Special Contribution

The proto-history of the molten salt system

Alvin M. Weinberg Former Director, Oak Ridge National Laboratory

Oak Ridge, TN 37831, U.S.A.

(Received 28 February 1997) It's a great pleasure for me to welcome our friends from Korea. I have visited Korea almost a half a dozen times. And of course, I'm very impressed with the enormous progress that this great country has made over the years. I suppose I was asked to speak here because, among the characteristics of the people who had been connected with the Molten Salt Reactor, (Gordon) Michaels has just said they're either retired or some members are still around; he properly neglected to say that many of them had died. And I'm still alive. I'm only 82 years old. I will try to tell you about the "proto-history" of the molten salt system. I'm an alumnus of the original Chicago Metallurgical Laboratory, which was the mother laboratory for the Oak Ridge National Laboratory. Oak Ridge and Argonne were daughters, siblings of the Chicago Metallurgical Laboratory, where Fermi established the first chain reaction, and Eugene Wigner designed the Hanford Reactor. I helped Eugene Wigner in designing the Hanford Reactors. After we had finished designing the Hanford Reactors, we didn't have very much to do. That was our main job. We organized a bi-weekly seminar which we called the New Piles seminar. People like Fermi and Wigner and Szilard and James Frank and some of the younger people—John Wheeler was one of them—we would discuss the possibilities for nuclear energy in the future. We allowed our imaginations to run riot. At that time the whole idea of a chain reaction was sort of bizarre; would the chain reaction work, how were we going to configure it, a chain reactor that was an engineering-ly feasible device? There were two people at the metallurgical laboratory, Harold Urey, the isotope chemist, and Eugene Wigner, the designer of Hanford, both Nobel Prize winners who always argued that we ought to investigate whether chain reactors, engineering devices that produced energy from the chain reaction, ought to be basically mechanical engineering devices or chemical engineering devices. And Wigner and Urey insisted that we ought to be looking at chemical devices—that means devices in which the fuel elements were replaced by liquids. Even in the earliest days people began thinking what kind of liquid would you have in a chain reactor. One of the liquids that was first looked at was slurry of uranium oxide in heavy water, but Wigner was not satisfied with that, and he had one of his people look into whether molten fluorides would be a possibility. So the ideas of molten fluorides first came into the chain reaction community by 1945. I was a disciple of Eugene Wigner's, and I was bitten by the 'homogeneous' bug, and I've never quite recovered from that, would you say, dreadful disease. My next connection with homogeneous reactors was, of course, here in Oak Ridge, where we thought we would build a homogeneous heavy water breeder. We did build two homogeneous reactors based on heavy water. But about the same time the laboratory was diverted from that path to try to develop a nuclear-powered airplane. The nuclear-powered airplane is, well, in English, the word is an oxymoron, that means a contradiction in terms. How can you have nuclear energy with its great big shield in a device that flies over wherever it flies and has a possibility of crashing, and what happens then will make Chernobyl look like a small-scale thing. But the Air Force was very interested in a nuclear-powered airplane because at that time it was not clear that inter-continental ballistic missile would work, and the nuclear powered airplane was looked upon as an alternative to the ICBM. The laboratory, then, for quite a while during the 1950's, before the 1960's when the MSBR project was instituted, the laboratory, therefore, was strongly involved in nuclear aircraft. In fact, that was the largest single project at the Oak Ridge National Laboratory for about ten years. The total amount of money that was spent by the United States altogether at Oak Ridge and at other places on the development of nuclear-powered airplanes was about a billion dollars, and thinking about it today, a billion dollars is what we would call real money. The leader of the nuclear-powered airplane group was a remarkable chemical engineer by the name of Ray Briant. He always insisted that it was impossible to have a heterogeneous sodium cooled reactor running at perhaps 650 degrees Centigrade, very, very high temperature. He would always say the reactor would come out looking like a bunch of spaghetti. So he leaned heavily towards the possibility of a liquid fuel, high temperature, high-power-

density reactor. There were others in the group who agreed with him and that's really how the Oak Ridge National Laboratory, with my blessing I must say, moved into development and investigation, I suppose investigation would be a better word, of Molten-Salt Reactors. We did have some success. I suppose the major success as I look back on it almost forty years is one, that we worked out the phase diagrams of many of the important molten fluorides, as well as some other salts; and secondly, perhaps even more important, we developed high temperature alloys that were compatible with these seemingly reactive uranium fluorides and lithium fluorides, and so on. And as I look back on what the Oak Ridge National Laboratory accomplished during these years I'd say that those two accomplishments by themselves were among the best things the laboratory did. The nuclear airplane project culminated at Oak Ridge National Laboratory in the design, construction, and operation of the device which was called the Aircraft Reactor Experiment, ARE. This was a small reactor which was moderated with beryllium oxide. The reactor originally was to be a heterogeneous reactor but was converted to a fluid-fuel reactor. It operated at a maximum power of about 2500 kilowatts. It achieved a temperature of about 1650 degrees Fahrenheit and it operated for about 200 hours. You must realize that a nuclear airplane was a military device, and the flight plan for the bomber was something that would last no more than thirty hours or something like that. So a few hundred hours was deemed to be sufficient for such a device. The Oak Ridge National Laboratory was very proud of having achieved the highest temperature of any reactor and having the thing last for as much as 200 hours. Dick (Engel), were you on the crew at that time? [No, I was on the aqueous system at that time. (Dick Engel)] Oh yes, yes. Well, I won't talk about that. But I must say that many of us at the Oak Ridge National Laboratory realized that a nuclear airplane was almost impossible. We could never quite prove that it was impossible, so we can always, [even if it wasn't, it was probably undesirable (Murray Rosenthal)], and that was part of it being impossible. So if problems did arise, we would say, well, we had ways of getting around it, and so I won't go into these details. These details are discussed in my auto-biography, which I published a couple years ago. But realizing that the nuclear airplane was the very edge of being feasible, and probably wasn't feasible, and deep in our hearts most of us had that feeling, we nevertheless felt that the molten salts for reactors was fundamentally a good idea, and that therefore we ought to try to exploit the technology which was gained from the aircraft in civilian power. That's how the molten-salt project, for civilian power, originated at the Oak Ridge National Laboratory. Now Murray Rosenthal, who was the leader of the project for quite a while, will tell you more in detail what success, what difficulties we've had. Let me close with the following observation. In 1962 when there was a big report put out by the Atomic Energy Commission about the future of nuclear energy; it was generally believed you that you would have burner reactors such as we have now or low conversion-ratio converters, and that these would then be gradually replaced by breeder reactors. But at that time we were very careful especially here at the Oak Ridge National Laboratory not to say fast breeder. The word was breeder reactors, not necessarily fast breeders. But Oak Ridge in a way was alone in insisting that thermal breeders, as well as fast breeders, ought to be considered. And there was indeed at that time a tendency within the Atomic Energy Commission to divide the breeder business into the plutonium breeder, which was generally viewed as a fast breeder, and the thorium breeder, and Oak Ridge elected to be the thorium breeder laboratory, because of our history and because the Molten-Salt Reactor lent itself so well to thorium breeding. Now this turned out in retrospect to be, I suppose you say, a political mistake, because the people in Washington were very much influenced by the bulk of the opinion which held that breeder meant fast breeder. Breeder meant sodium cooling. Breeder meant heterogeneous solid fuel element, and for the upstarts in the little town of Oak Ridge, Tennessee, to claim, now wait a minute, there's another way to go, and this is based on thorium, is based on liquid fuel, that was too far out of the mainstream, and so the thorium breeder, although it was mentioned quite prominently in this 1962 report by the Atomic Energy Commission the thorium breeder never received the political support and the organizational support within the Atomic Energy Commission that the fast breeder received. And therefore, the thorium breeder always has been, until I suppose rather recently, a second-class citizen. What is going to happen now, I don't know. I often have asked myself now why was it t his beautiful idea we really had, these wonderful things about these fluid fuels and so on, why is it that the powers that be in the Atomic Energy Commission never quite took the matter fully seriously. And I think the answer is, that the technology of fluid fuel is so different from the technology of solid fuel, the whole question of the maintainability of the system is so many orders of magnitude different than the problem of maintainability of the solid systems, that the people who were prepared to spend hundreds of millions of dollars on fast reactors just couldn't make that jump. And that basically was the reason why the system did not prosper the way it should have, although in those early discussions there were considerable arguments about which reactor was safer – the fast breeder reactor, heterogeneous, plutonium, the

molten salt breeder. And generally speaking, and this was a view that was supported by one of the very important commissioners, Tommy Thompson, (who died in an airplane crash, he was kind of a safety expert for the Atomic Energy Commission), always insisted that the Thorium Molten Salt Reactor was fundamentally safer, than the LMFBR (Liquid Metal Fast Breeder Reactor). Well, you'll excuse an old man who wakes up every now and then, every twenty years, and tells you what he remembers, telling you what it was like forty years ago. This special contribution is the speech of Dr. Alvin M. Weinberg to the Korean Scientific Delegation given at Oak Ridge National Laboratory on February 23, 1997.