Risk-Informed InService
Inspection (RI-ISI)
Using @RISK
Ching Guey
RI-ISI at Turkey Point
• Turkey Point’s 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.
Objectives
•
•
More Effective and Efficient Use of @Risk to
Streamlize the Analysis Work
Prioritize Turkey Point’s 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
Risk-Informed ISI
Process Overview
Expert Panel
Categorization
Consequence
Evaluation
Scope and
Segment
Definition
RiskEvaluation
Structural Element
Failure Probability
Assessment
@RISK
Element/
NDE
Selection
Feedback
Loop
Implement
Program
Turkey Point PRA LOCA Events and
Nominal Pipe Sizes
• Small-Small LOCA
• Small LOCA
3/8”< X  2”
2” < X  6”
• Medium LOCA
6” < X  13.5”
• Large LOCA
13.5” < X
• 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.
Failure Probability
Assessment Process
INDUSTRY EXPERIENCE
IDENTIFICATION OF
POTENTIAL FAILURE MODES
AND CAUSES
ENGINEERING
TEAM
PLANT INFORMATION
(LAYOUT, MATERIALS,
OPERATING CONDITIONS,
PLANT OPERATING
EXPERIENCE)
SRRA
TOOL
Engineering Team
ESTIMATED
LEAK AND BREAK
PROBABILITIES
–ISI/NDE Engineering
–Materials Engineering
–Design Stress Engineering
(Engineering Mechanics)
-Plant System Engineer
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
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
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.
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 @RISK .
• Assigned a preliminary risk category to each
segment based on the RRW.
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 riskrelated 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.
Results
• Weld Inspection Reduction 84%
• Radiation Dose Reduction
20 man-rem
• Role of @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
Other Benefits
• Avoidance of potential personnel injury by reduction
of:
- Scaffold Construction
- Asbestos Removal
- Insulation Removal
- Weld Grinding and Buffing
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.