Regional Basic Professional Training Course in Korea

that are important for safety and determination of importance factors.
❙ 563 ❙
9. Probabilistic Safety Analysis
Assessment of important dependencies between systems and between human
actions and systems.
9.1.4.2. Steps of a PSA level 1
Analysis of initiating events
The first step in performing a PSA is to identify possible initiating events and determine
their frequencies.
Internal initiators usually receive the most attention in a PSA, and their frequencies are
generally less difficult to estimate than the frequencies of external initiators. Internal
initiators are based on the operating experience and engineering analyses.
Following the studies performed in many countries, it appears that the internal events
must also include events at low power and shutdown. The associated risk is not negligible
because of the possibility of having a low water level in the primary circuit, no automatic
action of the safety systems, and unavailability of equipment for maintenance.
In addition to the traditional in-plant (internal) initiators, it appears that external initiators
(including internal fires) have to be taken into account, because the associated risks
generally are not negligible compared to the internal events.
The main external events to be studied are:
Internal fires (based on operating experience, and taking into account possibilities
for fires in the plant).
Seismic events for specific high level seismic sites based on historical operations
(if they exist) and studies of existing faults. This results in significant uncertainties