Risk Informed InService InspectionResults of Studies on Nuclear Power Plants and Heavy Water Plants

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Risk Informed In-Service Inspection-Results of
Studies on Nuclear Power Plants and Heavy Water
Plants

• Santhosh
• Reactor Safety Division, BARC
• Mumbai, India.

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Overview of Presentation

• Introduction
• RI-ISI Methodology
• Case Study 1 Heavy Water Plants
• Case Study 2 Indian PHWRs
• Conclusions

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Risk Analysis in PublicPolicy Decision

• Most of the public policy decisions that are made
  involve the allocation of resources.
• Available resources, whether natural or human,
  are finite. Therefore, resource allocation
  involves certain trade-offs.
• The outcome of many ,if not most, public policy
  decisions is uncertain.

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Risk Analysis in PublicPolicy Decision

• The best way to deal with such uncertainties is
  the risk analysis.
• Determining or estimating the probability of
  various outcomes gives a more complete picture
  than focusing only on the most likely (or the
  desired) outcome.
• The cost of resources allocated can be compared
  to the benefits to inform decisions.

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Risk Assessment in Industry
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RI-ISI Methodology
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RISK INFORMED IN-SERVICE INSPECTION

• CONSEQUENCE
• HEALTH
• DAMAGE
• TOXIC
• ECONOMIC

• LIKELIHOOD FREQUENCY
• SERVICE DATA ANALYSIS
• STRUCTURAL RELIABILITY ANALYSIS
• REMAINING LIFE MODELS
• EXPERT JUDGEMENT

• RISK MATRIX
• DEFINES THE INSPECTION CATEGORY,
• METHOD, FREQUENCY AND SCOPE OF INSPECTION

LIKELIHOOD FREQUENCY
RISK
INSPECTION
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Advantages of RI-ISI

• Decision making based on the risk criteria and
  deterministic information
• Better focus on allocating resources to high
  safety significant components.
• In- Service Inspection based on failure modes of
  components and associated risk

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Guides for RI-ISI

• NUREG 1661- RI-ISI for Nuclear Power Plants by US
  Nuclear Regulatory Commission
• API 581- Base Resource Document Mainly used by
  Chemical, Oil Petrochemical Industries
• ASME RBI Guidelines Vol. 3- for Fossil Fuel
  Plants
• EPRI- Risk Based Maintenance Guidelines
• Risk based Inspection MAintenance Procedures
  (RIMAP) Workbook

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Case Study 1RI-ISI of Heavy Water Plant
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Scope and Objective

• Apply RI-ISI for components connecting the
  CT1-HT1 towers, and compare with the existing ISI
  plan
• To demonstrate the reduction in inspection
  achieved through adopting RI-ISI without
  compromising the plant safety
• To ensure that the inspection will be focused on
  more safety significant components

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Schematic Diagram of HWP
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Steps Involved

• Scope boundary of the study
• Data collection on piping and equipment
• Finding the inventory in each component (mass of
  H2S in liquid gas)
• Evaluate the failure probability consequence
• Establish the categories for failure probability,
  consequence risk
• Conduct a risk impact analysis

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Piping Failure Analysis Structural Reliability

• Material properties for failure analysis
• Possible damage mechanisms
• Past inspection history for degradation rate
• ASME B31G-Failure Pressure Model for estimating
  the remaining strength of a corroded pipeline
• Limit state function is formulated for estimating
  the pipeline failure probability
• COMREL tool for computing the failure probability

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Equipment Failure Analysis Bayesian Updation
Technique

• Plant specific failure incidences were collected
  for various equipment from SRUORs
• Analyzed the failure incidences for identifying
  the exact failure mode
• Updated the generic information with the plant
  specific data using Bayesian technique
• Developed software for data base management and
  reliability analysis

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DAMAGE

• Chemical
• Quantity
• State
• Economic loss

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Consequence Category Specified in API 581
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Damage - Piping
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Damage - Equipment
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Health - Piping
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Health - Equipment
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Consequence Matrix
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Consequence Category- Equipment
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RISK MATRIX
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Risk Category - Equipment
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Risk Impact Analysis
CURRENT INSERVICE INSPECTION (ISI) PROGRAMME HAS
THREE INSPECTION CLASSES, VIZ. A, B C A
CARRYING H2S AND NON-ISOLATABLE B CARRYING H2S
AND ISOLATABLE C UTILITY FLUIDS Consequence is
considered in an implicit manner, but failure
potential are not considered
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Risk Impact Analysis
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Risk Impact Analysis-Results
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Case Study 2RI-ISI of Nuclear Power Plants
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Current PHWR ISI Programme

• Currently In-Service Inspection is based on CAN
  standards - CAN/CSA-N 785.4-94
• Inspection categories are defined as a function
  of fatigue usage factor, stress ratio and size of
  failure (RE)
• Fatigue usage factor is determined as per the
  rules given in ASME Boiler and Pressure Vessel
  Code

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INSPECTION CATEGORY FOR MEDIUM FAILURE SIZE
INSPECTION CATEGORY FOR LARGE FAILURE SIZE
Scope, frequency methods of inspection in
categories A, B, C1 C2 are decided by expert
group formed by utility
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RI-ISI for NPPs International Scenario
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Importance Measure in RI-ISI

• ASME-WOG SUGGESTED RRW RAW IN ISI PRIORITISION

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ASME-WOG on Indian PHWR

• Applied ASME-WOG ranking scheme on process and
  safety systems of Kaiga Generating Station using
  KGS PSA results
• RAW and RRW have been evaluated for initiating
  events considering the frequencies as 1/year and
  0/year respectively

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SYSTEM PRIORITISATION - ASME/WOG
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Application on Indian PHWRs

• RI-ISI was applied on piping segments connecting
  the Primary Heat Transport System Shutdown
  Cooling System
• Leak and rupture frequencies for piping segments
  were evaluated using Thomas model
• Possible degradation mechanisms were considered

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Application on Indian PHWRs

• Consequence is quantified using Conditional Core
  Damage Probability
• EPRI Risk matrix was applied for risk
  categorization
• Risk impact analysis was carried out comparing
  with the current ISI programme

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PHT of Indian PHWRs
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Component Failure Frequency

• STATISTICAL METHODS PAST EXPERIENCE
• DEGRADATION MECHANSIMS
• THOMAS MODEL WELDS, DIAMETER, LENGTH,THICKNESS,
  DESIGN, AGE, ETC
• MECHANISTIC MODELS - BY UNDERTSANDING THE
  PREVALENT PHENOMENA IN THE COMPONENT

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Classification of Degradation Mechanisms
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Possible Degradation Mechanisms in Indian PHWRs
Atmospheric Corrosion
Boiler
Erosion Corrosion
SCC, CF
Turbine
Generator
Calandria
Condenser
SCC, Pitting, etc
Hydriding,
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Risk Matrix
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ISI CATEGORIES FOR COMPONENTS IN PHT
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ISI CATEGORIES FOR COMPONENTS IN SDCS
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Code for Inspection Methods

• Visual examination 1
• Dimensional examination 2
• Surface examination 3A (LPT, MPT, ET)
• Volumetric examination 3B (UT,RT,ET)
• Integrative examination - 4

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RESULTS
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Suggested Inspection Method
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Conclusions

• RI-ISI was applied to HWPs and Indian PHWRs and
  demonstrated its advantages over the current ISI
  programme
• Reduction in quantum of inspection which in turn
  improves the plant down time and cost
• RI-ISI provides a basis of quantification for the
  inspection programme
• RI-ISI focuses inspections on most significant
  areas, where
• Degradation mechanisms are present
• Consequence of potential failure are greater
• Overall plant safety improvement is possible

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Thank you for your attention