DEFINITIONS OF PHENOMENA ASSOCIATED WITH NATURAL CIRCULATION4

Phenomena have been classified into two categories (a) phenomena occurring during interaction between primary system and containment; and (b) phenomena originated by the presence of new components and systems or special reactor configurations. This classification considers the information provided in the CSNI Report [25] which has been developed for the primary systems having in mind the safety assessment, and is intended to provide complementary aspects that are relevant to advanced water-cooled nuclear power plant designs, including containment designs. Therefore the descriptions given below are intended to supplement those in the CSNI Report.

Behaviour in large pools of liquid

Large pools of water (e. g. up to several thousand cubic meters) at near atmospheric pressure are incorporated into several advanced designs. These large pools provide a heat sink for heat removal from the reactor or the containment by natural circulation, as well as a source of water for core cooling. Examples include the pressure suppression pool (wet-well) of the ESBWR, the in­containment refuelling water storage tank of the AP-1000, the pool of the emergency condenser of the SWR-1000 and the gravity driven water pool of the AHWR.

Large pools may have a very wide spectrum of geometric configurations. Heat transfer in a limited zone in terms of volume (e. g. by condensing injected steam or by heat transfer from an isolation condenser) does not imply homogeneous or nearly homogeneous temperature in the pool. Three­dimensional convection flows develop affecting the heat transfer process, which results in a temperature stratification.

Steam generated by heat transfer or following injection may be released from the pool into the containment and influences the increase of the containment pressure. Compared to a homogeneous temperature distribution, the fluid at the top of the pool may reach the saturation temperature while the bulk fluid is sub-cooled. The evaporation from the top of the pool results in a pressure increase in the containment. Therefore the temperature stratification influences plant design. The three-dimensional nature of the temperature stratification requires appropriate modelling.

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