight door
Between T/B and R/BR/BT/B

Outer watertight door

<Scenario>• Inundated from T/B to R/B due toforgetting to close a watertight door
during evacuation (human error)<Evaluation policy>• Treat a door as an inundation route forR/B• Treat forgetting a door close asdifferent scenario

2022<Scenario>• Inundated from S/B to
R/B<Evaluation policy>• Consider impact due toflooding<Scenario>• Inundated from pipingpenetration<Evaluation policy>• Consider capacity against
water pressure

S/B

Underground penetration<Scenario>• R/B inundated when open the watertightdoor<Evaluation policy>• Consider capacity against tsunami waveforce

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PRA Evolution in Japan
•••

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•

PRA was not taken seriously prior to Fukushima.
Industry established the NRRC in 2014.
“To assist nuclear operators and the nuclear
industry in their continuous effort to improve thesafety of nuclear facilities, that is, to manage therelevant risks, by developing and employing
modern methods of Probabilistic Risk Assessment(PRA), risk-informed decision making and risk
communication.”International expert committees review the PRAs for
Ikata 3 (PWR) and Kashiwazaki-Kariwa 7 (BWR).
NRA establishes a Reactor Oversight Processsimilar to the risk-informed U.S. process (2020).
20227Utilities’ Strategic Plan (2018)

NRRC, with the help of electric utilities, is currently examining thetechnical basis for risk-informing on-line maintenance.
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Considerations on Performing Integrated Risk Informed
Decision Making (IAEA-TECDOC-1909, 2020)

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Risk-Informed Framework

Traditional“Deterministic”
Approach

RiskInformed
Approach• Unquantified
probabilities•Design-basis accidents•Defense in depth•Can impose
unnecessary
regulatory burden•Combinationof traditionaland riskbased
approaches
through adeliberative
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Risk-BasedApproach• Quantified
probabilities•Thousands ofaccident
sequences•Realistic

Safety Goals•••

They underlie risk-informed decision making.
The NRA has not declared formally any SGs.
Informally:- Core damage frequency: 10-4 per reactor year
- Containment failure frequency: 10-5 per reactor year
- The frequency of the release of Cs137 larger than 100
TBq during nuclear emergency should be less than oncein one million years (excluding those due to securityevents)••

Consistent with the JAEC statement: “Unremitting
effort to improve safety on the basis of nozero‐risk.”
An implicit admission that accidents may happen,albeit with very low probability.
202211Rebuilding Public Trust•••••••An essential requirement: No incidents or rule violations
should occur.
Openness is also essential (example: the NRRC publishes the(sometimes critical) reports of its Technical Advisory
Committee on its website).
Building trust requires telling the truth.
The language of truth in nuclear safety is risk (accident
consequences and probabilities).
The metrics core damage frequency (CDF) and large early
release frequency (LERF) are used routinely in the U.S. by theindustry, regulators, and public-interest groups.
It’s unclear what the Japanese public reaction to the risk
language will be (probably negative in the beginning).
But, it is the truth.

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