RiskInformed InService Inspection RIISI Using RISK

1
Risk-Informed In-ServiceInspection
(RI-ISI)Using _at_RISK

• Ching Guey

2
RI-ISI at Turkey Point

• Turkey Points application of RI-ISI is limited
  to Class 1 piping inside containment.
  
• The purpose of in-service inspection is to
  identify conditions, such as flaw indications,
  that are precursors to leaks and ruptures which
  violate pressure boundary integrity principles.
• In-service inspection (ISI) activities include
  ultrasonic testing, surface testing, and liquid
  penetrant testing.
  

3
Objectives

• More Effective and Efficient Use of _at_Risk to
  Streamlize the Analysis Work
  
• Prioritize Turkey Points ISI Activities by Risk
  
  
• Decrease cost of outage inspections, and
• Decrease personnel radiation exposure during
  outage inspections, while
  
• increasing or maintaining personnel safety
• increasing or maintaining unit reliability

4
Risk-Informed ISIProcess Overview
Expert Panel Categorization
Consequence Evaluation
Element/ NDE Selection
Scope and Segment Definition
Risk- Evaluation
Implement Program
Structural Element Failure Probability Assessmen
t
Feedback Loop
_at_RISK
5
Turkey Point PRA LOCA Events and Nominal Pipe
Sizes

• Small-Small LOCA 3/8
• Small LOCA 2
• Medium LOCA 6
• Large LOCA 13.5
• Any piping with a nominal diameter less than or
  equal to 3/8 is assumed not to result in a LOCA
  event, which could lead to core damage.

6
Failure ProbabilityAssessment Process
PLANT INFORMATION (LAYOUT, MATERIALS, OPERATING
CONDITIONS,
PLANT OPERATING EXPERIENCE)
IDENTIFICATION OF POTENTIAL FAILURE MODES AND CA
USES
INDUSTRY EXPERIENCE
ENGINEERING TEAM
SRRA TOOL

• Engineering Team
• ISI/NDE Engineering
• Materials Engineering
• Design Stress Engineering
• (Engineering Mechanics)
• -Plant System Engineer

ESTIMATED LEAK AND BREAK PROBABILITIES
7
Piping Segment Definition

• Segment
• Section of pipe for which a failure at any point
  has the same consequences
  
• Over 200 individual segments for each unit

8
Piping Failure Probability

• SRRA Code used to predict piping failure
  probability
  
• Factors used in SRRA Piping Failure Probability
  Calculation
  
• Pipe Segment Dimensions
• Material Stresses
• Pressure, Temperature

9
Consequences Evaluation

• Calculated using the Turkey Point Probabilistic
  Safety Assessment (PSA).
  
• Postulated different size breaks in each
  segment.
  
• Calculated Conditional Core Damage Probability
  (CCDP) and Conditional Large Early Release
  Probability (CLERP) for each segment and break
  size.

10
Risk Evaluation

• SRRA Code used to predict piping failure
  frequency.
  
• Calculated Conditional Core Damage Probability
  (CCDP) and Conditional Large Early Release
  Probability (CLERP) for each segment and break
  size using the PTN PSA model.
• Multiplied the segment piping failure frequencies
  from SRRA by their CCDPs and CLERPs to obtain the
  CDFs and LERFs for each segment.
  
• Calculated risk importance measures for each
  segment Using _at_RISK .
  
• Assigned a preliminary risk category to each
  segment based on the RRW.

11
Expert Panel

• Convened a panel of experts from ISI/NDE, Plant
  Engineering, Risk Assessment, Operations,
  Maintenance, etc.
  
• Provided each Expert Panel member with all of the
  risk-related and other information calculated
  and/or collected for each pipe segment.
  
• Reviewed, discussed, and eventually assigned a
  final risk category to each pipe segment.
  
• Documented the basis of the determination.

12
Results

• Weld Inspection Reduction 84
• Radiation Dose Reduction 20 man-rem
• Role of _at_RISK Methodology approved by the NRC
  requires doing Monte-Carlo Simulation and use
  mean values for the ranking of each segment
  
• A sensitivity study to compare Point Estimate and
  Monte-Carlo Generated Mean provides useful
  insights for the sensitivity of changes in the
  input parameters
• A function RiskMean() used saved many
  hours of manual work
  

13
Other Benefits

• Avoidance of potential personnel injury by
  reduction of
  
• Scaffold Construction
• Asbestos Removal
• Insulation Removal
• Weld Grinding and Buffing

14
Expert Panel

• Scope limited to changes in program new
  segments, modified segments, and segments whose
  risk ranking changed from Low to High or Medium
  to High.
• Risk ranking change due to changes in SRRA
  analysis of pipe segment failure frequency,
  changes in consequence assumptions, and/or
  changes in PTN PSA model.
• Expert Panel members will be provided with all of
  the risk-related and other information calculated
  or collected for the relevant pipe segments.
  
• Expert Panel will review, discuss, and eventually
  assign a final risk category to each pipe
  segment.