Sodium-cooled graphite reactors

Liquid metal has been used to cool thermal and fast reactors. Sodium-cooled graphite reactors are examples of thermal reactors. The sodium-cooled reactor experiment was... 

Hallam, a sodium cooled graphite moderated reactor plant built in HaUam, Nebraska, about 20 miles south of Lincoln. 

The Hallam plant was a sodium cooled, graphite moderated reactor with a 256 MWt output (Allen, 1977). It started up in 1962 and was permanently shut down in 1964. Atomics International, a subsidiary of Rockwell Aerospace, designed the core and coolant system for this plant. The aggressive effects of the sodium coolant on several alloys were not fully appreciated when the reactor started up. As a result, corrosion on the stainless steel cladding of individual fuel elements led to this plant s shutdown after just over 1 year of operation (Anon., 1972). 

Other newer designs include the advanced, gas-cooled reactor (AGR), Canadian deuterium reactor (CANDUR), sodium-cooled reactor (SCR), sodium-heated reactor (SHR), and fast breeder reactor (FBR). These reactors employ either natural or enriched uranium fuels that may be modified in some way (e.g., graphite-moderated fuels). 

There was, of course, the fast reactor, hut a commercial design was unlikely to be ready in time for operation in the mid-1960s. The fast reactor was cooled by liquid sodium, and this gave rise to another idea a graphite-moderated sodium-cooled reactor. The submarine USS Seawolf also used a sodium-cooled reactor, and although there were no major problems with the design, it was soon replaced with the light water reactor that would become standard in US Navy submarines. 

Compared with designs for a more advanced gas-cooled reactor, the sodium-graphite reactor appeared to offer serious technical problems combined with poor economics, which meant that it was soon dropped from the programme. 

In order to maximize the cross section for fission, which is greatest for low energy neutrons, the neutrons are slowed down or "moderated by a material (the moderator) that elastically scatters neutrons but has a small neutron capture cross section. In LWRs ordinary (but very pure) water serves the purpose of both moderation and cooling (in other reactor types the moderator may be a liquid like D2O, a solid material like graphite or absent and the cooling medium may be a gas or a metal like lead, mercury or sodium). 

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