The role of natural circulation systems

IAEA-TECDOC-626 Safety Related Terms For Advanced Nuclear Plants. 

With respect to plant safety, application of passive systems/components is intended to simplify the safety systems and to improve their reliability, to mitigate the effect of human errors and equipment failures, and to provide increased time to enable the operators to prevent or mitigate severe accidents. Natural circulation systems typically do not require or accommodate repair or maintenance work during power operation.Therefore, a reduced number of safety system trains may be needed to perform the designated safety function with the required reliability. An IAEA conference [1] included discussions on the safety of future plants, and noted that the use of passive safety systems is a desirable method of achieving simplification and increasing the reliability of the performance of essential safety functions, and should be used wherever appropriate. 

However, natural circulation systems have their own advantages and drawbacks in comparison with forced flow systems, both in the areas of plant safety and plant economics. Passivity itself does not mean that a natural circulation system should be automatically considered as more reliable with regard to fulfilment of the designated safety function. Therefore a reasonable balance of traditional systems and new passive means is adopted in many future reactor concepts as a possible way to improve safety and public acceptability of nuclear power, and at the same time to keep nuclear power competitive with conventional power technologies. Many considerations govern this balance and define the final design decision, such as  

All the above aspects are being taken into account by nuclear power plant designers, and as a result, both novel and more or less proven passive systems and features are proposed in many new designs [2]. Some designs have only added a few passive components to the traditional systems. Some other designs make use of the passive systems/components and natural circulation phenomena for power production in normal operation, or to fulfil a number of safety functions, intended to prevent severe accidents and mitigate their consequences. 

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