The purpose of the reactor coolant system is to maintain cooling during normal operation and also during transients. There must be sufficient water inventory and safety water injection systems to ensure that water reaches the core. Heat is then transferred by circulation (forced or natural circulation) to the ultimate heat sink (Fil et al., 1999).

In many advanced plants, water to replenish any reduced water inventory in the primary circuit is stored entirely inside the containment. This provides additional protection against external events and other types of accident, e. g. containment bypass. Other features that are included to ensure protection of the primary circuit inventory include:

— pressurizer relief to a water storage tank;

— heat rejection to a water storage tank via heat exchangers;

— water storage tank joined with the containment sump;

— water storage tank, located high above the core for gravity driven injection, and;

— core make-up tanks (CMTs) at full circuit pressure to provide high pressure injection.

High-pressure passive injection systems are not present in currently operating reactors. The CMTs provide this function for AP1000 and AP600, (written AP1000/600 in this section). If the initiation set points are reached, valves open and cold water from the CMT flows into the reactor coolant system. If the CMT water level falls too low, then stepwise depressurisation of the reactor coolant system is initiated to ensure that medium and low pressure systems initiate.

Passive injection from accumulators is available in advanced passive designs, as it is in present generation plant. Modern designs have been optimised to increase system reliability and to broaden the pressure window of operation. Examples of such plants are AP1000/600, Mitsubishi APWR and Indian HWR designs. In addition, the Russian W-392 and W-407 designs adopt this principle. The Mitsubishi APWR accommodates an advanced accumulator system which eliminates the need for low pressure injection.

Passive low-pressure injection from the water storage tank is placed at high elevation across the core. Discharge can only take place when the reactor system pressure is at the last stage of depressurisation. Examples of such designs are AP1000/600 and the VVER-640/W-407 designs.

In advanced systems, sufficient heat transfer is attained provided there is sufficient water to cool the core. It is ensured by natural circulation from the heat source (core) to the heat sink, e. g. water storage tank in the AP1000/600 designs or the SGs in the Russian VVER-1000/W-392 design. These paths can exist in single — or two-phase water/steam modes. Different designs can make use of a range of different natural circulation paths.

Under accident conditions, heat is transferred to water tanks inside or outside the containment. Heat is then transferred to the surrounding atmosphere either via the containment shell or via a special heat exchanger, discussed below.

Passive feedwater systems have been considered in connection with the CANDU reactor design. There is an elevated tank above the boilers. Valves are opened to depressurise the boilers and allow flow by gravity.