Although the highest priority was given to LMFBRs, several other types of breeders were considered, and reached various stages of development in the United States. In addition to the LMFBR, these included the gas (helium) cooled fast breeder, and two thermal-neutron reactor types, the light-water breeder reactor and the molten-salt breeder reactor (MSBR). The fast-neutron breeder reactors were designed to breed plutonium from uranium-238, while the thermal-neutron breeder designs were optimized to breed uranium-233 from thorium-232.

Perhaps the most interesting alternate concept explored in this early work was the molten-salt breeder, which still has advocates.47 In this reactor, the fuel and coolant are combined in a molten mixture of fluoride salts. The salt flows through the reactor core, through an intermediate heat exchanger, and then back to the reactor core. Molten-salt reactors were first proposed by Ed Bettis and Ray Briant of Oak Ridge National Laboratory (ORNL) during the post-World War II attempt to design a nuclear-powered aircraft.48 Two molten-salt reactors were built at ORNL. The first was a prototype aircraft reactor, the 1.5 MWt Aircraft Reactor Experiment (ARE), which operated for 100 hours in October 1954. The second, the graphite-moderated 8 MWt Molten Salt Reactor Experiment (MSRE), operated between June 1965 and December 1969, demonstrating the technical feasibility of the molten-salt breeder concept.

In 1972, ORNL proposed a major development program that would have culminated in the construction and operation of a demonstration reactor called the Molten Salt Breeder Experiment. The total program cost was estimated at $350 million over a period of 11 years.49 Those who would have had to approve the funding of the program were already heavily committed to the LMFBR, however. The ORNL proposal was rejected by the AEC partly because it wished to reduce the number of breeder candidates to be developed and because the breeding ratios projected for the molten-salt reactor were low compared to those foreseen for the fast-neutron reactors.50 In January 1973, ORNL was directed to terminate MSBR development work. The program was reinstated a year later, and in 1974 ORNL submitted a more elaborate proposal calling for approximately $720 million to be spent over an 11-year period. This proposal was also rejected, and, in 1976, ORNL was again ordered to shut down the MSBR program "for budgetary reasons."51

The Shippingport Atomic Power Station was converted in 1975 into a marginal breeder using a thorium-uranium-233 fuel cycle.52 The Shippingport plant had begun commercial operations on May 26, 1958 and was the first nuclear power station in the United States to generate commercial electricity. It also was a major milestone in the development of light-water power reactors because it pioneered the use of uranium-oxide fuel in a water-cooled reactor.53

The gas-cooled, fast breeder reactor (GCFBR) was promoted by General Atomics, which had developed and was marketing the high temperature gas-cooled reactor (HTGR) in the United States.54 The first HTGR demonstration plant was built at the Fort St. Vrain, Colorado Nuclear Generating Station. Fort St. Vrain was connected to the grid on December 11, 1976, and was shut down on August 29, 1989 due to continuing problems.55 The GCFR would have had the same helium coolant technology, and its fuel would have had much in common with that of the HTGR. However, it would have lacked the graphite moderator of the HTGR and the safety advantage of its large thermal heat capacity.