Early Reactor Concept Development

Eugene Wigner (left), Enrico Fermi (right)

Some of the best scientist of the 20th century hung around around the “Metallurgical Laboratory” of the University of Chicago. The “Metallurgical Lab” was a cover for the nuclear research establishment which Author H. Compton set up at the University of Chicago during World War II. The Metallurgical Lab was to later give birth to Argonne National Laboratory. The Lab was dominated by two extremely brilliant men, who both undoubtedly made tremendous contributions to establishing the nuclear age. They were the Italian Enrico Fermi, already the holder of a Nobel Prize in Physics, and Eugene Wigner, a Hungarian engineer/physicist who was later to receive a Nobel Prize.

Although no one speaks of a rivalry between the two men, scientists associated with Metallurgical Lab began to form networks around them. After the war Fermi was to lead Argonne National Laboratory, while Wigner took a brief stint as director of the nascent Oak Ridge National Laboratory. It was a young Wigner associate from the Metallurgical Laboratory, Alvin Weinberg, who was to lead ORNL during its years of glory.

During the war, the atomic energy research establishment quickly expanded. J. Robert Oppenheimer, of the University of California, organized the Los Alamos team. Fermi was clearly a player in the Los Alamos project. Wigner focused on reactor development in Oak Ridge and Hanford. Thus the division of labor between Fermi and Wigner thus made Fermi responsible for the bomb, and Wigner responsible for the reactors.

Weinberg was drawn to Wigner, and became his assistant at the Metallurgical Lab. Both had minds that were not compartmentalized in one discipline. Wigner was educated as an chemical engineer, and had made significant contributions to theoretical chemistry, but although Wigner never formally studied physics, his is a great name in the history of the discipline. Weinberg’s PhD crossed the lines between physics, math, and biology, and for a brief time before he got caught up in the Manhattan Project, he appeared headed for a career in biology. Weinberg, who had studied cell devision from the viewpoint of physics and math, was well equipped to understand nuclear fission and the chain reaction. For 35 years, Weinberg stood on the stage of history, and because he was an observant and thoughtful man, he left us an invaluable record of that history.

It is a mark of Wigner’s genius that between 1942 and 1945 Wigner headed both the theoretical-physics research and reactor-engineering and development at the Metallurgical Laboratory. Weinberg was a close Wigner associate. As a reactor developer, Wigner thought like an engineer. Both the original Stagg Field reactor and the ORNL pile reactor were air-cooled. It was suggested that the larger Hanford reactors be cooled with helium. He objected to the use of helium as a reactor coolant, because the then existing reactor-building materials would not tolerate the high heat found in helium-cooled reactors. Wigner decided that the Hanford reactors should be water-cooled. Weinberg observed, “making that proposal was a very brave thing for him to do because, at that early time, it had not yet been experimentally demonstrated that any chain reaction was possible. But Wigner had so much confidence in the accuracy of his calculations (backed, of course, by Fermi’s experiments) that he insisted that, though the presence of water in the reactor would reduce the multiplication factor by perhaps 3%, enough reactivity would be left to make a reactor of modest size. Moreover, Wigner emphasized that a water-cooled machine could be built much more quickly than a helium-cooled reactor.”

A.H. Compton backed Wigner’s plan to build the Hanford reactors with water rather than helium cooling. And the Hanford reactors were designed and built as large, water-cooled, graphite moderated piles.

The British, despite their participation in the World War II Chalk River project, which developed heavy-water reactor technology, were to follow the path which Wigner rejected. Their first reactors were air-cooled graphite piles. They were to pay dearly for this technological regression. The infamous Windscale fire, one of the three great reactor accidents in reactor history, was a result of the British use of this primitive technology. The British air- and helium-cooled, graphite moderated Magnox reactors, have proven far less durable than American Water cooled reactors, and far more expensive to decommission.

After the Hanford reactor design project was complete. Weinberg states, “there wasn’t much left for Wigner’s group to contribute to the project, . . .” In another account of the the period, Weinberg reports,

We organized a New Piles Committee which met weekly for three months during the spring of 1944. Here the senior luminaries, such as Fermi, Wigner, Szilard, and Franck, together with a few younger assistants like myself, discussed various ideas for reactors: for power, for submarines, for production of plutonium, even for inducing endothermic chemical reactions. Our imaginations ranged widely as we considered various moderators, coolants, and configurations. Inventing a new reactor was an everyday occurrence, simply because no one else had thought about these matters. At that time we were under the impression that uranium would always be scarce.

What happened next was one of the most amazing episodes in the history of science. “Wigner obtained 37 engineering patents on various kinds of reactors: reactors moderated with heavy-water, homogeneous reactors, fast reactors, air-cooled research reactors, and water-cooled compact reactors enriched with uranium-235. In all those instances, Wigner served as the spark behind the designs.”

Weinberg, of coursed patented the the pressurized-water reactor, the basis of naval and civilian power reactors. Farrington Daniels, a chemist in Wigner’s reactors group, patented the pebble bed reactor in 1945. Among the reactors that Wigner patented, along with Harry Soodak was the liquid-metal fast breeder reactor. Developing the homogeneous reactor became a Fermi project at Los Alamos.

Wigner advocated a homogeneous thorium breeder reactor over his own invention, the Liquid-Metal Fast Breeder Reactor. Fermi preferred the fast breeder. The advantage of the homogeneous reactor concept was that it allowed continuous processing of nuclear fuel, and the extraction of fission products, in an environment in which nuclear breeding is possible. Wigner’s background in chemical engineering played a critical role in his preference. A liquid core reactor would solve many of the chemical processing problems associated with nuclear breeding. Thorium held numerous advantages over uranium in nuclear br
eeding. The possibility of a shortage of uranium was one of the Metallurgical Lab concerns, and breeders were the agreed on solution to that problem. Of course uranium turned out to be anything but in short supply.

The New Piles Committee was anything but short sighted. During a April 26, 1944 meeting, Fermi outlined the plans for a Pu-239 fast breeder that would supply fissionable fuel for other reactors, he noted, “There may be nontechnical objections to this arrangement, for example, the shipment of Pu-239 to smaller consuming plants offers the serious hazard of its falling into the wrong hands.”

Weinberg also states that Fermi warned in a New Pile Committee meeting that “for the first time mankind would be confronted with enormous amounts of radioactivity; we must not assume that this will be accepted easily by society.”

The devision between Fermi and Wigner about breeding technologies had long term institutional consequences. Fermi’s Argonne National Lab developed and championed the LMFBR concept, while Wigner’s lab, ORNL, under the leadership of his former assistant, Alvin Weinberg, experimented with homogeneous reactors until the late 1950’s before abandoning the technology for the Molten-Salt Reactor concept.

The idea for the Molten-Salt Reactor originated with V.P. Calkins, Kermit Anderson, and Ed Bettis in about 1947. By 1950, Alvin Weinberg, Warren Grimes, R.C. Briant, and my father, C.J. Barton, Sr. were supporting the project. Although technologically more challenging, the use of liquid salt fuel solved many of the problems associated with the homogeneous reactor. The MSR/LFTR concept thus is very much a part of the intellectual heritage of Eugene Wigner, even though he did not invent it.

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