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Critical safety functions (CSF)

If "Control of reactivity" safety function is affected (failure of the scram system) two other safety systems are triggered: the quick boron injection system (passive system) and the emergency boron injection system (active system).

In impacts to the "core cooling" safety function, if the active systems for emergency core cooling are inoperable, water supply to the primary circuit is performed by passive systems (hydro accumulators: first and second stage).

In design basis and beyond design basis accidents, heat removal from the secondary side is performed by the steam generators (SG) emergency cooldown system (active system). In unavailability of that system, SGs cooldown is carried out by the passive heat removal system (PHRS).

Even in total loss of AC power, safety functions are fulfilled by the passive systems and the Unit could be shut down and maintained in a safe state for a long time.

Even in loss of the main cooling water and loss of the spray pools, the PHRS is capable of cooling down the reactor installation.

Even in a combination of a beyond design basis accident (rupture of a primary pipe with a maximum diameter) with simultaneous loss of all on-site power sources during the first 24 hours it does not lead to severe accident.

In case that, within this time frame, the power supply of the systems is not restored, the accident may lead to severe accident conditions. However, this does not lead to loss of containment integrity.

Analyses of all postulated failure scenarios, that lead to severe accidents, show that, with the operation of the passive safety systems, containment design pressure will not be exceed, which guarantees preservation of its integrity and functions.

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