overview of candu ri-isi - ndt.net · •cog wg includes bruce power, nb power, hydro quebec, opg,...
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Overview of CANDU
RI-ISI
Prepared By: Kasia IzdebskaJune 15, 2010
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Outline
• Background
• Risk-Informed In-Service Inspection (RI-ISI) Programs
• Inspection Requirements for CANDU
• CNSC RI-ISI Guideline
• RI-ISI Pilot Study
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Background
• Traditional in-service inspection programs are based on
deterministic rules
• Inspection programs are based on defense-in-depth concept to
prevent accidents by detecting degradation
• Deterministic inspection programs are mostly based on ASME BPVC XI
• Augmented inspection programs have been developed for
specific degradation mechanisms such as erosion, corrosion, and inter-granular stress corrosion
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Background (continued)
• Initiation of risk-based approach and incentives to reduce dose
exposures and costs is moving the industry toward risk-informed
inspections
• There have been widespread adaptations of RI-ISI concept by
the operators and regulatory bodies globally
• The CNSC is interested in seeing development of risk-informed
methodology to provide a consistent framework covering CSA
N285.4 inspection programs as well as in-service inspection
programs
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RI-ISI Programs
• RI-ISI methodologies have been developed in several countries,
however Electric Power Research Institute (EPRI), Westinghouse
Owners Group (WOG), and ASME methodologies are widely
applied methods
• Based on risk insights, operating experience, and engineering
judgment
• Involves establishing scope, carrying out engineering analyses,
identifying execution and monitoring program, and providing
feedback mechanism
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RI-ISI Programs (continued)
• Despite different approaches to RI-ISI, they are based on
common principles:
• Assessment of potential of failure
• Assessment of consequence of failure
• Risk Ranking
• Selection of inspection locations
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Potential of Failure
• Assessment of potential of failure is either qualitative or
quantitative:
• Qualitative: degradation mechanism evaluation, operational
characteristics, dynamic loads and flaw distribution
• Quantitative: structural reliability models
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Consequence of Failure
• Consequence of failure is based on Core Damage Frequency
(CDF) and Large Early Release Frequency (LERF)
• Evaluates impacts of events originating from pressure boundary failure on overall plant risk
• Impact is either direct, indirect or combination
• Evaluation done using Probabilistic Safety Assessment (PSA)
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Risk Ranking
• Performed using risk of pipe segment failure evaluated on the
basis of expected failure potential and consequence
• Potential of Failure level can be High, Medium, or Low
• Consequence of Failure level can be High, Medium, Low or
None
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Risk Ranking
• Final ranking of piping segments may be conducted by expert panel
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Inspection Location Selection
• Percentage of elements selected for inspection depends on the
risk category
• Focus is on higher risk areas (e.g. 25% for categories 1,2,3, 10%
for categories 4,5)
• Selection is based on but not limited to, plant history, past inspection results, active degradation mechanism, accessibility,
radiation exposure, and inspection distribution among systems
and components
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Inspection Location Selection
• Elements not inspected under existing programs could be
included in scope
• May result in scope with same risk but different level of radiation
exposure
• In absence of degradation mechanisms, may focus on high stress and/or high fatigue locations
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Change-in-Risk Evaluation
• Performed prior to implementation of program
• Identifies whether changes from existing ISI program to a RI-ISI
program are in compliance with acceptance criteria by the
regulator as determined by CDF and/or LERF
• Supplementary locations added if original selection produces non-compliant impacts on risk
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Implementation
• Significant reduction in inspection scope achieved by
implementing RI-ISI approach on ASME XI programs
• Risk levels did not change or increased within allowable limits
• Inspection focused on higher risk areas
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Inspection Requirements for
CANDU
• Canadian utilities follow CSA N285.4 and CSA N285.5
• In CSA N285.4, inspection requirements (categories) are
determined based on failure size, fatigue usage factor, and
stress intensity
• CSA N285.4 provides an implicit risk-related rationale for reducing the inspection scope based both on low likelihood of
failure and low consequence
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Inspection Requirements for
CANDU (continued)
• Inspection scope minimized by reducing inspection on areas
with low failure potential and consequence
• Identical pipe runs are grouped as “family”
• Reduction factors are applied on number of identical pipe runs
in each family
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Inspection Requirements for
CANDU (continued)
• Single weld with highest fatigue usage factor and stress intensity
is selected from each pipe run
• The above sampling is different from RI-ISI methodologies where
a percentage of welds in each category is selected
• No requirement to use PSA analysis to justify assumptions
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CNSC RI-ISI Guideline
• CNSC prepared “Guidelines for Risk-Informed In-Service
Inspection for Piping”
• Provides instructions and acceptance criteria for licensees to
develop a RI-ISI program
• Provides high level expectations for integrating risk insights and traditional analysis to establish alternate piping inspection
programs
• Enhances regulatory stability in the review, approval, and implementation of licensees RI-ISI programs
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CNSC RI-ISI Guideline (continued)
• Not intended to preclude use of other validated methodologies
for the incorporation of risk insights, plant experience and
knowledge of RI-ISI program
• Licensees can suggest use of other methods with exception that
acceptable levels of quality and safety are maintained
• Does not establish specific licensing requirements and its use by
licensees is voluntary
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CNSC RI-ISI Guideline (continued)
• Full scope and partial scope applications are both acceptable
for RI-ISI programs for piping
• Partial scope applies when an alternative program is already in
place (e.g. CSA N285.4 program)
• Piping exempt from CSA N285.4 may not be exempt from RI-ISI
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CANDU Utilities
• In response to CNSC RI-ISI Guideline, utilities have established a
piping RI-ISI Working Group (WG) in COG
• COG WG includes Bruce Power, NB Power, Hydro Quebec,
OPG, and AECL
• Objectives of WG are to:
• Establish common Canadian utility position to apply RI-ISI to CANDU
• Develop common approach for CANDU to meet ongoing
challenges that would facilitate introduction of risk based solutions
in a manner compatible with CSA N285.4
• Provide forum for discussion of common RI-ISI issues, sharing of
knowledge, experience, and lessons learned among members
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RI-ISI Pilot Study
• A pilot study is being performed to:
• Develop a CANDU best fitted methodology as expected by CNSC
• Evaluate risk levels between current inspection and RI-ISI programs
• Darlington Unit 2 selected
• Scope includes:
• Main and Auxiliary PHT, SDC, and ECI systems
• Two non-nuclear safety related systems (Boiler Feed, Main Steam -
Boiler Feed currently inspected under FAC/MIC program)
• EPRI/Structural Integrity Associates Inc. (SI) selected through
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RI-ISI Pilot Study (continued)
• As part of this project, the following could also be explored:
• Reduction in risk levels within the current scope by relocating
inspection locations
• Significant reduction in risk levels by adding small number of
inspection points
• The RI-ISI application is in CSA N285B Technical Committee
agenda (future) and COG project could be used as seed
document to develop a new RI-ISI CSA standard
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