Gravity driven cooling provides emergency core cooling water by gravity draining, in events with loss of coolant. This system requires a large volume of water above the core, plus additional depressurization capacity, so that the primary coolant system can be depressurized to allow for gravity flow from the elevated suppression pool. Since there are no large reactor vessel pipes at or below the core elevation, this design ensures that the core will remain covered by water during all design basis accidents. In general, gravity driven cooling concept is mainly based on the depressurization of the reactor pressure vessel to sufficiently low pressures to enable reflood of the core by gravity feed from an elevated pool. When the gravity driven cooling operates, the gravity drain flow rate to the reactor pressure vessel depends on the piping geometry, the state of the fluid, and the pressure conditions in both the water pool and the reactor pressure vessel. Flow entering the reactor pressure vessel during the later stages of blowdown during a postulated loss-of coolant accident (LOCA) must be sufficient to keep the nuclear core flooded. The system which provides gravity driven cooling is a simple and economical safety system.

The following is a listing of the gravity driven cooling related phenomena:

• Depressurization of the reactor pressure vessel by discharging through depressurization valves into the drywell and increase of pressure in the upper part of containment;

• Evaporation in the reactor pressure vessel due to depressurization;

• Friction in the gravity driven cooling system and injection lines including the valves in these lines;

• Large amounts of cold water immediately floods the lower parts of the reactor pressure vessel, causing:

— Collapse of voids

— Condensation of steam

— Suppression of boiling

— Increase of water level inside the reactor pressure vessel;

• Condensing of steam out of the reactor pressure vessel and drywell gas space until air accumulates on the primary sides of the passive containment cooling system, resulting in termination of steam condensation.

As examples for gravity driven cooling system can be given gravity driven cooling system (GDCS) of ESBWR and passive core flooding system (HA-2 hydraulic accumulators of the second stage) of WWER-1000/392 and passive core flooding systems (ECCS tank)of WWER-640/407.