Risk-Informed In-Service

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 ISIProcess Overview

Scope andSegmentDefinition

ConsequenceEvaluation

Expert PanelCategorization

Risk-Evaluation

Element/NDESelection

ImplementProgram

Structural ElementFailure ProbabilityAssessment

FeedbackLoop@RISK

Turkey Point PRA LOCA Events and Nominal Pipe Sizes

• Small-Small LOCA 3/8”< X 2”

• Small LOCA 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 ProbabilityAssessment Process

INDUSTRY EXPERIENCE

IDENTIFICATION OFPOTENTIAL FAILURE MODES

AND CAUSES

PLANT INFORMATION(LAYOUT, MATERIALS,

OPERATING CONDITIONS,PLANT OPERATING

EXPERIENCE)

ENGINEERINGTEAM

ESTIMATEDLEAK AND BREAK

PROBABILITIES

SRRATOOL

Engineering Team–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 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.

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.