outside the containment building. The site was shut down for repairs and restarted in July 1970. It ran for only a short period before closing again on 27 November 1972, and was officially decommissioned on 31 December 1975. History Breeder reactors Conventional reactors normally run on fissile 235U fuel. In nature, this is found mixed with a much greater amount of non-fissile 238U. There is so much 238U that the 235U atoms are physically separated by an amount that makes a chain reaction impossible. In most cases, this is overcome with two methods. One is to "enrich" the fuel, concentrating the 235U so the neutrons are more likely to hit them. The result of the enrichment process is a leftover mass of 238U, referred to as depleted uranium. The other is to slow the neutrons down, which makes them more likely to be captured by the 235U. This is accomplished using a "moderator", normally water. Slowing the neutrons causes the water to heat up, which can then be used to run a steam turbine. Thus, the water serves dual purposes, moderating the neutrons and cooling the reactor while extracting energy. Although 238U cannot sustain a chain reaction, it can capture a neutron of sufficient energy and may be converted to plutonium. 239Pu is fissile, and can be used for fuel. This is the basis of the breeder concept, which uses a "blanket" of depleted uranium around a neutron source which converts the blanket into plutonium. The neutron source is typically another reactor, although fusion reactors and particle accelerators have also been proposed. As the energy needed to trigger this reaction is higher than a moderated neutron, the reactor cannot be of the same design as the conventional water-cooled type. Instead, these designs typically use much more highly enriched fuels which can continue the chain reaction even with non-moderated neutrons. As water would result in undesired moderation, some other form of coolant is needed. Sodium metal makes an excellent coolant in these designs, as it has a very low chance of reacting with the neutrons, thus improving the neutron economy. It also makes an excellent heat transfer material. The efficiency of heat transfer is a function of the difference between the maximum and minimum working temperatures. In sodium this is the melting point of 371 Kelvin (K) and boiling point of 1156 K. In contrast, water's equivalent limits are 273 K