Wigner was a great deal more that a chemical engineer. He made notable contributions to both chemistry and physics. Wigner was probably the greatest of all reactor designers. And eventually the chemical engineer came to the fore in Wigner. The fluid core reactor actually predates Fermi’s CP-1. Fermi was interested enough in the early Cavendish reactor experiment to develop a fluid core reactor at Los Alamos during World War II. For Fermi the experiment was a dead end, but Wigner realized that the fluid core reactor concept had promise.

The great flaw of the Fermi reactor was that it used fuel that required expensive and technically difficult chemical processes. Indeed the fuel required chemical and mechanical processing both before it entered the reactor and after it left the reactor if the fuel was to be reprocessed. The Fermi solid core reactor concept is at the heart of the so-called problem of nuclear waste.

Wigner, the trained chemical engineer, saw the potential of Fermi’s fluid core reactor and along with associates Alvin Weinberg, Gale Young and Harold Urey developed the concept of a fluid core reactor with a thorium blanket, that could produce electricity at a price that was competitive with the price of coal. The development of the molten salt reactor concept represented a major advance over Wigner’s initial heavy-water fluid reactor, but it did not significant departure from the concept.

Fermi was also interested in nuclear breeding, but, unlike Wigner, Fermi saw the problem as a pure physicist, not as a chemical engineer. Fermi had the poor judgement to pick liquid sodium as his cooling fluid and the uranium breeding cycle. Fermi’s rationale was that liquid sodium was a poor moderator, thus a sodium-cooled reactor would be a reactor would produce fast neutrons. Plutonium functions best as a nuclear fuel in a fast neutron environment. The down side to a fast neutron reactor is that it takes a lot of fissionable materials to get a chain reaction going inside one of them. A more significant disadvantage was the difficulty of working with sodium. Fermi’s Lab, Argonne National Laboratory managed to solve, or at least believes that it solved, the sodium problem, but others were unimpressed with the Argonne solution, and billion of dollars of have been wasted on unsuccessful sodium cooled reactor projects.

Fermi chose the sodium cooled fast breeder approach as a physicist, primarily because of its neutron economy. The downside of the LMFBR is that it uses solid fuel that must be processed at a separate and very expensive chemical reprocessing plant, in an expensive and messy chemical process. The power produced by the LMFBR would never compete in price with the price of coal. No doubt the chemical engineers at Argonne do what they are told by the physicists, and don’t talk back.

It is an utter illusion to think that the LMFBR could ever produce electricity at as low a price as the LFTR could. Nor will the LMFBR ever be as safe. There is no more reason to assume core breach in a LFTR as in a LMFBR, yet I have demonstrated that a number of safety features could prevent a core breach of a LFTR from ever turning into a disaster. Not so with the LMFBR. If there is ever a LMFBR core breech, there would be hell to pay. The notion that Sodium cooled reactors like the Integral Fast Reactor are inherently safe are illusions as long as they use a fluid sodium coolant.

Liquid metal fast breeders will never hope to compete on construction or electrical production cost with LFTRs , as Eugene Wigner and Alvin Weinberg understood. And Liquid Fluoride salts would never, never, never catch fire.

Many years ago, Alvin Weinberg pointed out the issue of LMFBR safety:

We have no real estimates of accident probabilities for liquid metal fast breeder reactors (LMFBR’s). The Rasmussen estimate (one in 20,000 per reactor year with an uncertainty of five either way) 6 would lead to a meltdown every 3 years. This is probably an unacceptable rate; an accident rate at least ten times lower, and possibly 100 times lower may be needed if the system is to be acceptable.

Weinberg pointed to another, very significant problem with the LMFBR, the issue of nuclear waste:

Each 5,000 MW LMFBR produces about 75 cubic feet of high-level solidified waste per year, contained in about 50 steel cans. According to present plans, these would occupy about 1.5 acres of burial space. Thus the entire system of 7,000 reactors would require about 15 square miles of burial space per year. After 1,000 years, 15,000 square miles level wastes will have decayed sufficiently to allow fresh wastes to be will have been used up; by that time, the radioactivity in the high layered over the older wastes. Thus the 15,000 square miles devoted to high-level wastes might be usable for much longer than 1,000 years.

To summarize, although we cannot identify physical limits that make a world of 7,000 large LMFBR’s impossible, one would have to concede that the demands on the technology would be formidable. Two issues appear to me to predominate: first, the acceptable accident rate will probably have to be much lower than the Rasmussen report suggests. If one uncontained core meltdown per 100 years is acceptable (and we have no way of knowing what an acceptable rate really is), then the probability of such an accident will have to be reduced to about one in 1 million per reactor per year. This is the design goal for the LMFBR project in the United States. Second, a nuclear world such as we envisage will have long since had to make peace with plutonium. Ten tons of plutonium per day is mind-boggling. It is hard to conceive of the enterprise being conducted except in well-defined, permanent sites, and under the supervision of a special cadre -perhaps a kind of nuclear United Nations.

I for one do not morn the passing of the Integral Fast Reactor. Sodium cooled reactors are bad ideas as Weinberg subtly suggested. It would be most unfortunate if a single cent more is wasted on Fermi’s folly. Let’s nail the coffin of the sodium-cooled fast breeder shut once and for all.