The complex set of thermal-hydraulic phenomena that occur in a gravity environment when geometrically or materially distinct heat sinks and heat sources are connected by a fluid can be identified as Natural Circulation (NC). No external sources of mechanical energy for the fluid motion are involved when NC is established.

The above definition includes the situations of a heater immersed into a fluid, of an open flame in the air, of a chimney driven fire, of insurge of hot fluid into a pool of cold liquid, and of a heat source and sink (e. g. heater and cooler) consisting of separated mechanical components connected by piping and situated at different gravity elevations. Natural circulation also drives the occurrence of stratification in horizontal pipes.

Within the scope of this document, this phenomenon involves the following system configurations:

(a) Heat source and sink of primary loop constituted, respectively, by core and steam generator, or boiler, or primary side of heat exchanger, with core located at a lower elevation;

(b) Heat source and sink inside the pressure vessel, constituted, respectively, by core and (typically annular-like region of) vessel downcomer. ‘Steady-state’ NC between core and downcomer occurs owing to continuous cooling of the downcomer fluid by a heat exchanger (boiler or steam generator) or by continuous inlet of feed-water liquid at a temperature lower than core outlet temperature;

(c) Cooling of the containment atmosphere by a closed loop.

In the current generation of nuclear plants, the NC core power removal capability is exploited for accident situations to demonstrate the inherent safety features of the plant (with the noticeable exception of the Dodewaard commercial BWR unit, shutdown in 1997). The natural circulation is also occurring during various phases of the refuelling.

In future generation of nuclear plants, NC is planned to be used for ensuring the nominal operating conditions and for achieving safe cooling following accidents in a wider spectrum than foreseen for current generation reactors.