increased demand led mining companies to expand supply channels, which came online just as the rate of reactor construction stalled in the mid-1970s. The resulting oversupply caused fuel prices to decline from about US$40 per pound in 1980 to less than $20 by 1984. Breeders produced fuel that was much more expensive, on the order of $100 to $160, and the few units that reached commercial operation proved to be economically unfeasible. Advantages Fast reactors are widely seen as an essential development because of several advantages over moderated designs. The most studied and built Fast reactor type is the Sodium-cooled fast reactor. Some of the advantages of this design are discussed below; other designs such as the Lead-cooled fast reactor have similar advantages. A fission event creates more neutrons than in the thermal reactor. This gives flexibility and allows breeding of uranium. As becomes slightly reactive to fast neutrons, a significant percentage of the fission events in the reactor occur with this isotope. There is a fine balance between the production of neutrons from fission on the one hand, and the many processes that remove them from the equation on the other. If the temperature increases in a fast reactor, this will have two effects: Doppler broadening of the neutron spectrum, and a very small increase in the physical size of the reactor core. These two effects serve to reduce the reactivity because it allows more neutrons to escape the core, as was shown in a demonstration at EBR-II in 1986. In this test, the additional heat was readily absorbed by the large volume of liquid sodium, and the reactor shut itself down, without operator interference. Because sodium has a boiling point of , and lead has a boiling point of but reactors operates typically around to , there is a large margin where the metals will stay liquid, and thermal increases can be easily absorbed, without any pressure increase. As no water is present in the core at high temperatures, the reactor is essentially at atmospheric pressure. Most often, an inert gas blanket at a modest pressure (e.g. 0.5 atmospheres) is present to ensure that any leak results in mass transport to the outside of the reactor. This means that there is no pressure vessel with associated problems (high pressure systems are complex), nor will a leak from the reactor emit high pressure jets. The entire vessel being at