Initiating Event AnalysisInitiating Event Analysis

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

Workshop InformationWorkshop InformationIAEA WorkshopIAEA Workshop City , Country

XX - XX Month, YearCity , Country

XX - XX Month, Year

LecturerLesson IV 3_2.1

LecturerLesson IV 3_2.1

IAEA Training Course on Safety Assessment of NPPs Assist Decision Making 2

Context of Initiating Event AnalysisContext 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 modesanalysed 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 AnalysisInitiating Event Analysis

IAEA Training Course on Safety Assessment of NPPs Assist Decision Making 4

Selection of Initiating EventsSelection of Initiating EventsAPPROACHES

• 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 Assessment of Safety Functions and System RequirementsSystem RequirementsPURPOSE

• 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 EventsGrouping 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 Safety Functions and Corresponding Front Line SystemsLine 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 PWRExamples 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 Example of Front Line Systems and Support System Dependences for a PWR PlantDependences for a PWR Plant

Table VII - Safety Series No. 50-P-4

IAEA Training Course on Safety Assessment of NPPs Assist Decision Making 10

ExampleExample ofof GroupingGrouping ofof InitiatingInitiating EventsEvents

Table VIII - Safety Series No. 50-P-4

IAEA Training Course on Safety Assessment of NPPs Assist Decision Making 11

ExampleExample ofof InitiatingInitiating EventEvent GroupingGrouping forfor a a CanduCandu Nuclear Nuclear PowerPower PlantPlant

Table IX - Safety Series No. 50-P-4

IAEA Training Course on Safety Assessment of NPPs Assist Decision Making 12

ExampleExample ofof InitiatingInitiating EventEvent GroupingGrouping forfor a a CanduCandu Nuclear Nuclear PowerPower PlantPlant

Table IX (Cont.) - Safety Series No. 50-P-4

IAEA Training Course on Safety Assessment of NPPs Assist Decision Making 13

ExampleExample ofof InitiatingInitiating EventEvent GroupingGrouping forfor a a CanduCandu Nuclear Nuclear PowerPower PlantPlant

Table IX (Cont.) - Safety Series No. 50-P-4

IAEA Training Course on Safety Assessment of NPPs Assist Decision Making 14

IE Mutually Exclusive CriteriaIE Mutually Exclusive Criteria

• Initiator development must focus on “Muaually Exclusive”

• Sequences are Mutually Exclusive because:

• Freq. [ Sequencei] = Freq. (Sequencei)Ui

Σi

– 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 TrainsExample of NPP with 2 Feedwater Trains

Operating Experience

IAEA Training Course on Safety Assessment of NPPs Assist Decision Making 16

Typical Under Typical Under -- Estimation MistakesEstimation 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

TypicalTypical RangesRanges ofof InitiatingInitiating EventsEvents