Initiating Event Analysis IAEA Training Course on Safety Assessment of NPPs to Assist Decision Making Workshop Information IAEA Workshop City, Country XX - XX Month, Year Lecturer Lesson IV 3_2.1 Lecturer Lesson IV 3_2.1

IAEA Training Course on Safety Assessment of NPPs Assist Decision Making 2 Context of Initiating Event Analysis Procedural tasks for the identification of sources of radioactive releases and accident initiator (IAEA Safety Series No. 50-P-4 Fig. 3)

IAEA Training Course on Safety Assessment of NPPs Assist Decision Making 3 OBJECTIVE A comprehensive list of all possible initiating events during plant operational modes analysed in the PSA (Power Operation, Shutdown, Refuelling, etc.). INITIATING EVENT An event that creates a disturbance in the plant that has the potential to lead to core damage,depending on the successful operation of required mitigating systems in the plant. Initiating Event Analysis

IAEA Training Course on Safety Assessment of NPPs Assist Decision Making 4 Selection of Initiating Events APPROACHES Engineering Evaluation (e.g. FMEA). Reference to Previous Lists (e.g. NUREG/CR-5750). Deductive Analyses (e.g. Master Logic Diagram). Operational Experience. CLASSIFICATION Internal Events: Plant hardware failures or faulty operations. External Events: Events (hazards internal or external to the plant) that create beyond design basis environmental conditions to plant systems: earthquakes, floods, fires, etc.

IAEA Training Course on Safety Assessment of NPPs Assist Decision Making 5 Assessment of Safety Functions and System Requirements PURPOSE Grouping of those initiating events that place the same demands on safety functions and system requirements. SYSTEM RELATIONSHIPS Functional relationships between front-line and support systems. Degrading impact of initiating events on mitigating system operability. SYSTEM SUCCESS CRITERIA Conservative (e.g. FSAR) and best estimate analyses support required minimum system performance to fulfil safety functions. Time windows available to perform required human actuations. Allowed plant system configurations.

IAEA Training Course on Safety Assessment of NPPs Assist Decision Making 6 Grouping of Initiating Events The same success criteria on front-line systems. The same contextual conditions (e.g. Challenges to the operator, automatic plant signals, etc.). The same degrading impact on mitigating systems. Mutually exclusive initiating event groups: Each initiating event in a single initiating event group.

IAEA Training Course on Safety Assessment of NPPs Assist Decision Making 7 Safety Functions and Corresponding Front Line Systems Table V - Safety Series No. 50-P-4

IAEA Training Course on Safety Assessment of NPPs Assist Decision Making 8 Examples of Front Line Systems for a PWR Table VI - Safety Series No. 50-P-4

IAEA Training Course on Safety Assessment of NPPs Assist Decision Making 9 Example of Front Line Systems and Support System Dependences for a PWR Plant Table VII - Safety Series No. 50-P-4

IAEA Training Course on Safety Assessment of NPPs Assist Decision Making 10 Example of Grouping of Initiating Events Table VIII - Safety Series No. 50-P-4

IAEA Training Course on Safety Assessment of NPPs Assist Decision Making 11 Example of Initiating Event Grouping for a Candu Nuclear Power Plant Table IX - Safety Series No. 50-P-4

IAEA Training Course on Safety Assessment of NPPs Assist Decision Making 12 Example of Initiating Event Grouping for a Candu Nuclear Power Plant Table IX (Cont.) - Safety Series No. 50-P-4

IAEA Training Course on Safety Assessment of NPPs Assist Decision Making 13 Example of Initiating Event Grouping for a Candu Nuclear Power Plant Table IX (Cont.) - Safety Series No. 50-P-4

IAEA Training Course on Safety Assessment of NPPs Assist Decision Making 14 IE Mutually Exclusive Criteria Initiator development must focus on “Muaually Exclusive” Sequences are Mutually Exclusive because: Freq. [ Sequence i ] = Freq. (Sequence i ) –If and only if the set of sequences are mutually exclusive. –Initiatiors are mutually exclusive. –Event trees end states represent mutually exclusive pathways. –Definitions of initiators. –Calculations of initiator frequency. Accident Sequence Quantification

IAEA Training Course on Safety Assessment of NPPs Assist Decision Making 15 Example of NPP with 2 Feedwater Trains Operating Experience

IAEA Training Course on Safety Assessment of NPPs Assist Decision Making 16 Typical Under - Estimation Mistakes Examples: Plant unique LOCA location or leakage size issue might lead to direct core melt (such LOCAs need to be treated separately). Total loss of instrument air.

IAEA Training Course on Safety Assessment of NPPs Assist Decision Making 17 Typical Ranges of Initiating Events