Issues, Guidance and Recommendations for: Tsunami Risk Assessment and Reduction for Nuclear Facilities

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Issues, Guidance and Recommendations
forTsunami Risk Assessment and Reduction for
Nuclear Facilities

• Dr. Robert T. Sewell
• Dr. George Pararas-Carayannis
• Fifth International Tsunami Symposium ofThe
  Tsunami Society International
• European Commission Joint Research Centre
• Ispra - Italy, 5 September 2012

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Topics

• Definition and Importance of External Hazards
  (Including Tsunamis) for NPPs
• External Hazards Assessment for NPPs
• Special Considerations Unique Challenges
• Applicable Existing Safety Guidance for NPPs
• Screening and Evaluation Approaches and Bases
• Deterministic Assessment for External Hazards
• Probabilistic Assessment for External Hazards
• Recommendations and Discussion

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External Hazards IAEA Definition

• External hazards originate from sources located
  outside of the site of the nuclear power plant.
• Examples of external hazards include
• Seismic hazards
• High winds and wind-induced missiles
• External floods including tsunamis
• Other severe weather phenomena (e.g., snow, ice)
• Off-site transportation accidents
• Off-site explosions
• Releases of toxic chemicals from off-site storage
  facilities
• External fires (e.g. fires affecting the site and
  originating from nearby forest fires)

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Importance External Hazards

• External Hazards can often be the dominant
  contributor to the risk of nuclear plant failure
  (e.g., core damage, or significant radiological
  release)
• For example, seismic events (earthquakes) are a
  particularly severe challenge to NPPs, and
  typically cannot be ruled at any location for
  return periods of interest (i.e., up to 10
  million years)
• Recent experience at the Fukushima-Daiichi NPP
  has demonstrated tsunamis to be an external
  hazard requiring improved awareness and risk
  management

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Special Considerations and Unique Challenges for
External Hazards

• High Severity Common Cause
• Scenarios have the potential to adversely affect
  many components or, often, the entire plant
• As in the Fukushima catastrophe
• High Uncertainty
• Experience data is often lacking uncertainties
  must be systematically quantified.
• Broad and Diverse Phenomena
• Covers several disciplines and areas of expertise
• Leads to temptation to minimize or ignore the
  threat (e.g., it is common that analysts or
  decision makers prematurely eliminate or
  screen-out events outside of their immediate
  expertise)

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Special/Unique Considerations (contd)

• Spatial relationships (e.g., Locations and
  Proximities) usually are of particular Importance
  in the assessment of external hazards.
  (Knowledge of threats outside of the site
  boundary of an NPP needs to be obtained.)
• For tsunamis, the source event can be caused by
  substantially different phenomena (e.g.,
  earthquakes, landslides, meteor impacts, large
  releases of gases)
• As events of very long return period must be
  considered (i.e., 10 million years median
  hazard, or 1 million years mean hazard),
  analysis (with associated uncertainties) is
  typically required to fill-in for data gaps

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Example External Hazard (Tsunami) at NPP
(Fukushima Daiichi)
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Example External Hazard (Seismic, No Tsunami) at
NPP (Kashiwazaki)
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Applicable Safety Guidance External Hazards
(incl. Tsunamis)

• IAEA Safety Guides
• Several applicable guides to overview
• ASME/ANS Standard
• Update to New ASME / ANSI Standard for Use in
  USNRC Programs in Response to Fukushima
• US NRC
• Best Practices from IPEEE Implementation
  Experience

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Applicable IAEA Safety Guides Periodic Safety
Review Process

• NS-G-2.10
• Logistics of Safety Assessment in the Context of
  the Periodic Review Cycle
• Relevance of External Hazards to the PSR

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Applicable IAEA Safety Guides General Safety
Assessment

• NS-G-1.2
• Guidance for Safety Assessment for Design and
  Design Modifications
• Relevance of External Hazards in a Design or
  Retrofit Safety Assessment
• Overviews Deterministic and Probabilistic
  Approaches for Safety Assessment

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Applicable IAEA Safety Guides Deterministic
Safety Assessment

• SSG-2
• Detailed Guidance on the Deterministic Safety
  Assessment Approaches
• Treatment of External Hazards in a Deterministic
  Safety Assessment

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Applicable IAEA Safety Guides Probabilistic
Safety Assessment (L1)

• SSG-3
• Detailed Guidance on the Probabilistic Safety
  Assessment (PSA) Approaches for a Level-1
  Analysis
• Treatment of External Hazards in a Level-1 PSA

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Applicable IAEA Safety Guides Probabilistic
Safety Assessment (L2)

• SSG-4
• Detailed Guidance on the Probabilistic Safety
  Assessment (PSA) Approaches for a Level-2
  Analysis
• Treatment of External Hazards in a Level-2 PSA

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Applicable IAEA Safety Guides External Hazards
(Except Seismic)

• NS-G-1.5
• Guidance on the Safety Assessment of External
  Hazards, Excluding Seismic

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Applicable IAEA Safety Guides Seismic Hazard
Analysis

• NS-G-3.3
• Guidance on Procedures for Seismic Hazard Analysis

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Applicable IAEA Safety Guides Flood Hazards

• NS-G-3.5
• Guidance on Procedures for Flood Hazard Analysis
• Includes Tsunamis, Seiche, Storm Surges, in
  Addition to River Flooding

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Applicable IAEA Safety Guides Geotechnical
Hazards

• NS-G-3.6
• Guidance on Geotechnical Considerations in NPP
  Siting and Design
• Relevant to Identifying and Evaluating External
  Hazards Associated with Landsliding, Settlements,
  Ground Collapses, Etc.

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Other Guidance / Standards

• ASME/ANS RA-S-2008
• Freely Available on Internet
• http//www.engineeringcodes.com/download/ASME_RA-
  S-2008_cont_0670.pdf
• Covers Internal Hazards and External Hazards
• This document is current being updated for use in
  a US External Hazards Safety Program in Response
  to the Fukushima Event
• http//cstools.asme.org/csconnect/pdf/CommitteeFi
  les/27524.pdf

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Other Guidance / Standards

• NUREG-1407
• Not much guidance for tsunamis
• Implementation Experience in the IPEEE Program
  Has Been Important to Defining and Improving the
  State of the Art
• Event Categories and Screening Criteria in the
  Earlier Guidance of NUREG/CR-2300 Has Become
  Substantially Obsolete

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External Hazards Safety Assessment Basic
Approach (incl. Tsunamis)

• Comprehensively and Conservatively Identify and
  List All General Types of External Hazards
• This is not a screening step every general
  hazard should be included
• See Spreadsheet for typical list for NPPs
• Identify and Characterize all Plant-Unique
  External Hazards
• Perform an initial review of plant information
• Perform a GIS survey of the plant zone, vicinity,
  region
• Perform an initial walkdown of the plant and
  vicinity
• This is not a screening step every
  plant-specific hazard should be included

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• Research and Collect the Necessary Plant,
  Engineering and Scientific Data Sufficient to
  Perform a Conservative Qualitative Screening
  Analysis of Each External Hazard
• Past experience reveals that most analysts tend
  to be overly aggressive in this Qualitative
  Screening phase. Some common problems
• Insufficient research is done to rigorously
  defend the reasons for screening out a hazard
• e.g., Scientific literature and the relevant body
  of scientists or experts having expertise with
  the particular hazard are not fully consulted
• Analysts tend to baseline their judgment on the
  types of extreme events observed during a
  generation or two (what they have heard about
  anecdotally or experienced themselves in their
  lifetimes), rather than on sufficiently rare
  phenomena
• Insufficient attention to uncertainties and
  diverse viewpoints within the Informed Technical
  Community (ITC)

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• For Each Hazard Not Qualitatively Screened-Out
  
• Perform Additional Walkdowns, Research and Data
  Collection Sufficient to Perform a Conservative
  (Simplified) Quantitative Screening Analysis
• Hazard-based (or location-based) screening
• Fragility-based (or vulnerability-based)
  screening
• Exposure-based (or Conditional Core-Damage
  Probability CCDP-based) screening
• Simplified Risk-Based Screening (combining two or
  more of the preceding)
• The effective screening threshold in each case
  should be no less stringent than
• Mean annual frequency of 10-6 (1,000,000-yr RP)
• Median annual frequency of 10-7 (10,000,000-yr RP)

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• For Each Hazard Not Screened-Out with a
  Simplified Conservative Quantitative Analysis
• Perform Additional Walkdowns, Research and Data
  Collection Sufficient to Perform a Detailed
  Analysis
• Detailed hazard analysis
• Detailed fragility analyses
• Detailed CCDP analysis or hazard-specific plant
  logic analysis and/or
• Detailed risk analysis
• Always Perform a Detailed Analysis for
  Earthquakes at Every NPP (and Tsunamis for
  coastal NPPs)
• Report the Total Risk from External Hazards
• All contributions for hazards analyzed inSteps 4
  through 6

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Deterministic Approach

• Conceive and Apply Maximum Credible Scenarios
• Often conceived as (or believed to be) a worst
  case
• Significant disadvantage of this approach is that
  the event frequency (and its level of
  conservatism) are not known, and can be
  significantly misunderstood and misrepresented
• Use of Conservative Assumptions and Relationships
• For example, equivalent to a mean one-sigma
  (or 84th-Fractile) level
• Expert Uncertainty Is Often Not Formally
  Addressed
• Significant disadvantage of this approach
• Could be somewhat mitigated by using the more
  conservative / stringent expert interpretations

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Framework of Probabilistic Approach

• Conceptual Elements of Risk for External Hazards
• Risk Evaluation
• Direct (statistical analysis of data) and derived
  (analytical modeling of phenomena) approaches
• Random Process Modeling
• Rate, occurrence model, and time
• Aging, forecasting, time dependencies
• Uncertainty Analysis Framework
• Aleatory variability and epistemic uncertainty

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Conceptual Elements of Riskfor External Hazards
Hazard
Vulnerability
Risk
Location /Proximity
Exposure
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Generalized Direct Formulation of Hazard (or Risk)
TH Hazard State e.g., X?x TC Component
Damage State e.g., RWST Fails TS
System-Level Risk State e.g., CCW System
Fails TP Plant-Level Risk State i.e.,
Plant Fails
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Generalized Derived Formulation of Probabilistic
Hazard (or Risk)
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Risk Evaluation (for a Failure State)
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Common Hazard Fragility Formulation
Hazard Curve
Fragility Curve (Component, System, or Plant)
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Random Process Modeling

• Rates, ?, are long term values (events per
  year)
• Assumes that the current snapshot of processes
  continue indefinitely (e.g., millions to hundreds
  of millions of years, not only 50 years)
• Does not consider changes in processes themselves
  that can occur over these long time frames
• A stochastic occurrence model (e.g., Poisson,
  Renewal, Time-Predictable, etc.) is needed to
  evaluate probabilities for each category of
  external event
• In general, different occurrence models may be
  appropriate for different external hazards

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Poisson Occurrence Model (Common)
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Aleatory and Epistemic Variations

• Aleatory Variation (inherent randomness) Derives
  from differences, even under seemingly identical
  conditions, in the way nature behaves and is
  manifest.
• Usually addressed by classical methods of
  probability and statistics
• Epistemic Variation (professional uncertainty)
  Derives from data limitations and differences in
  models, judgments, and use of data by credible
  experts
• Usually addressed by methods of subjective
  probability and Bayesian analysis

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Accounting for Aleatory and Epistemic Variations

• Develop an epistemic logic tree (E-LT) model to
  describe possible epistemic variations
• Requires development of multiple, alternative
  aleatory models and their subjective
  probabilities
• Simulate epistemic scenarios from the E-LT
• For each epistemic scenario, develop an aleatory
  logic tree (A-LT) model to describe possible
  aleatory variations
• Requires a behavioral model, its basic random
  variables and their probability distributions

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Uncertainty Analysis Framework
EpistemicModel ofHazard
Aleatory Model ofScenarios
Epistemic Model ofOutcomes
Aleatory Model ofOutcomes
UncertainRiskScenarios
HazardAnalysis
Vulnerability Analysis
RiskResults
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Elements of Tsunami Risk Assessment for NPPs

• Seismic Probabilistic Safety Assessment
  (PSA)a.k.a. Seismic Probabilistic Risk
  Assessment (PRA)
• Fragility analysis approach for capacity
  assessment of components and structures
• Full event-tree / fault-tree quantification of
  plant systems response from component responses
• Full treatment of random failures and human
  errors
• Point-estimate or full uncertainty analysis
• Tsunami core-damage frequency (CDF)
• Tsunami large-early release frequency (LERF)

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Tsunami Hazard fromSubmarine Landsliding

• l(W?w) ? nLS ?? PTypeSize ? PVelocity ?
  PTrajectory ?PW?w Type, Size, Velocity,
  Trajectory
• (Lack of Site-Specific Empirical Data Suggests
  the Need forModeling and Paleo-Tsunamic Data,
  Which Are Readily Accommodated Here)

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Tsunami Hazard Scenario

• One scenario (numerical simulation) is
  illustrated here
• Multiple such scenarios and their likelihoods can
  be estimated to evaluate the probabilistic hazard
• Latin Hypercube simulation considering aleatory
  and epistemic variations is an efficient approach
  for this type of hazard study

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Location
Carmel AreaNear Pt. Lobos(CA State Reserve Area)
Latitude36.51965 N
Longitude121.92126 W
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Location
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Location
Site
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About 160 Miles Further South
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LandslideHazard
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Submarine Landslide Scenario Animation
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Tsunami Scenario Animation
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T0.0 min.
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T0.5 min.
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Recommendations (Discussion)

• In the US, there exists a PMEL-USGS-NRC(SITAG)
  joint program on tsunami hazard for NPPs, which
  is a step in the right direction, provided that
  the program embraces an epistemic assessment
  similar to the rigorous Level-4 (or, in some
  cases, Level-3) guidelines of the Senior Seismic
  Hazard Analysis Committee (SSHAC) that have set a
  precedence and standard for probabilistic hazard
  assessment at NPP sites.
• i.e., The diverse viewpoints within the entire
  Informed Technical Community should be addressed,
  requiring the participation of additional experts

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Recommendations (Discussion)

• Beyond-design margin for tsunamis needs to be
  better understood.
• A 10,000-yr design basis (as has been applied as
  a standard for NPP design for other external
  hazards) has not been systematically applied for
  the tsunami threat.
• Even the 10,000-yr design level may not be
  sufficient if there exist "cliff edge" effects
  with the tsunami margin (i.e., if failure is
  incipient with minimal exceedance of the design
  level).

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Modern Tsunami Evaluation Methods
Recommendations (Discussion)

• Probabilistic safety/risk assessments (PSAs/PRAs)
  is the modern approach for safety assessment of
  NPPs. Correspondingly, the probabilistic nature
  of the tsunami hazard and tsunami fragility of
  NPPs need to be better evaluated or understood.
• The major knowledge base developed over the past
  30 years in improvement of methodology for
  seismic risk assessment of NPPs should be
  leveraged for rapid improvement in tsunami risk
  assessment of NPPs

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Recommendations (Discussion)

• Consistent with screening criteria for external
  hazards (e.g., in an external events PSA for an
  NPP), probabilistic tsunami hazard studies should
  cover hazard values down to 1E-6/yr mean
  exceedance frequency and 1E-7/yr median
  exceedance frequency.
• This can be expected to represent a significant
  challenge for some tsunami experts, who may have
  focused in the past on characterizing the hazard
  for much shorter return periods (e.g., typical of
  inundation studies or studies for public warning
  and evacuation zoning, rather than extremely rare
  events).

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Recommendations (Discussion)

• Development and application of procedural
  methodology for epistemic assessment should be
  undertaken in a manner similar to a SSHAC Level-4
  (or Level-3) approach for tsunami hazard, with
  tsunami source, wave propagation and run-up being
  analogous (respectively) to seismic source
  modeling, ground-motion modeling, and site
  response analysis.

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Recommendations (Discussion)

• Implementation of a robust and rigorous
  probabilistic tsunami hazard assessment (PTHA)
  methodology for the aleatory assessment is also
  needed. Such a PTHA methodology was developed by
  the first author (in concert with Dr. C. Mader)
  in 2002 for an LNG facility, and adapted the
  well-established PSHA (probabilistic seismic
  hazard analysis) methods originated by Cornell
  (1968), as well as generalized the probabilistic
  treatment of tsunamigenic sources for all tsunami
  causes (not just earthquakes).
• Based on the authors experience, such a
  methodology can be successfully applied and
  improved. Although similar approaches have since
  been used by others, it may still take
  significant time and effort to fully acquaint the
  ITC of tsunami scientists with these methods.

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Recommendations (Discussion)

• Sufficient coverage and undertaking of the
  marine, geophysical, geological, paleo, etc.,
  studies is needed to properly understand and
  characterize the tsunami threat, bathymetry, etc
• Every coastal NPP should perform these according
  to a rigorous consistent standard (which itself
  needs to be developed)
• Adaption of physical models (finite elements, 3D
  slip surface having minimum factor of safety,
  etc.) to define the extent and behavior of
  submarine landslides in consideration of physical
  geological/geotechnical properties, rather than
  reliance solely on geometric models that are
  related primarily to marine geomorphology, is
  needed for describing the landslide-tsunami
  threat to NPPs.

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Recommendations (Discussion)

• A sufficient description/characterization of the
  full scope of possible tsunami threats
  including characterization for both direct
  effects and collateral effects and damages from
  tsunamis is needed.
• Development of local tsunami warning systems for
  NPPs is needed.
• Whereas this step may seem a severe measure to
  some, oil companies have sought such measures for
  protection of LNG plants and other facilities.
  Also, local tsunami warning systems are
  considered for warning communities of silent
  tsunamis from causes unrelated to earthquakes
  (e.g., Skagway, AK). It can be easily argued that
  the need to protect NPPs in areas of significant
  tsunami hazard is at least as great as for
  petroleum facilities and for communities having
  high visibility as vacation destinations.

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Recommendations (Discussion)

• A reproducible set of policies and procedures
  (consistent with overall safety management)
  pertaining to how to conduct site-specific
  tsunami hazard studies, tsunami risk studies, and
  reviews of such tsunami studies for NPPs should
  be developed.
• Procedures are needed including comprehensive
  walkdown methods and field studies of NPPs to
  suitably understand and assess the deterministic
  and probabilistic vulnerability, design, and risk
  evaluation, and to ensure adequate
  fragility/resistance of NPPs to tsunamis
  (including impacts on structures, systems,
  components and operators).