OI/OIG Seminar
RISK ASSESSMENTRISK ASSESSMENT
HRTDHuman ResourcesTraining & Development
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RISK ASSESSMENTRISK ASSESSMENT
OI/OIG Seminar
Objectives
• Understand the basics of risk application
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• Understand how NRC applies risk in regulating power reactors
OI/OIG Seminar
What is Risk?• Arises from a “Danger” or “Hazard”
• Always associated with undesired
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• Always associated with undesired occurrences
• Involves both:
• Likelihood of undesired occurrences
• Severity (magnitude) of the consequences
OI/OIG Seminar
Definition of “Risk Triplet”• What can happen?
• Possible scenario
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• How likely is it?
• Frequency, probability
• What will be the outcome?
• Consequences
OI/OIG Seminar
Basic Terminology
• Probability - Likelihood of an event occurring
• Frequency or rate - number of occurrences of some event
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Frequency or rate number of occurrences of some event of interest in some defined interval of time
• Consequence - The ultimate result of an event or sequence of events in terms of impacts on public and employee health and safety, impacts on the environment, and costs or damage
OI/OIG Seminar
Ways to Assess Risk
“Objectively”• Numerical Approaches
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Subjectively• Expert Panels
OI/OIG Seminar
Ways to Use Risk Information
Licensing• Evaluate designs for vulnerabilities• Evaluate designs for adequate safety
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g q y• Remove requirements that offer minimal safety
benefit
Inspection• Select inspection areas with risk (safety) significance• Properly evaluate the significance of inspection
findings• Monitor day-to-day risk management
OI/OIG Seminar
Risk-Informed versus Risk-Based Regulation
• Risk-Informed – Using risk to supplement deterministic methods of regulation
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• Risk-Based – Basing regulatory decisions on risk numbers alone
OI/OIG Seminar
Numerical Approaches
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Numerical Approaches
OI/OIG Seminar
Basic Terminology
Risk Magnitude of ConsequencesUnit of Time =[ ]
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Frequency XScenario
Unit of Time
Consequences Magnitude Scenario
[
[
]
]
OI/OIG Seminar
Consequence FrequencyAlternate Frequency
Super Bowl 1/yr. 1.14e-4/hr.
Considering Frequency…
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Prednisone Purchase for Boudreaux
1/mo. 12/yr.
Paydays (NRC) 1/2wks. .071/day
Saturday Morning 1/wk. 9.92e-5/min
Commute 2/day 10/wk.
Going to Bed 1/day 7/wk.
OI/OIG Seminar
Determining & Using Frequency – What are the Odds?
What are the odds
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odds
that a coin toss
will come
up heads?
OI/OIG Seminar
Determining & Using Frequency – What are the Odds?
In reactor applications, the “Consequence” can be:
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What is the “Consequence?”
“Consequence” can be:
• An initiating event
• The failure of a component
• The failure of a fission product barrier
OI/OIG Seminar
Determining & Using Frequency – What are the Odds?
The math works out like this:
• Event of interest = Heads
F ½ T
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What is the “Frequency?”
• Frequency = ½ per Toss
• .5 heads per toss
Carrying this out further, if you tossed the coin 30 times per hour, the frequency could be expressed as:
15 heads per hour360 heads per day131,400 heads per annum
OI/OIG Seminar
Determining & Using Frequency – What are the Odds?
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The Reactor Version
Frequencies of Interest
• Scrams/Yr.
• Component Failure/Start
• Component Failure/Mission
OI/OIG Seminar
The Next Level…
What are the odds of tossing heads twice in a row?
First, the “consequence” is tossing heads t ice consec ti el
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Multiple Events
heads twice consecutively…
Probabilities are combined by multiplying, so…
.5 heads/toss x .5 heads/toss = .25/attempt
What are the odds of tossing heads thrice in a row?
.5 x .5 x .5 = .125/attempt
OI/OIG Seminar
Let’s Talk Hardware…
You’re outside Wal Mart and you decide to get a 25 cent coke. What can go wrong?
• You can drop your only quarter under the machine
• You can put your quarter in the machine and it gets stuck before being counted
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• Your quarter is counted, but the machine doesn’t allow you to select a drink
• You can select a drink but the can gets stuck before coming out of the machine
• The “Make Another Selection” lights are all burned out and the coin return doesn’t work
Can we calculate the odds of getting a coke?
What information do we need?
OI/OIG SeminarFrom your extensive and frustrating previous attempts at getting drinks at Wal-Mart, you compile the following consequences and frequencies:
Consequence Frequency of Occurrence Frequency of Success
Dropped/Lost Quarter 1 in 20 Attempts (.05/Attempt) 19 in 20 (.95/Attempt)
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Stuck Quarter 1 in 15 Attempts (.067/Attempt) 14 in 15 (.93/Attempt)
Quarter Counted/No Delivery 1 in 100 Attempts (.01/Attempt) 99 in 100 (.99/Attempt)
Stuck Coke 1 in 50 Attempts (.02/Attempt) 49 in 50 (.98/Attempt)
No Coke/No Coin Return 1 in 3 Attempts (.33/Attempts) 2 in 3 (.66/Attempt)
Now, how many different ways can things go wrong?
OI/OIG SeminarRepresenting the case graphically…
I Want a CokeDropped/Lost
QuarterStuck Quarter
Quarter Counted/No
DeliveryStuck Coke
No Coke/No Coin Return
Outcome
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Succ
ess
Fai
lure
OI/OIG SeminarConverting Graphics to Numbers…
I Want a CokeDropped/Lost
QuarterStuck Quarter
Quarter Counted/No
DeliveryStuck Coke
No Coke/No Coin Return
Frequency
99/
.98/attempt
.67/attempt
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Succ
ess
Fai
lure
.05/attempt
.95/attempt
.93/attempt
.067/attempt
.99/attempt
.01/attempt
.02/attempt
.33/attempt
OI/OIG SeminarSame Approach for a Reactor Plant
Loss of Offsite Power
Reactor TripMain
Feedwater?Heat Sink?
Pressure Control?
Volume Control?
End State
Stable RCS
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Succ
ess
Fai
lure
Level Prob.
Press Prob.Temp Control
Temp Control
ATWS
OI/OIG Seminar
Event FrequenciesLevel 1
Types of Risk Assessments
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Event Progression, Barrier Response
Release of Radioactive Material
Consequences
Level 2
Level 3
OI/OIG Seminar
Numerical Methods Share Commonalities• Identify Possible Scenarios• Estimate the Likelihood of Occurrence• Estimate Consequences
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Differences in Methods Exist• Scope
• CDF as Surrogate for Consequences• Individual Plant Examination vs. Individual Plant
Examination for External Events• Complexity• Calculational Methodology
OI/OIG Seminar
Advantages of PRA• Rigorous, Systematic Tool• Information Integration (multidisciplinary)• Considers Complex Interactions• Develops Qualitative Design Insights
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p Q g g• Quantitative Decision-Making Possible• Uncertainty Analysis Possible
Limitations of PRA• Adequacy of Data• Sensitivity to Analytic Assumptions• Frequently not Comparable Site to Site• Not “Living Documents”
OI/OIG Seminar
Seminal Moments in the History of Risk Assessment in Nuclear Power
PRA Policy Statement (ca. 1995)• Held that probabilistic approaches enhanced traditional deterministic approaches• Probabilistic approaches seen as improving decision-making• Offered the possibility of reduced regulatory burden
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Risk-Informed and Performance-Based White Paper (ca. 1998)• Held risk-informed approach as preferred• Maintained value of defense-in-depth• Espoused performance-based approaches to regulation where possible
Risk-Informed Regulation Regulatory Guides and Standard Review Plans• Issued to allow PRA use for specific applications
Maintenance Rule (1999)• First regulation to require licensees to manage risk at plants
OI/OIG Seminar
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Questions?