The accident at the Fukushima Daiichi Nuclear Power Plant underlined the need to assess the nuclear safety of multi-unit sites considering the accident sequences involving more than one reactor units on site. The objective of this Safety Report is to provide a methodology for the development of a Multi-unit Probabilistic Safety Assessment (MUPSA). It provides practical examples and an overview of the actual state of practice in this area. The publication provides a detailed description of Level 1 MUPSA methodology, the principles of development of Level 2 MUPSA models and the path forward for multi-unit consequence analysis (Level 3 MUPSA). In addition, it summarizes the experience available in Member States in the area of MUPSA. The scope of this Safety Report includes consideration of various hazards and plant operational states normally considered in PSA development in the multi-unit context.

• 1. INTRODUCTION
  • 1.1. Background
  • 1.2. Objective
  • 1.3. Scope
  • 1.4. Structure
• 2. OVERVIEW OF MUPSA TECHNICAL BASIS
  • 2.1. Limitations and challenges
  • 2.2. Overview of available experience
    • 2.2.1. Previous IAEA experience
    • 2.2.2. Previous international experience
    • 2.2.3. Experience with scoping approaches
  • 2.3. Risk metrics
    • 2.3.1. Multi-unit Level 1 risk metrics
    • 2.3.2. Multi-unit Level 2 risk metrics
    • 2.3.3. Multi-unit Level 3 risk metrics
  • 2.4. Iaea/nsni project on mupsa
    • 2.4.1. Phase I. MUPSA methodology
    • 2.4.2. Phase II. MUPSA case study
• 3. MUPSA METHODOLOGY: PRINCIPLES AND ASSUMPTIONS
• 4. GENERAL APPROACH FOR LEVEL 1 MUPSA
  • 4.1. Level 1 mupsa scope and risk metrics selection
    • 4.1.1. Scope selection
    • 4.1.2. Risk metrics selection
  • 4.2. Review and refinement of level 1 supsa model
    • 4.2.1. Model simplification
    • 4.2.2. Model refinement
    • 4.2.3. Development of models for all units
  • 4.3. Ie analysis for multi-unit context
    • 4.3.1. Identification of various types of MUIEs
    • 4.3.2. Review of operating experience
    • 4.3.3. Grouping of MUIEs
    • 4.3.4. Frequency assessment of MUIEs
  • 4.4. Level 1 mupsa logic model development
    • 4.4.1. Screening of MU combinations
    • 4.4.2. Accident sequence analysis
    • 4.4.3. System analysis
    • 4.4.4. HRA
    • 4.4.5. Data, CCFs and fragility analysis
  • 4.5. Level 1 mupsa model integration and quantification
    • 4.5.1. Model integration roadmap
    • 4.5.2. Model quantification
  • 4.6. Analysis and interpretation of the level 1 mupsa results
    • 4.6.1. Review of results
    • 4.6.2. Importance analysis
    • 4.6.3. Sensitivity analysis
    • 4.6.4. Uncertainty analysis
    • 4.6.5. Interpretation of results
  • 4.7. Documentation of the results
• 5. LEVEL 2 AND LEVEL 3 MUPSA
  • 5.1. Level 2 mupsa scope and risk metrics selection
    • 5.1.1. Scope selection
    • 5.1.2. Risk metrics selection
  • 5.2. Review and refinement of level 2 supsa model
  • 5.3. Ie analysis for mu impacts
  • 5.4. Level 2 mupsa logic model development
    • 5.4.1. Level 1–Level 2 interface
    • 5.4.2. Accident progression analysis
    • 5.4.3. Source term analysis
  • 5.5. Level 2 mupsa model integration and quantification
  • 5.6. Documentation of level 2 mupsa results
  • 5.7. Level 3 mupsa
• 6. PATH FORWARD FOR MUPSA
  • 6.1. Complexity and size of mupsa model
  • 6.2. Small modular and advanced reactors
  • 6.3. CCF analysis
  • 6.4. MU HRA issues
  • 6.5. Level 2 MUPSA factors
  • 6.6. Multiple pos
• Appendix ILEVEL OF CHANGES NECESSARY FOR TYPICAL PSA TASKS IN MU CONTEXT
• Appendix IIMUPSA CASE STUDY
• Appendix III ACCIDENT SEQUENCE MODELLING APPROACHES
• REFERENCES
• ANNEXES: COUNTRY REPORTS
• Annex I CANADA/COG — APPROACH TO MUPSA AT CANADIAN NPPs
• Annex II FRANCE: IRSN — APPROACH TO MUPSA
• Annex IIIHUNGARY/NUBIKI — DEVELOPING A SITE RISK MODEL FOR THE PAKS NPP
• Annex IV REPUBLIC OF KOREA — EXPERIENCE WITH MUPSA
• Annex V UNITED KINGDOM — ASSESSING AND REGULATING SAFETY ON MU FACILITY SITES
• Annex VI UNITED STATES OF AMERICA — APPROACH TO MUPSA
• ABBREVIATIONS
• CONTRIBUTORS TO DRAFTING AND REVIEW