IAEA Training Course on Safety Assessment of NPPs to Assist Decision Making PSA Applications Methodologies and Approaches. Status in OECD Countries Workshop

IAEA Training Course on Safety Assessment of NPPs to Assist Decision Making

PSA Applications MethodologiesPSA Applications Methodologies and Approaches. Status in and Approaches. Status in

OECD Countries OECD Countries

Workshop InformationWorkshop InformationIAEA WorkshopIAEA Workshop City , Country

XX - XX Month, Year

LecturerLesson IV 3_11.1


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TopicsTopics

– PSA applications – what is it

– PSA applications-methods and approaches

– PSA applications at utilitities

– PSA applications at regulatory bodies

– Risk Informed Regulation

– Overview of regulatory applications in selected OECD countries


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PSA Applications: Complement to PSA Applications: Complement to deterministic regulationdeterministic regulation

– Application of PSA represents an extension and enhancement of traditional regulation rather than a separate and different technology

– Allows consideration of risk in a more coherent and complete manner treating multiple and common cause failures

– When implemented in a consistent and predictable manner, it promotes regulatory stability, efficiency and enhances safety


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PSA applications-an overviewPSA applications-an overview– PSA models contains a wealth of information on plant

operation and design

– This information (which complements deterministic insights), makes PSA an ideal basis for optimization processes

– PSA applications is the best way to make a full use of efforts invested in PSA development and review

– For applications, PSA has to be of high quality

– PSA applications use broadly similar methods as PSA study

– PSA applications offers an extension of deterministic rules, by supporting quantitative optimization


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PSA applicationsPSA applications– Various PSA applications considered since early eighties

– Earliest applications focused on Tech.Spec relaxation

– Other one-off applications, like design modifications attempted

– Regulatory applications less developed, focus on precursors

– Regulatory reviews of utility submittals

– Regulatory prioritization of issues, a non-systematic application

– Since mid nineties under the concept of “Risk informed regulation” PSA applications enter in the mainstream


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USNRC PSA Policy StatementUSNRC PSA Policy Statement– Use of PSA technology should be increased in all

regulatory matters to the extent supported by the state-of-the-art in PRA methods and data and in a manner that complements the NRC’s deterministic approach and supports the NRC’s traditional defense-in-depth philosophy

– PSA and associated analyses (e.g., sensitivity studies, uncertainty analyses, and importance measures) should be used in regulatory matters, where practical within the bounds of the state-of-the-art, to reduce unnecessary conservatism associated with current regulatory requirements,...


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USNRC PSA Policy Statement (Cont’d)USNRC PSA Policy Statement (Cont’d)

– PSA evaluations in support of regulatory decisions should be as realistic as practicable and appropriate supporting data should be available for public review

– The commission’s safety goal for nuclear power plants and subsidiary numerical objectives are to be used with appropriate consideration of uncertainties in making regulatory judgments for proposing and back-fitting new generic requirements


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PSA applications infrastructurePSA applications infrastructure

– PSAs of high quality and appropriate coverage

– Models (updated and accurate!!!) and full documentation shall be available, preferably on a fast computational platform (i.e. Risk Spectrum)

– Infrastructure to support multiple applications

– Training and guidance to enable understanding strengths and limitations of PSA and application methodologies

– Procedures to assure consistency and reproducibility of applications


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PSA Application MethodsPSA Application Methods

– Identification of application needs

– Adequacy of the PSA model and scope

– Use PSA model to assess impact

– Complement with traditional engineering analysis

– Conduct sensitivity and uncertainty analyses

– Define acceptability consistent with acceptability guidelines

– Define implementation and monitoring requirements


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Defining Application NeedsDefining Application Needs

– Define the type of application to be conducted

– Extent of the reliance of the decision on the PSA results:

• deterministic analysis supplemented with PSA insights

• PSA insights supplemented with deterministic analyses

– Required level of resolution/specificity of PSA results

• broad categories of safety significance

• sufficient characterization of PSA results to assess acceptability using risk acceptance criteria


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PSA Quality and ScopePSA Quality and Scope– Ensure that PSA is complete to appropriate level of detail

• Consideration of relevant initiating events, plant systems, and operator actions

• Analysis reflects operating experience, design features, etc. All calculations are documented

– PSA methodology and associated input• Assess adequacy of applicable assumptions and data used • Define needs for modeling improvements and additional data

assessments– Review and QA process

• Peer review• Certification• Standards


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PSA Analysis for the ApplicationPSA Analysis for the Application– PSA analysis results

• Baseline CDF/LERF of the plant and changed CDF/LERF of the plant

• PSA Importance Measures for ranking– Analysis of the results

• effect of the change on the dominant sequences• assessment of the change; significant contributors to

the change– Identification of key modeling assumptions that

impact the application• identify key assumptions needed to support the application


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Traditional Engineering AnalysisTraditional Engineering Analysis

– Defense-in-depth is maintained• A reasonable balance among prevention of core damage,

prevention of containment failure, and consequence mitigation is preserved

• Over-reliance on programmatic activities to compensate for weaknesses in plant design is avoided

• System redundancy, independence, and diversity are preserved commensurate with the expected frequency and consequences of challenges to the system (e.g., no risk outliers)

• Defenses against potential common-cause failures are preserved and the potential for introduction of new common-cause failure mechanisms is assessed


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Traditional Analysis (Cont’d)Traditional Analysis (Cont’d)– Defense-in-depth is maintained (cont.)

• Independence of barriers is not degraded

• Defenses against human errors are preserved

• The intent of the General Design Criteria are maintained

– Sufficient safety margins are maintained

• Codes and standards or alternatives approved for use

• Safety analysis acceptance criteria in the licensing basis (e.g., FSAR, supporting analyses) are met, or proposed revisions provide sufficient margin to account for analysis and data uncertainty


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Sensitivity and Uncertainty AnalysisSensitivity and Uncertainty Analysis

– PSA strength is in quantifying uncertainties associated with analysis

– Conduct parameter uncertainty analysis if new parameters are being introduced

– Conduct sensitivity analysis with respect to key modeling assumptions; qualitative analysis may be used assessing impact on the contributors

– Sensitivity analysis with respect to human errors and common-cause failures


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Implementation and Monitoring Implementation and Monitoring ConsiderationsConsiderations– Assure that the impact of any change continues to

reflect the actual reliability and availability of SSCs– Slower, phased approach to implementation when

uncertainty is high and programmatic changes are being made

– Monitoring program to track the performance of equipment

– Consideration of compensatory measures, as defined and adequate provisions to assure that compensatory measures are appropriately used


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Regulatory vs. utility applicationsRegulatory vs. utility applications– Regulatory PSA applications

• Those which are directly supporting regulatory functions• Analysis and evaluation preformed within regulatory

organization• Support for the regulatory decision making• Models and tools, incl. methods to be available within

organization– Utility PSA application

• Those applications which are of specific interest to utilities• Applications focused on streamlining or easing operations,

cost optimization• Analysis by utility, or consulting support• Methods and tools often similar for both applications


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Regulatory vs. utility applicationsRegulatory vs. utility applications

REGULATORY APPL.REGULATORY APPL.RI rules and requirementsEffectiveness of rules & requirementsTraining of regulatory staff Risk follow-up Risk-based safety indicatorsImpact of ageingShutdown safety guidance Regulatory use of risk monitorsOperational events analysisPeriodic safety review Assessment of deviations Ranking of safety issues Risk informed inspectionResponse to emergency conditions Prioritisation of regulatory research

UTILITY APPLICATIONSDevelopment of EOPsTech. Spec optimisationChanges & exemptions to Tech. Spec Justification for continued operationModification of licensing basisTraining for Operators & Tech. StaffsPlant changes and backfitsControl Room HabitabilityDirecting ISI & ISTGraded QA QCCorrective Action ProgramMaintenance ruleSurveillance program planningAppendix R and P


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PSA Application examplesPSA Application examples(by subject area)(by subject area)

– PSA in rules and regulations

– PSA application in operation

• ATWS rule• LOOP rule• Maintenance Rule• Risk-informing 10 CFR Part 50

• Assessment of operational events• Configuration control

(equipment outages)• Maintenance planning and

prioritization• Operator training• Outage planning


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PSA Application examples PSA Application examples (by subject area)(by subject area)

– Assessment of changes to licensing basis

– Regulatory oversight

• Technical Specification changes (AOTs, STIs, transition requirements, etc.)

• Changes to ISI• Changes to IST• Changes to QA requirements• Risk-informed inspection of

NPPs• Generic issue resolution• Periodic safety review• Regulatory response to

operational events• Emergency planning


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PSA Applications: Summary ObservationsPSA Applications: Summary Observations– Enhancement of safety decision making by complementing

the traditional deterministic approach with a systematic probabilistic approach that is consistent and predictable

– Efficient use of regulatory resources and reduction in unnecessary burden on licensees

– Continued focus on aspects that contribute to risks of NPP operation

– Requires maintaining updated risk analyses models, tools, and understanding of risk implications of plant design features and processes incorporating operating experiences.


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RIR internationallyRIR internationally– PSA being required as a part of safety case universally– The coverage and the quality varies considerably– Uses of PSA to support decision making in many organizations

but not systematic– Some countries (i.e. UK) established probabilistic criteria– Risk Informed Regulation Implementation Plan by USNRC

specified the issues and established the principles– A set of regulatory guides established (i.e RG 1.174) defining

rules for applications– RGs to assure observance of risk informed and engineering

insights– In several European countries RIRs but not fully formalized


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RIR internationallyRIR internationally– USNRC RIRIP establishes plans and provide guidance

• Applying criteria to select regulatory requirements and practices which could become risk informed

• Risk-informing selected requirements and practices• Developing the necessary data, methods, guidance, and

training• Communicating with internal and external stakeholders

– Risk Informed Inspection and USNRC’s new Oversight process• Highly visible risk informed activity• Aim to define guidance and provide plant-specific risk

information for specific inspection to focus on risk-significant aspects


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Elements of Risk-Informed PSA ApplicationsElements of Risk-Informed PSA ApplicationsIntegrated Decision Framework Integrated Decision Framework (USNRC RG 1.174)(USNRC RG 1.174)

TraditionalAnalysis PRA

DefineChange

PerformEngineeringAnalysis

DefineImplementation/MonitoringProgram

SubmitProposedChange

Element 1 Element 2 Element 3 Element 4


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Guidance Documents for PSA ApplicationsGuidance Documents for PSA Applications– CSN Guide 1.4– NRC Reg. Guides 1.174 An Approach for Using PSA in

Risk-Informed Decisions on Plant-specific Changes to the Licensing Basis

– NRC Reg. Guide 1.175 Risk-Informed Decision Making : In-Service Testing

– NRC Reg. Guide 1.176 Risk-Informed Decision Making : Graded Quality Assurance

– NRC Reg. Guide 1.177 Risk-Informed Decision Making: Technical Specifications

– NRC Reg. Guides 1.178 Risk-Informed Decision Making: In-service Inspection


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Acceptance Guidelines for CDFAcceptance Guidelines for CDF

10-5

10-6

10-5 10-4

Acceptance Guidelines* for Core Damage Frequency (CDF)

REGION II

REGION III

REGION I

Region I - No Changes Allowed Region II - Small Changes - Track Cumulative Impacts Region III - Very Small Changes - More Flexibility with respect to baseline CDF - Track Cumulative Impacts Region IV - Not risk significant

10-7

REGION IV {Not significant to public health and safety.}


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Acceptance Guidelines for LERFAcceptance Guidelines for LERF

10-6

10-7

10-6 10-5

Acceptance Guidelines for Large Early Release Frequency(LERF)

REGION II

REGION III

REGION I

Region I - No Changes Allowed Region II - Small Changes - Track Cumulative Impacts Region III - Very Small Changes - More Flexible with Respect to Baseline LERF - Track Cumulative Impacts Region IV - Not risk significant

10-8

REGION IV {Not significant to public health and safety.}


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RIR internationallyRIR internationally– Finnish regulatory body STUK one of European leaders

• Requirements for a living PSA to be used at utility and regulatory body

Utility uses to support decision making on operational safety issues

Regulatory use include risk follow up, event analysis, resolution of safety issues

• Strict requirements on coverage and quality of PSA• PSA recognised as effective safety management tool, but

limitations to be understood and observed in applications– STUK to use probabilistic and deterministic reviews in

parallel


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RIR internationallyRIR internationally– In Belgium, PSAs performed as a part of Periodic Safety

Review– Regulatory review integral part of PSA performance

(inspectors assigned to PSA teams)– Detailed knowledge on strengths and weaknesses of each

PSA– Use of PSA for design evaluation in relation with PSRs– Discussion with utilities on applications and associated

criteria– Regulatory applications strong in PSAEA, risk follow up– Expected initiation of RII soon


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RIR internationallyRIR internationally

– SKI is preparing long term plan for RIR to be announced next year

– PSAs reviewed in detail before possible uses– NII’s use of PSA to support a variety of decisions– German TUVs increasingly use PSAs for decision support– In France, event analysis and risk follow up systematic,

some use for regulatory decision support– RIR planed in every regulatory organization. Actual

application depends on national conditions


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RIR internationally-SPAINRIR internationally-SPAIN– CSN has a long term policy of active PSA requirements– Very extensive requirements on PSAs (level 1 internal and

external events, Level 2, other modes)– Reviews of PSAs by CSN staff and consultants– PSA group of 7 people each responsible for one plant (group

also handles other issues like human factors)– PSA application in a non coordinated manner since nineties

• PSA based event analysis • Tech Spec evaluation• Ad hoc decision support

– Since 2000 a comprehensive project to enhance PSA applications