Molten salt reactors are nothing new and have been operational as research and test plants since the 1950s. One of the earliest molten salt reactor experiments was operated at Tennessee's Oak Ridge from 1965 until 1969, after going critical. Even though the The Molten Salt Reactor Experiment did end in a critical event, it was well known and respected throughout the nuclear research community as a success. However, later studies found the reactor only operated for just 13,172 hours over those four years, or only around 40 percent of the time and experienced 171 unplanned shut downs. These interruptions were documented by the research team as technical problems, including: “chronic plugging” of the pipes that led into charcoal beds intended to capture and remove radioactive materials so the reactor could operate; failures of the blowers that removed the heat produced in the reactor; and fuel draining through the so-called freeze valve safety system intended to prevent an accident. Corrosion is a result of the reactor’s nature, which involves the use of a fuel consisting of uranium mixed with the hot salts for which the reactor is named. As anyone living near a seashore knows, chemically corrosive salt water eats most metallic objects.
Fast breeder reactors “burn” {{chem|235|U|link=Uranium-235}} (0.7% of [[natural uranium]]) as fuel, but they also convert [[fertile material]]s such as {{chem|238|U|link=Uranium-238}} (which makes up 99.3% of natural uranium) into [[fissile]] {{chem|239|Pu|link=Plutonium-239}}. This newly produced plutonium can then be used as nuclear fuel.<ref name="world-nuclear1"/> The [[traveling wave reactor]] proposed by [[TerraPower]] is designed to "burn" the fuel it breeds in situ, without requiring its removal from the reactor core or further reprocessing.<ref>Wald, M. [http://www.technologyreview.com/energy/22114/?a=f "TR10: Traveling Wave Reactor"] {{Webarchive|url=https://web.archive.org/web/20111011040125/http://www.technologyreview.com/energy/22114/?a=f |date=11 October 2011 }}, [http://www.technologyreview.com ''Technology Review'']</ref>
