We have been tacitly assuming that the reactors under consideration are what are sometimes known as heterogeneous, in which the fuel, coolant, and moderator (if any) are distinct physical entities. All reactors used today for power generation are of this form. However, in the early days of nuclear power, there was considerable exploration of an alternative configuration, the homogeneous reactor, defined as "a reactor whose small-scale composition is uniform and isotropic" [2, p. 378]. Homogeneity can be achieved if the fuel is in liquid form, where the liquid is circulated for heat transfer to a steam generator.
One variant of this reactor type was known as the aqueous homogenous reactor because the fuel was mixed with water (H2O or D2O). In the so-called homogeneous reactor experiment, two small reactors, called HRE-1 and HRE-2, were built at Oak Ridge National Laboratory (ORNL) in the 1950s.1 For HRE-2, the fluid was uranyl sulfate (UO2SO4) in heavy water (D2O), with the uranium highly enriched in 235 U. This program had the potential of developing a thermal breeder reactor, but although HRE-2 operated uninterruptedly for over 100 days at 5 MWe, some difficulties developed, and the program was dropped in favor of alternative liquid-fuel projects.
One alternative was the molten-salt reactor. The fluid was a mixture of fluoride compounds, including the fissile component 235 UF4 and the fertile component ThF4. After initial operation, 233UF4 was successfully tried as an
1 For a description of the history of this program and the ORNL program on molten-salt breeders, see Chapter 6 of Ref. . Technical aspects of fluid-fuel reactors are also discussed in Ref. [4, pp. 403-413].
alternative to 235UF4. Like HRE-2, this reactor was designed to be a thermal breeder reactor. Again, there was initial success in the reactor operation, but a decision was made in the 1960s to abandon development of thermal breeders in favor of fast breeder reactors.2 A further homogenous reactor approach, a liquid metal thermal breeder using uranium compounds in molten bismuth, was also investigated but was abandoned without construction even of a test reactor.
At present, there are no electricity-generating homogenous reactors, and we will not consider them further in the chapter. Instead, we will restrict consideration to heterogeneous reactors, which are so dominant that it is unusual to include the specification "heterogeneous." Nonetheless, some interest remains in homogenous reactors, particularly in molten-salt reactors (see, e.g., Ref. ).3 In fact, one of the reactors selected for possible long-term development under the Generation IV program is a molten-salt reactor (see Chapter 16). Overall, although we focus in this chapter on reactors of the sort in actual use or in immediate prospect, it is well to remember that on a longer time scale, a wide array of variants are possible. Some of these are being tentatively explored in the thinking underway in the United States and elsewhere on nuclear options for the future, as discussed in Chapter 16.
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