|Fig. 1: The breeding process. (1) A neutron impinges on a plutonium nucleus. (2) The nucleus fissions, releasing additional neutrons. (3) One of the new neutrons approaches a nucleus of U-238. (4) The neutron has been captured, making U-239. (5) The nucleus has beta decayed to Np-239, releasing an electron. (6) The nucleus beta decays a second time into Pu-239.|
The Clinch River Breeder Reactor was a planned liquid metal fast breeder reactor to be built near Oak Ridge, Tennessee. It was to serve as a prototype for commercial breeder reactors in the US, delivering 350 megawatts of electrical power to the neighboring community. The project was authorized in 1970 and proceeded until 1983 when its funding was cancelled by the US Congress.
A breeder reactor produces more nuclear fuel than it consumes. It does this by using free neutrons produced by fission to transmute non-fissile isotopes of nuclear material into fissile isotopes. A fissile isotope is one that is capable of sustaining a chain reaction, a prerequisite for power production. In a fast breeder, the non-fissile isotope is U-238. When an atom of U-238 captures a free neutron, it will either fission or become U-239. If it becomes U-239, it will eventually decay into Pu-239, a fissile isotope.  The plutonium can then be extracted through fuel reprocessing. Through this process, a much greater amount of energy can be extracted from a given amount of natural uranium, when compared to conventional reactors.
Fast breeders generally have two types of fuel in the core - enriched uranium or plutonium to power the chain reaction, and natural or depleted uranium to be bred. At Clinch River, the enriched fuel would have varied from 18-24% plutonium, and the breeding material would have been depleted uranium.  Uranium is bred into plutonium much more efficiently by fast neutrons than slow ones, so fast breeders typically lack the neutron moderator present in many other reactor designs. 
The most common nuclear reactors today use light water as a coolant. Clinch River could not use water, because it also acts as a neutron moderator. Thus liquid sodium was chosen instead. Liquid sodium has the advantage that its boiling temperature is significantly higher than the operating temperature in a reactor, eliminating the need to pressurize coolant. However, sodium is highly reactive on contact with air and water, making reactor leaks a potentially significant fire hazard.  Special care had to be taken to prevent corrosion and leaks. Sodium leaks in other breeder reactors, such as Japan's Monju, have led to costly damage and extended periods of shutdown and repair.
In 1971, President Nixon announced that fast breeder technology was the United States' highest priority research and development effort. However, Clinch River faced strong opposition from several fronts. In the public, a strong anti-nuclear sentiment was growing. The 1979 accident at Three Mile Island only served to bolster this. Many argued that the costs could not be justified. While a breeder reactor could use nuclear fuel much more efficiently, the cost of uranium was not nearly high enough to offset the extra costs involved in the production of the reactor. Clinch River's initial estimated cost was $400 million, but by 1983 the estimated cost had ballooned to $8 billion or more, many times more than a light water reactor of comparable size.  President Carter objected to the project on the basis of its economics, design, and its potential negative impact on the policy of non-proliferation.  Despite all these, development continued until 1983, when opposition in Congress grew strong enough to pass a bill canceling funding for the project.
Breeder reactors are said to represent a proliferation risk because the necessary nuclear reprocessing can potentially produce weapons grade plutonium. President Carter's protests against Clinch River were in part because of the reprocessing involved. Carter issued a moratorium on U.S. reprocessing in 1977, hoping to prevent the spread of reprocessing technology. However, other nuclear powers around the world continue to operate facilities for reprocessing of material from light water reactors. The International Fuel Cycle Evaluation report in 1979 concluded that the breeder fuel cycle is no more of a risk to proliferation than the light water reactor cycle. 
© Aaron Coleman. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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