Summary of technical developments

The important feature of all these systems, however, is not their degree of passivity but their performance and reliability in carrying out the function for which they were designed. All reactors have to achieve the same standards of safety as a minimum but the passive systems may be able to achieve this standard more easily provided their performance and reliability can be demonstrated. The driving forces of natural convection are generally lower than those of pumped circulation systems and 

The power range of nuclear heating reactors is generally lower than SMR power reactors. They are rated between 2 to 500 MWth. Apart from the high temperature reactors which are discussed in section 3.2.5., their supply temperature is aimed mainly at district heating or sea water desalination and does not exceed 130 C. This corresponds to a primary circuit temperature of around 200 C, and a power density ranging from 2 to 60 kW/1. 

The smaller size and lower pressure resulting from these requirement leads to simplification of the overall design and allows for the maximum utilization of natural processes. 

Simplifications have been achieved through a less massive RPV, through integration of the primary circuit in the RPV, and in the safety sterns and containment. Further simplifications have been made in the use of natural circulation for normal heat removal (made possible by the large safety margins in the NHP design) and by the use of passive safety systems. 

Over a dozen reactor designs are known worldwide, most of which have originated in developing Member States. The economics of these reactors, however, can only be justified in remote regions isolated fiom a national grid. Only a few of the concepts have been constructed (e g. AST-500 in Russia, HR5 in China and SLOWPOKE in Canada). As a result operational experience has been limited. It is not expected that all currently proposed designs will be implemented. 

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