Nuclear Damage Compensation and Decommissioning Facilitation Corporation

Challenges in Risk-Informed Approach
to Safer Fuel Debris Retrieval

July 3, 2017
Kazuki Hida
Nuclear Damage Compensation &Decommissioning Facilitation Corporation©Nuclear Damage Compensation and Decommissioning Facilitation Corporation

Safer Fuel Debris Retrieval
 NDF’s fundamental policy for decommissioning of Fukushima
Daiich NPS (1F) To reduce continuously and promptly the radiological risks
that resulted from the accident Safety is of highest priority among five guiding principles
 Safe, proven, efficient, timely, field-oriented

 Risk-informed approach to safer fuel debris retrieval
 Risks posed by fuel debris• Existing baseline risk
– Radioactive release originated from initiating events• Additional risk during retrieval– Changes in facilities & fuel debris characteristics– Events caused by operation
 Use of risk management process to understand risks & toreduce level of risks

2©Nuclear Damage Compensation and Decommissioning Facilitation Corporation

Risk Management Process
Risk management processin ISO 31000-2009 *Risk assessment

Continued review & update

Identification* Referenced in “For understanding of risk assessment”

Atomic Energy Society of Japan

Risk sources: fuel debris, initiating events
Events: loss of safety functions forcriticality, cooling & confinement
Consequence: uncontrolled airborne & waterborne release

AnalysisAnalysis of consequence & likelihood of occurrence- Semi-quantitative expert judgment- Quantitative method adapted from PRA

Evaluation

Comparison of level of risk with criteria (or goals)- Setting safety goals- Consideration of 1F reality

Treatment

Reduction of level of risk
- Prevention, mitigation & their combination- Practicably achievable measures

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Risk Analysis by Expert Judgment

Occurrence Semi-quantitative analysis of consequence & likelihood ofoccurrence (collaboration with U.S. PNNL) Based on judgment by experts
Group of events
who experienced TMI-2 or Hanford
Airborne releasedecommissioning
Heavy load drop
 Risk identification
Waterborne release Potential events based on
Criticality
assumed facilities & operation
Hydrogen combustion
 Risk analysis & evaluation
 Five categories for consequence &likelihood of occurrenceH• Effective dose rate estimatedby assuming fuel debrisMcharacteristics & release paths
 Five categories for level of riskL
 Risk treatment
LM
H Prevention & mitigation measures
Consequence
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Quantitative Risk Analysis Adapted from PRA
 Events
 Events identified by experts & loss of safety functions
 Analysis method
Example probability
 Event trees & fault trees
by expert judgment
 Frequencies & probabilities
Certain1.0
 Hazard curves for quake & tsunami
Likely0.9
 Fragility & random failure from database
Indeterminate0.5
 Human error
Unlikely0.1
 Natural phenomena by expert judgment
Highly Unlikely 0.001
 Source Term = MAR x DR x LPF x ARF x RFImpossible
0.0 MAR (Material-At-Risk)• Fuel debris, FPs, contaminated water DR (Damage Ratio) & LPF (Leak Path Factor)• Expert judgment
 ARF (Airborne Release Fraction) & RF (Respirable Fraction)• Database5©Nuclear Damage Compensation and Decommissioning Facilitation Corporation

Preliminary Event DiagramUncontrolled release
Failure of confinement
PCV
damageHydrogencombustion
Inert gas
supplyfailurePipe
failureWater level
reversed
Pump
failureIncrease in release amountGas control
failureVaporization ofFPs dispersed
at accident

Pressure
increase

FPs

Dispersive
fuel debris

Criticality

Heavy
load
drop

Temperature
increase
Cooling
failureFire

Debrisaccumulation

Operation

Pipe
failureCutting Water
fuellevel
debris increase

Lift ofheavy
load

External events (quake, etc.) & internal events (random failure, human error, etc.)

Changeovertime

Core of risk management  continued information collection & update
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Example of Event Tree & Fault Tree
Quake
Frequency

PCVdamagePowersupplyCoolingPressure
increase

Gas control

Large

Yes
NoRelease

Success－

Success
Failure－No

Yes

FailureSuccess

Small

FailureMedium－Success
FailureNo－

Yes

MediumCooling failure
OR

Pump failure

Line failure

Tank failure

AND

Regular pump

RecoveryEmergency pump

OR

Example of hazard curve
Damageby quake

Randomfailure7©Nuclear Damage Compensation and Decommissioning Facilitation CorporationExample of Results & Risk Treatment
Scenario
AAnalyze ET/FT foreach scenario↓
Identify
effective measures

GPrevention

Frequency (/Year)CBFE

HMitigation

DRelease (converted to mSv)
8
©Nuclear Damage Compensation and Decommissioning Facilitation Corporation

Safety Goals
 1F reality important for goal setting & their impacts
 Uncertainty  conservative safety measures & extended
preparation/operation period• Extension of time at risk posed by fuel debris
 Contamination  operation under high dose environment(implementation, maintenance, etc.)• Increase in occupational exposure Example of goal setting & safety evaluation
 Goal setting• Reduce level of risk during retrieval as low asreasonably practicable• Consider time at risk posed by fuel debris &occupational exposure Safety evaluation• Practical definition of representative agent

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Concluding Remarks
 Risk-informed safe & prompt fuel debris retrieval
 Expert knowledge collected Quantitative risk analysis method developed & applied Lessons learned from preliminary study Benefits of risk-informed approach• Understanding of existing risk & risk during retrieval• Prioritization of risk treatment• Development of risk reduction measures
 Challenges in method• Challenges: probabilities for natural phenomena &release scenarios• Short term solution by expert judgment & mid-to-longterm solution by R&Ds
 Continued review & update are critical• Progress in facility design & operation planning• New information regarding fuel debris characteristics

10©Nuclear Damage Compensation and Decommissioning Facilitation Corporation