1. Low Power and Shutdown PSA 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_10 Lecturer Lesson IV 3_10

2. IAEA Training Course on Safety Assessment of NPPs to Assist Decision Making 1  Need to select those that pose the most significant risk. Plant Operating States  Low power and shutdown include many different plant conditions.  A Plant Operating State (POS) is a determined plant condition that can be unmistakably distinguished in respect of its characteristics as far as carrying out a PSA is concerned.

3. IAEA Training Course on Safety Assessment of NPPs to Assist Decision Making 2 Characteristics to define plant operating states and to select those that represent a greater potential risk of fuel damage: Plant Operating States Definition  Success criteria to assure safety functions.  Similarity of initiating events within the same operating state.  Level of existing residual heat.  Water inventory.  Reactor vessel head and vent status.

4. IAEA Training Course on Safety Assessment of NPPs to Assist Decision Making 3 Selection of Initiating Events  Analysis of specific initiating events in low power and shutdown modes.  Analysis of initiating events of the full power PSA. Analysis of events which have occurred in similar reactors. Analysis of the actual plant operating experience. Qualitative analysis of possible initiating events at the plant.

5. IAEA Training Course on Safety Assessment of NPPs to Assist Decision Making 4 Grouping of Initiating Events  The accident sequences they generate are the same.  The consequences they generate are the same.  The requirements and availability of systems are the same.  The success criteria of the safety functions are similar.  The responses of the operating groups are identical.  The state of the containment is the same. Criteria for grouping Initiating Events

6. IAEA Training Course on Safety Assessment of NPPs to Assist Decision Making 5 Scenario Definition Scenario  Group of initiating events that could arise in a given POS. Selection Criteria: Availability of systems in each POS. Permanence time in each POS. Level of residual heat in each POS. Consequences which can be extrapolated from the full power PSA.

7. IAEA Training Course on Safety Assessment of NPPs to Assist Decision Making 6 Accident Sequences and Success Criteria  Similar methodology than in a full power PSA to delineate Accident Sequences.  Success Criteria : Use of success criteria from the full power PSA when applicable. Performance of simple calculation for scenarios with reactor vessel head off.

8. IAEA Training Course on Safety Assessment of NPPs to Assist Decision Making 7 System Modelling  Consistent methodology with that used in the full power PSA.  Use as much as possible models developed for the full power PSA.  New models for systems not analysed in the full power PSA.  Some systems could be operating with different alignment than in the full power PSA.

9. IAEA Training Course on Safety Assessment of NPPs to Assist Decision Making 8 Data Analysis  Permanence time in each Plant Operating State (POS)  Use of component reliability data and common cause failure obtained in the full power PSA.  Use of plant-specific data whenever available.  Component unavailability data based on schedules established for plant shutdown.

10. IAEA Training Course on Safety Assessment of NPPs to Assist Decision Making 9 Human Reliability Analysis (1/3) Differences with respect to HRA in full power operation: Need to actuate equipment manually. Low availability of procedures. Lack of training or experience. Instrumentation and indications not available.

11. IAEA Training Course on Safety Assessment of NPPs to Assist Decision Making 10 Human Reliability Analysis (2/3)  Type 1 actions: In general full power PSA analysis remains valid. Identification of basic events whose HPE can be affected by the scenario, a new basic events not modelled at full power.  Type 2 actions: Full power PSA: integrated in frequency of the initiating events.. SPSA: two categories:  IEs with dependencies in subsequent actions.  IEs that not present dependencies with subsequent actions.

12. IAEA Training Course on Safety Assessment of NPPs to Assist Decision Making 11 Human Reliability Analysis (3/3)  Type 3 actions: Evaluations of the impact of the scenario on the way the task are carried out.  Type 4 actions: In full power PSAs normally are not modelled. In SPSA generation of a confusion matrix in accordance with NSAC 60 or expert judgement.  Type 5 actions: Evaluations of the impact of the scenario on the way the tasks are carried out.

13. IAEA Training Course on Safety Assessment of NPPs to Assist Decision Making 12 Area Initiating Events Internal Fires  Increased human presence: Increased fire frequency. Greater detection and suppression capacity.  Greater number of activities with fire risk.  Presence of larger amounts of transient fuel.  Possible opening of fire barriers.  Possible shutoff of automatic detection and suppression systems.

14. IAEA Training Course on Safety Assessment of NPPs to Assist Decision Making 13 Area Initiating Events Internal Flooding  Activities involving the transfer of liquids.  Greater number of maintenance activities.  Possible opening of barriers delimiting flood zones.  Possible activation of automatic fire extinguishing systems.