Natural circulation systems to cope with design basis accidents

NATURAL CIRCULATION SYSTEMS TO COPE WITH DESIGN BASIS ACCIDENTS 

To cope with Design Basis Accidents (DBAs) means are provided to ensure the fulfilment of the basic safety functions, such as reactivity control (reactor trip) and fuel cooling. The systems based on natural phenomena are implemented in many new designs to fulfil these 

The safety function fuel cooling during transients and accidents is ensured by provision of sufficient coolant inventory, by coolant injection, by sufficient heat transfer, by circulation of the coolant, and by provision of an ultimate heat sink. Depending on the type of transient or accident, a subset of these functions or all of them may be required. Various passive systems/components are proposed for future reactor concepts to fulfil these functions. It is a feature of many new concepts that the water for replenishment of primary coolant inventory is entirely stored inside the containment. This ensures protection against external events and reduces the risk of loss of coolant accidents with containment bypass. Additional features implemented in some new designs to improve the replenishment of primary coolant inventory function include  

Most of the new concepts incorporate a combination of different passive and active means to ensure the function coolant injection . Passive injection systems at high primary pressure are new in comparison to systems in operating reactors. AP-600 is an example of a design where this function is provided by core make-up tanks (CMT). Pneumatic isolation valves in the injection lines open automatically if one of the initiation setpoints (e g. low primary pressure. 

The function ultimate heat sink for accident conditions in the advanced concepts is mainly ensured either by the water stored in tanks (located inside or outside the containment) or by heat transfer directly to the surrounding atmosphere (via special heat exchanger or via 

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