The objective of this Safety Guide is to provide recommendations for meeting the requirements of GSR Part 4 (Rev. 1) in the development and application of Level 1 Probabilistic Safety Assessments (PSAs) for nuclear power plants (NPPs). The recommendations provided in this publication promoting technical consistency among Level 1 PSA studies, in order to provide reliable support for applications of PSAs and risk informed decision making, particularly to support the design of NPPs and decision making during plant commissioning and operation. The revised Safety Guide's scope encompasses the main methodological aspects of PSA and in particular has been updated to reflect developments in specific areas, such as passive systems reliability, computer based systems reliability, combinations of hazards, human reliability analysis and to expand the scope of PSA to include site level risk considerations such as multi-unit and spent fuel pool PSA.

• 1. INTRODUCTION
  • Background
  • Objective
  • Scope
  • Structure
• 2. GENERAL CONSIDERATIONS RELATING TO THE PERFORMANCE AND USE OF PSA
  • Scope of the PSA
  • Validation and review of the PSA
  • Living PSA
  • Probabilistic safety goals or criteria
  • Use of PSA in decision making
• 3. PROJECT MANAGEMENT AND ORGANIZATION FOR PSA
  • Definition of objectives and scope of the PSA project
  • Project management for PSA
  • Selection of methods and establishment of procedures
  • Team selection and organization
  • Establishing a quality assurance programme for PSA
  • General aspects of PSA documentation
    • Objectives and content of PSA documentation
    • Organization of documentation
• 4. FAMILIARIZATION WITH THE PLANT AND COLLECTION OF INFORMATION
• 5. LEVEL 1 PSA FOR INTERNAL INITIATING EVENTS FOR POWER OPERATION
  • General aspects of Level 1 PSA methodology
  • Initiating event analysis
    • Identification of initiating events
    • Transients
    • Loss of coolant accidents
    • Grouping of initiating events
  • Accident sequence analysis
    • Core damage
    • Safety functions and success criteria
    • Analysis to support the specification of success criteria
    • Modelling of accident sequences
    • End states of accident sequences and plant damage states
  • Systems analysis
    • Fault tree analysis
    • System information
  • Analysis of dependent failures
  • Analysis of common cause failures
  • Human reliability analysis
    • Identification and definition of human failure events
    • Qualitative assessment of human failure events
    • Quantitative assessment of human failure events
    • Treatment of dependencies between human failure events
    • Integration of human failure events in the PSA model
  • Other modelling issues
    • Passive systems
    • Software based systems
  • Data for a Level 1 PSA
    • Frequencies of initiating events
    • Component failure probabilities
    • Component outage frequencies and durations
  • Quantification of the analysis
  • Importance analysis, sensitivity studies and uncertainty analysis
    • Importance analysis
    • Types of uncertainty
    • Sensitivity studies
    • Uncertainty analysis
• 6. GENERAL METHODOLOGY FOR LEVEL 1 PSA FOR INTERNAL HAZARDS AND EXTERNAL HAZARDS
  • Introduction
  • Analysis process
  • Collection of initial information
  • Identification of hazards
  • Screening of single and combined hazards
• 7. SPECIFIC ASPECTS OF LEVEL 1 PSA FOR INTERNAL HAZARDS
  • Introduction
  • Bounding assessment for Level 1 PSA for internal hazards
  • Analysis of internal fire
    • General
    • Data collection and assessment of potential for internal fire
    • Analysis of fire compartments
    • Selection of equipment for Level 1 PSA for internal fire
    • Screening by impact
    • Screening by frequency
    • Detailed analysis of fire
    • Quantification of risk of internal fire
    • Documentation for Level 1 PSA for internal fire
  • Analysis of internal flooding
    • General
    • Data collection and assessment of potential for internal flooding
    • Identification of internal flooding scenarios
    • Screening by impact
    • Screening by frequency
    • Detailed analysis of flooding
    • Quantification of risk of internal flooding
    • Documentation for Level 1 PSA for internal flooding
  • Analysis of other internal hazards
    • Analysis of the collapse of structures and heavy load drops
    • Analysis of turbine missiles
    • Analysis of internal explosion
    • Analysis of other credible internal hazards
• 8. SPECIFIC ASPECTS OF LEVEL 1 PSA FOR EXTERNAL HAZARDS
  • Introduction
  • Bounding assessment for Level 1 PSA for external hazards
    • General aspects
    • Natural hazards
    • Human induced hazards
  • Parameterization of external hazards
    • General aspects
    • Natural hazards
    • Human induced hazards
  • Detailed analysis of external hazards
  • Frequency assessment for external hazards
    • General aspects
    • Natural hazards
    • Human induced hazards
  • Fragility analysis for structures, systems and components
    • General aspects
    • Natural hazards
    • Human induced hazards
  • Integration of external hazards in the Level 1 PSA model
    • General aspects
    • Natural hazards
    • Human induced hazards
  • Documentation and presentation of results
    • General aspects
    • Natural hazards
    • Human induced hazards
• 9. LEVEL 1 PSA FOR SHUTDOWN STATES
  • General aspects of Level 1 PSA for shutdown states
  • Specification of outage types and plant operational states
  • Initiating events analysis
  • Accident sequence analysis
    • Safety functions and success criteria
    • Analysis to support the specification of success criteria
    • Modelling of accident sequences
    • Accident sequence end states and plant damage states
  • Systems analysis
  • Analysis of dependent failures
  • Human reliability analysis
    • Type A human failure events — pre-initiator human failure events
    • Type B human failure events — human failure events that might cause an initiating event
    • Type C human failure events — post-initiator human failure events
  • Data assessment
  • Quantification of accident sequences
  • Uncertainty analysis, importance analysis and sensitivity studies
  • Documentation and presentation of results
• 10. SPECIFIC ASPECTS OF LEVEL 1 PSA FOR THE SPENT FUEL POOL
  • Undesired end states
  • Plant operational states
  • Initiating events
  • Accident sequence analysis
  • Human reliability analysis
  • Quantification of the analysis
  • Interpretation of the results
• 11. LEVEL 1 MULTI-UNIT PSA
  • Scope of multi-unit PSA
  • Risk metrics for multi-unit PSA
  • Plant operational states
  • Initiating events analysis
  • Systems analysis
  • Human reliability analysis
  • Common cause failure and hazard fragility correlations
  • Quantification of the multi-unit PSA risk profile
• 12. USE AND APPLICATIONS OF LEVEL 1 PSA
  • General aspects of PSA applications
  • Scope of Level 1 PSA applications
  • Risk informed approach
  • Use of PSA for design evaluation
    • Use of PSA to support decision making during the design of a nuclear power plant
    • Use of PSA in the licensing process
    • Comparison of design options
    • Use of PSA in the periodic safety review
    • Optimization of protection against internal hazards and external hazards
  • Use of PSA for optimization of inspections, testing and maintenance
    • Risk informed technical specifications
    • Determination and evaluation of surveillance test intervals
    • Risk informed in-service testing
    • Risk informed pre-service and in-service inspection
  • Risk informed classification of structures, systems and components
  • Monitoring and managing risk configuration
    • PSA model and software for a risk monitor
    • Limitations of risk monitors
  • Risk based safety performance indicators
  • PSA based event analysis (precursor analysis)
  • Risk informed regulations
  • Risk informed oversight and enforcement
  • Use of PSA insights to develop or enhance emergency operating procedures
  • Use of PSA insights for risk informed training of plant personnel
    • Improvement of the training programme for operating personnel
    • Improvement of the training programme for maintenance personnel
  • Use of PSA to address emerging issues
• REFERENCES
• Annex I EXAMPLE OF A GENERIC LIST OF EXTERNAL HAZARDS
• Annex II EXAMPLES OF FIRE EVENT TREES AND SEISMIC EVENT TREES
• Annex III SUPPORTING INFORMATION ON PSA FOR SHUTDOWN STATES
• CONTRIBUTORS TO DRAFTING AND REVIEW