Diablo Canyon NPP RiskInformed Inservice Inspection

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Diablo Canyon NPPRisk-Informed In-service
Inspection
IAEA Training Course on Safety Assessment of NPPs
to Assist Decision Making
Lecturer Lesson IV 3_11.3

• Workshop Information

IAEA Workshop
City , CountryXX - XX Month, Year
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Purpose of In-service Inspection

• To identify conditions, such as flaw indications,
  that are precursors to leaks and rupture, which
  violate pressure boundary integrity principles.

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RI-ISI benefits

• Enhance or maintained plant safety (CDF/LERF)
• Enhanced component reliability for high safety
  significance components (HSSCs)
• Reduce nondestructive exams (NDE)
• Reduced man-rem exposure
• Other unquantifiable benefits
• Reduced costs of engineering analysis (flaw
  evaluations, etc.)
• Reduced outage time
• Reduced chance of complicating plant operations
  (scaffolding, leakage, etc.)

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ASME Section XI Enhanced by Risk-Informed ISI
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Overall Risk-Informed ISI Process
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Segment Definition

• Full Scope Definition
• All Class 1, 2, and 3 piping systems in ASME
  Section XI
• Piping fluid systems modeled in PSA
• Various balance of plant (non-nuclear code class)
  fluid systems of importance
• Systems included under scope of Maintenance Rule
  determined to be risk-significant
• Systems included in program are reviewed by
  expert panel for concurrence
• Partial Scope Definition
• Subset of piping classes such as ASME Class 1
  piping only (includes piping exempt from current
  requirements)

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Segment Definition

• Segment defined based on
• Piping which have same consequence (loss of train
  A of RHR, loss of RWST, inside or outside
  containment consequences)
• Where flow splits or joins (traditional PSA
  modeling points)
• Includes piping to a point in which a pipe
  failure could be isolated (e.g., check valve,
  MOV, AOV, no credit for manual valves)
• Pipe size changes
• Failure probability expected to be markedly
  different due to material properties
• Iterative process with Consequence Evaluation

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Segment Definition

• Subdivided system into piping segments
• Assigned numerical identifier
• Based upon similar consequence
• Marked PIds field isometrics
• Determined failure modes effects analysis (FMEA)
• Without operator action
• With operator action

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Consequence Evaluation

• Both direct and indirect (spatial) effects are
  considered
• PSA is used to quantify impact
• Consistent with EPRI PSA Applications Guide
• Calculations for CDF and LERF
• Conditional probability/frequency given piping
  failure
• Considers multiple impacts
• Initiating event impact
• Single/multiple component/train/system impacts
• Combinations of impacts

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Direct Effects Evaluation

• Failure effect based on disabling segment
  function leak
• PRA and system information used to determine if
  piping failure causes
• An initiating event (e.g. LOCA, Reactor Trip)
• Loss of train or system
• Loss of multiple trains or systems
• Combination of the above

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Overview Of Indirect Effects Evaluation

• Purpose of Evaluation
• To review any issues in identifying potential
  indirect effects/consequences from piping
  failures
• Identify indirect effects that would
  differentiate piping segments from each other

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Indirect Effects

• Considerations
• Flooding, spraying, dripping should be
  primarily addressed by the PSA internal flooding
  analyses for all plant areas
• Pipe Whip, jet impingement concern is primarily
  for high-energy fluid system piping

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Indirect Effects Process

• Prewalkdown
• Review existing documents which examine the local
  effects of pipe breaks for the systems in the
  risk-informed ISI program
• Identify other systems/trains affected by a
  failure in each area
• Identify plant areas for plant walkdown
• Document evaluation
• Develop walkdown sheets for key areas
• Walkdown
• Perform walkdown and document results, actions,
  issues
• Post Walkdown
• Evaluate results
• Resolve actions

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Failure Probability Assessment Process

• Industry failure experience
• Identification of potential failure modes and
  causes
• Specific-plant information layout, materials,
  operating conditions and experience
• Use of tools or data to calculate failure
  probability
• Estimation of leak and break probabilities by
  engineering team

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Failure Probability Assessment Process
Engineering Team
-ISI/NDE Engineering -Materials
Engineering -Design Stress Engineering
(Engineering Mechanics) -Plant System Engineer
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RI-ISI Expert Panel Process
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Mapping of Surry Segments on Structural Element
Selection Matrix
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Presentation Format

• Overview of RI-ISI approach
• Detailed comparison
• Scope and segment definition
• Consequence evaluation
• Failure probability assessment process
• Risk evaluation
• Selection of elements and NDE methods (expert
  panel)
• Change in Risk calculations
• RI-ISI implementation (not addressed here)

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EPRI-RI-ISI Process
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Segment Definition

• Segment definition guidelines (similar in both
  methodologies)
• Piping which have same consequences
• Where flow splits or joins
• Pipe size changes
• Change in piping material
• Isolation capability
• EPRI uses the above plus same failure mechanism
  criterion

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Consequence Evaluation

• Deterministic evaluation of piping
  failure-induced impact (both methodologies)
• Direct impact (e.g. loss of a train)
• Indirect impact (e.g. damage caused by flooding,
  jet impingement)
• Multiple impacts (e.g. initiating events
  Accident mitigation)
• Probabilistic evaluation
• EPRI uses a bounding worst case evaluation (using
  matrix or calculation)
• WOG uses surrogate(s) to quantify condition CDF
  (CDP) and LERF (LERP) for spectrum of failure
  modes (leak, disabling leak, double ended break)
  utilizing internal events PSA model

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Structural Reliability Assessment

• Both methodologies evaluate potential for pipe
  failure
• EPRI qualitatively classifies potential for pipe
  rupture as High, Medium, or Low based on
  degradation mechanisms, in-service data, expert
  knowledge (no code).
• WOG uses SRRA code (stays with the user) to
  quantify leak/rupture frequency/probability based
  on in-service data, potential failure mechanisms,
  and plant specific information (e.g. layout,
  materials, operating and conditions, etc.)

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Risk Evaluation

• EPRI uses risk matrix to separately categorize
  piping segments in the high, medium, or low
  classifications using prescriptive criteria for
  the consequence and rupture potential elements
  (risk is not calculated). It uses plant staff to
  review the results and concur with the risk
  ranking results
• WOG methodology uses standard approaches for
  CDF/LERF calculation (ie. Frequency CCDP) and
  risk ranking process (RAW and RRW).
  Additionally, expert panel discussions are held
  to review PSA results and include other potential
  risk contributors (e.g. shutdown risk, external
  events, etc.)
• WOG methodology allows credit for aumented
  programs

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Element Selection

• Both methodologies inspect for cause
• EPRI methodology uses prescriptive rules (fixed
  percentages) to determine the population of
  elements to be inspected
• WOG methodology uses a combination of
  prescriptive and statistical rules to determine
  the population of elements to be inspected.

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WOG Matrix
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EPRI Risk Matrix
Consequence Assessment
Failure Potential Assessment
DEGRADATION CATEGORY Pipe Rupture Potential
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Change in Risk Calculations

• EPRI methodology uses a progressively more
  quantitative evaluation to assess the changes in
  Risk
• Qualitative
• Bounding
• Simplified
• Complex
• WOG methodology calculates the change in Risk
  based on the change in pipe failure frequency
  (probability) due to the proposed change in the
  Inspection program. The calculations are
  consistent with those performed to calculate the
  Risk.