Reflections on Risk-InformedDecision Making
Richard A. Meserve
Executive Adviser, Nuclear Risk Research Center
rmeserve@carnegiescience.edu

NRRC Symposium, TokyoFebruary 8, 201811

Introduction:
Scope of Presentation••••

Why Risk-Informed Decision Making?
International Guidance on RIDM Application
The Challenge of External Events
Institutional Strength in Depth2Why Risk-Informed Decision Making?
•Traditional approach was deterministic:Ø Design Basis Accidents
Ø Supplemented by:• Defense in Depth• Redundant and diverse means to respond to events• Avoidance of vulnerability to single equipment failure• Conservative engineering design and application of conservativeengineering codes• Stringent Quality Assurance in construction• Attention to configuration management, training, maintenance, andoperational requirements• Application of lessons from operational experience.
• Strong safety cultureØ Result has been a strong safety record for those that followed thismethodology
3Why Risk-Informed Decision Making (2)• RIDM requires deterministic analyses to besupplemented by probabilistic analyses• Benefits of probabilistic analysis
Ø Provide a realistic view of possible evolution of an accident,
thereby revealing vulnerabilities and enabling safetyenhancement.
Ø Quantitative results provide means to set priorities.
Ø Enhances performance, flexibility, and cost-effectivenessØ Enables identification and reduction of unnecessary
requirements
Ø Provides capability to monitor safety status as equipment takenout of service

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Risk Informed Decision Making (3)• Why not evolve to a risk-based approach?
Ø Strong experience and history with deterministic approach.
Ø There can be large uncertainties with Probabilistic Risk
Assessments (PRAs)
Ø Reliability of PRAs dependent on modeling scope, choices byanalysts, and availability of information.
Ø Probabilistic assessments do not capture all potential risks• Conclusion: Combine deterministic and probabilistic
analyses. Get the best of both approaches.

5International Guidance on theApplication of RIDM••••

Available at:
http://www-pub.iaea.org/books/IAEABooks/8577/
A-Framework-for-an-Integrated-Risk-InformedDecision-Making-Process

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Guidance for achieving a balance ofdeterministic and probabilistic
considerations.
Explicit consideration of all effects
because improvements in one area
may have adverse effects in otherareas.
Multidisciplinary teams should be
involved in the decision process toreconcile diverse inputs with different
measures, thereby balancing different
risks.
Combine results from deterministic
analyses with risk information toensure all relevant factors areappropriately evaluatedINSAG 25: The Process

7External Events – a Special Challenge• All engineered structures, including NPPs, arevulnerable to natural external events – typhoons,
earthquakes, tsunamis, volcanism, and the like.
• External events have large uncertainties as tofrequency/magnitude of threat.
• External events may wipe away layers of defense indepth – consider Fukushima accident.
• Some external events have cliff-edge effects – nonlinear increase in risk from slight changes incircumstances.
• Special challenge for both deterministic andprobabilistic analyses.
8Institutional Strength in Depth
••

•

•
•
Available at:
http://www-pub.iaea.org/books/IAEABooks/11148/
Ensuring-Robust-National-Nuclear-Safety-SystemsInstitutional-Strength-in-Depth

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INSAG Guidance on the building
a robust nuclear safety system.
Recognize and build oninteractions among and withinsubsystems (regulators,operators, and stakeholders) toreinforce safety obligation
Build interfaces to reinforce
safety obligation within andamong subsystems.
Complementary to Defense inDepth
Should be component of theimplementation of RIDM

Elements of Strength in Depth

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Example: Industry Sub-system

11Elements of a Positive Safety Culture• Leaders demonstrate commitment to safety inbehaviors and decisions• Issues impacting safety are promptly identified,analyzed, and addressed• All individuals take personal responsibility for safety• Engage in continuous learning to improve safety• Personnel are free to raise safety concerns without
retaliation• Communications focus on safety• Trust and respect each other• Individuals avoid complacency and maintain aquestioning attitude
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Conclusion

RIDM provides an integrated means to improve safety,
enhance decision making, and build confidence.

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