EBR-1, which started operation in the United States in 1951, was a small fast reactor cooled with sodium. Its operation was largely successful and was widely reported. Possibly for these reasons it set a trend for sodium cooling, and designers of subsequent civil fast reactors kept to a coolant that was known to work rather than risk trying something else. As a result almost all fast reactors have been sodium — cooled. The only exceptions have been the USSR submarine reactors mentioned earlier for which sodium could not be used because of the impossibility of venting sodium-water reaction products.

Because there is such a preponderance of sodium design and oper­ating experience, and because at the time of writing all operating and planned fast reactors are sodium-cooled, the rest of this chapter deals only with the sodium systems of power-producing fast reactors. It con­centrates on the primary and secondary coolant circuits. The associated steam plant, apart from the steam generators, is very similar to that in a conventional fossil-fuelled power station so it is mentioned only briefly here, and the electrical equipment, which is entirely conventional, is not dealt with at all.

Chapter 3 emphasises the way in which the designer of the reactor core is constrained within very narrow limits by the properties of the materials so that there is a marked similarity between all fast reactor cores. The same is not true for the coolant circuits. The use of sodium dictates the size of the heat transfer surfaces but not the form of the circuits or the heat exchangers. There are for example two very differ­ent approaches to the layout of the primary circuit, leading to either a “loop” or a “pool” reactor. Similarly many different steam gener­ator designs are possible, employing U-tubes, straight tubes or helical tubes, with either once-through steam flow or separate evaporators and superheaters.

So far all fast power reactors have had a secondary liquid metal coolant circuit because the risk of having water and radioactive primary sodium in the same heat exchanger has been judged unaccept­able. With increasing experience of designing and operating sodium — heated steam generators, and of preventing leaks in them, the situation may change in the future, because the capital cost would be reduced if the secondary sodium circuits could be dispensed with. But because the present purpose is to describe existing and imminent systems only designs incorporating secondary sodium circuits are considered here.