A Brief History of the Fluid Fuel Reactor: The Aqueous Homogeneous Reactor

The history of the nuclear reactor usually begins with the first successful operation of Enrico Fermi’s Stagg Field reactor on December 2, 1942. In fact scientist working at the Cavendish Laboratory at Cambridge, England built the first successful reactor in 1940. British scientists mixed 112 liters of heavy water with U308 powder inside an aluminum sphere that was 60 cm (2 feet) in diameter. The mixture was mud like and was called slurry. The aluminum sphere was immersed in a bath of heavy mineral oil to serve as a neutron reflector. The British researchers had already found that although a chain reaction was not possible if the U3O8 mixture was suspended in ordinary water, but they were able to witness signs of a chain reaction with heavy water.

During World War II the Canadians were able to successfully produce heavy water so in 1943, Harold Urey and Enrico Fermi suggested repeating the Cavendish experiment. Eugene Wigner became interested in the experiment and began working on an optimal design for a reactor. The original Wigner design called in which the slurry was pumped through a lattice of tubes immersed in a heavy water moderator.

After the value of heavy water as a moderator was better understood, researchers suggested building a large heavy water-uranium slurry reactor as an alternative to the Hanford pile reactors for Plutonium production. The reactor provide top be too great a technological project to be useful as a wartime project. Wigner and other researchers however noted the advantage of using fluoride-uranium salts, rather than U3O8 in the reactors. Of course the mixture of fluoride-uranium salts and heavy water would have created a significant corrosion problem in the reactors.

Beginning in 1943 developed a small aqueous homogeneous reactor was built and operated at Los Alamos. Like the early Cavendish experiment the core of the little reactor was a pot into witch a mud like mixture of water and a uranium compound was poured. Several similar reactors were developed Los Alamos during and after World War II, but eventually Los Alamos scientists began to see the project as a dead end.

Oak Ridge scientists were also interested in the aqueous homogeneous reactor concept during World War II. Meanwhile, Eugene Wigner, who was still in Chicago, had become interested in breeder reactors, and their siblings, converter reactors. Wigner became intrigued by the potential of a thorium breeding cycle. Wigner concerned about future uranium supplies envisioned a reactor that would burn Pu239 and would be surrounded by a blanket of Th232. The reactor would produce U-233, a fissionable nuclear fuel. But, it was noted that the cost of fuel reprocessing for such a reactor would make it not competitive with coal as a power source.

At that point Wigner and Harold Urey realized that the aqueous homogeneous reactor offered a solution to the problem of fuel reprocessing costs. Unlike Fermi who was strictly a classical physicist, Wigner was trained as a chemical engineer. For Wigner, the fluid fuel approach meant that the fuel could be withdrawn without difficulty from the reactor, reprocessed, and returned in a process that would cost much less than the cost of reprocessing solid reactor fuel. It is a tribute to the genius of Eugene Wigner, that he understood the problem that would create nuclear waste in conventional civilian power reactors, and that started the process of developing a solution to the problem.

Wigner, and his bright young assistant, Alvin Weinberg, together with engineer Gale Young, wrote a report outlining the concept in the spring of 1945. Thus the notion that the aqueous homogeneous reactor could serve as a basis for a civilian power industry remained a focus of Wigner for some time. Weinberg, both a research director and later as general director of Oak Ridge National Laboratory, championed Oak Ridge research on the aqueous homogeneous reactor until the end of the 1950’s.

Critics of nuclear power often depict nuclear scientists, as lacking in vision or a concern for human well being, and impractical. In fact the opposite is the case. Eugene Wigner was a scientist who could look long into the future and anticipated resource shortages. He was practical enough to see that low cost power was highly desirable, and as someone who had actually worked as a chemical engineer, he applied a sound chemical engineering approach to the reprocessing of nuclear fuel, and worked that approach back into the design of the reactor. Alvin Weinberg, Wigner’s young assistant, was to learn from Wigner’s long vision, and was to elaborate it during the coming years. Both Weinberg and Wigner were profoundly concerned about human well being, and both saw the possibility that nuclear power could be directed from war to the improvement of the quality of human life.

Sources
ORNL Review, History of Oak Ridge National Labratory, Chapter 4.

Alvin M. Weinberg, "The First Nuclear Era: The Life and Times of a Technological Fixer"

James A. Lane and W. E. Thompson, Fluid Fuel Reactors, Part I, Chapter 1, Addison-Wesley, 1958