The Concept of Risk and its Role inRational Decision Making on Nuclear
Safety Issues
George Apostolakis
Head, Nuclear Risk Research Center
apostola@criepi.denken.or.jp
NRRC Symposium
September 2, 2015

2015

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Risks in Society• Hazard: A source of danger
 Industrial facilities
 Activities, e.g., driving a car• Risk:The possibility that something bad or
unpleasant (such as an injury or a loss)
will happen• Uncertainty is an integral part of risk
• Risk:2015

Probability and adverse consequences2

Safety vs. Residual Risk

20153Why do we tolerate Residual Risks?
• Because each facility or activity provides benefits• For individual voluntary activities in which a person
feels in control the residual risk may be relatively
high (the risk in general aviation is about 1,000 times
greater than that in commercial aviation)• For industrial facilities, it is society through its
representatives, government and regulatory
agencies, that decides• Risk-Benefit tradeoffs are rarely quantitative; benefit
is much harder to quantify than risk
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Managing Uncertainty in Nuclear Safety (1)• Traditional “conservative” approach A bottom-up approach A limited number of potential accidents is considered Uncertainty is not quantified Unquantified uncertainty is managed by conservatism via
defense in depth and safety margins• Defense-in-Depth is a safety philosophy that employs
successive compensatory measures to prevent accidents or
mitigate damage if a malfunction, accident, or naturally caused
event occurs at a nuclear facility.
• Safety Margin: The imposed stress on a component orstructure is maintained well below the onset of damage.
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Major Elements of Defense in Depth

Accident Prevention

Safety SystemsContainment
Accident Management

Emergency Plans

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Managing Uncertainty in Nuclear Safety (2)• Probabilistic Risk Assessment
 A top-down approach Thousands of potential accident sequences areinvestigated Uncertainty is quantified and managed
 More realistic depiction of what can go wrong•

Probabilistic Risk Assessment (PRA) supports
Risk Management by answering the questions:

What can go wrong? (thousands of accident sequencesor scenarios)
How likely are these scenarios?
What are their consequences?
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Problems with the Traditional Approach• There is no guidance as to how much defense indepth is sufficient (unreliable regulations)• Qualitative approaches are used to ensure system
reliability (the single-failure criterion) when more
modern quantitative approaches exist• Human performance is stylized (e.g., operators areassumed to take no action within, for example, 30
minutes of an accident’s initiation)• Difficult to reflect operating experience andmodern understanding• Industry-sponsored PRAs showed a variability inrisk of plants that were licensed under the same
regulations.
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8Reactor Safety Study Insights(WASH-1400; 1975)
Prior Beliefs:
1.
2.
3.

Protect against large loss-of-coolant accident (LOCA)
Core damage frequency (CDF) is low (about once every
100 million years, 10-8 per reactor year)
Consequences of accidents would be disastrous

Major Findings
1.
2.

3.
4.

Dominant contributors: Small LOCAs and Transients
CDF higher than earlier believed (best estimate: 5x10-5,once every 20,000 years; upper bound: 3x10-4 per reactor
year, once every 3,333 years)
Consequences significantly smaller
Support systems and operator actions very important

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Some Results from a PRA for a GEN II Plant
Core Damage Frequency (CDF): about 5 events
per 100,000 years of operation
Initiator Contribution to CDF Total:• Internal Events (losses of coolant; transients): 56%• External Events:

44%

 Seismic Events
 Fires Other

24%
18%
2%

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Further Results• Functional Internal-Event Sequences
Contribution to CDF

 Transients - Station Blackout/Seal LOCA
 Transients - Loss of Support Systems/Seal LOCA
 Transients - Loss of Feedwater/Feed & Bleed
 LOCA - Injection/Recirculation Failure
 ATWS - No Long Term Reactivity Control
 ATWS - Reactor Vessel Overpressurization

111145%
29%
12%
7%
6%
2%

Regulatory Decision Making• Regulatory decision making (like any decision) should
be based on the current state of knowledge and should
be documented (clear and reliable regulations) The current state of knowledge regarding design, operation,and regulation is key.
 PRAs do not “predict” the future; they evaluate and assess
future possibilities to inform the decision makers’ current
state of knowledge.
 Ignoring the results and insights from PRAs results indecisions not utilizing the complete state of knowledge.
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The Deliberation (NUREG-2150)
Options

TechnicalAnalysis

Decision
Criteria

one or more
techniques

Assumptions,
Uncertainties
andSensitivities

ResourceandSchedule
Constraints

Deliberation
Other
Factors

Stakeholder
Input

Decision &Implementation

Figure 3-2 Deliberations
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U.S. Quantitative Health Objectives, 1986
(Residual Risk Goals)• Early and latent cancer mortality risks to anindividual living near the plant should not exceed
0.1 percent of the background accident or cancer
mortality risk, approximately
5x10 -7/year for early death and2 x10 -6/year for death from cancer
The prompt fatality goal applies to an average individual living in
the region between the site boundary and 1 mile beyond thisboundary.
The latent cancer fatality goal applies to an average individual livingin the region between the site boundary and 10 miles beyond thisboundary.
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Risk-Informed Framework

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

Risk-informed Regulation“A risk-informed approach to regulatory
decision-making represents a philosophy
whereby risk insights are considered
together with other factors to establish
requirements that better focus licensee andregulatory attention on design andoperational issues commensurate with theirimportance to public health and safety.”[Commission’s White Paper, USNRC, 1999]
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Confidence Building• Industry-sponsored PRAs for Zion and Indian PointNPPs
 Reviewed extensively by the USNRC
 Identified the significance of earthquakes and fires Failure modes with easy fixes identified• Early applications of risk-informed decision making
 South Texas Project Experience Allowed Outage Times extended from 3 days to 14 daysfor emergency AC power and 7 days for EssentialCooling Water and Essential Chilled Water systems.
 Actual experience: Less than 5 days.

• PRA standards
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Major Successes• Maintenance Rule
 Objective - Structures, systems and components (SSCs)
important to safety of nuclear power plants shall bemaintained so that they will perform their intended
function when required.
 PRA identifies SSCs important to safety• Reactor Oversight Process
 Regulatory and industry response to “violations” iscommensurate to their risk significance• Risk-informed In-service Inspection
 Focused on risk significant piping segments
 Reduces cost and man-rem to workers• Fire Protection Rule
 Realistic assessment of fire risks
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Concluding Remarks• The question of what is acceptable or tolerable risk
cannot be avoided• Decision making regarding reactor safety issues
must be based on the totality of available
information• Too many people focus on the P of PRA. It’s theaccident scenarios that provide the most benefit.
• Risk insights have been used successfully to focus
attention (industry and regulators) on what is
important to safety201519