A Brief History of the Fluid Fuel Reactor: Bettis and Weinberg

It is now clear that the MSR began with conceptual studies of a fluid salt fueled reactor conducted by a group of Oak Ridge scientists, in the late 1940’s. It is not clear what the original goal of this project, or even that there was a formal project, but in 1950 that original seed was to suddenly take root. ORNL had received a research project from the Air Force to participate in crazy project, called Aircraft Nuclear Propulsion (ANP). The Air Force had decided that it wanted a reactor powered aircraft. The whole business was insane, because reactor shielding is very heavy. Thus a reactor powered aircraft will either kill its crew with radiation, or be too heavy from radiation shielding to get off the ground.

Alvin Weinberg attributes the idea of a reactor powered aircraft to Gordon Simmons, a K-25 engineer. Weinberg described Simmons as an aggressive, fast talking optimist, who viewed difficulties of reactor powered flight as technical problems that could be overcome by research. Simmons convinced Fairchild Aircraft of the correctness of his views, and through Fairchild the Air Force and Congress. ANP was originally a K-25 project, and Gordon was its first head. Ed Bettis and his associates were part of the ANP project.

Eventually ANP research was transferred to ORNL, but it carried a K-25 legacy. A K-25 physicist Cecil Ellis was in charge of the project. Ellis favored a Liquid Metal cooled reactor. Weinberg was not satisfied with Ellis’s performance, and replaced him with the brilliant industrial chemist, Raymon C. Briant .

Briant was to smart to believe in nuclear powered flight, but he saw the project as an opportunity to do research high temperature reactors. But he was dissatisfied with the liquid metal reactor concept, that had emerged from the project under Cecil Ellis’s leadership.

The problems of the Liquid Metal cooled reactor were explained by Ed Bettis some time later, “a group of engineers and physicists at ORNL started design work
on a solid-fuel-pin sodium-cooled reactor, with the fuel consisting of 235U (as UO2) canned in stainless steel. It was decided to make this a thermal reactor and to use BeO blocks as the moderator. The circulating sodium was to extract heat from the fuel pins and at the same time to
remove heat from the moderator blocks. The design of this solid-fuel-pin, BeO-moderated, sodium-cooled reactor proceeded to the point of purchase of the BeO moderator blocks. . . .”

“The solid-fuel-pin thermal reactor design was found to possess a serious difficulty when the design concept was projected to cover a relatively high-power reactor. The problem was the positive temperature coefficient of reactivity associated with the cross section of xenon at
elevated temperatures. This xenon instability was considered to be serious enough to warrant abandoning the solid-fuel design concept, because of the exacting requirement placed on any automatic control system by this instability”.

Bettis’s explanation requires a translation for the 99% of people who know nothing about reactor physics. The positive temperature coefficient of reactivity means as the reactor gets hotter processes inside the reactor’s power level goes up as it gets hotter. As reactor power goes up, more heat is produced, which further increases the reactor’s power. Thus a reactor with a positive temperature coefficient of reactivity is difficult to control and potentially dangerous. In addition, if you are flying an atomic airplane and you want to increase your speed, you withdraw heat from the reactor. With a positive temperature coefficient of reactivity that decreases reactor power and heat production which makes the engine loose power, and the aircraft slow down.

The Xenon problem also needs to be explained. When U-235 encounters a neutron inside a reactor, most of the time it splits into two large atomic fragments and some left over bits including two or three neutrons. Xenon-135 is frequently one of those fragments. Xenon-135 is the Chuck Norris of neutron absorbers. Xenon atoms might also be described as the NFL linemen of reactors. Think of U-235 atoms as the quarterbacks of the reactor, and neutrons as pass rushers. Xenon-135 atoms are very big for rushing neutrons. When neutrons hit Xenon 135 atoms, they are blocked from hitting U-235 atoms. When neutrons hit U-235 atoms inside a reactor, more blockers, that is more xenon atoms enter the game. Xenon builds up as more and more fissionable atoms are split, and thus more and more neutrons are blocked by Xenon. The Xenon blocking, tends to slow down chain reactors, thus Xenon poisoning makes reactors more difficult to control.

It is highly likely that in 1950 Ed Bettis explained these problem and how the liquid salt reactor concept would solve them to Ray Briant and later to Alvin Weinberg. Although the MSR posed significant technological difficulties, they were not as difficult as making a reactor powered airplane fly.

Hot liquid salts expand as they heat. Suppose you have a one gallon pot on the stove and you fill it up with hot liquid salt. Now you turn up the heat under the salt pot. What will happen? As the heat goes up the liquid salt expands and starts running over the top of the pan. Now imagine that the hot salt includes a uranium salt that is enriched with U-235. You don’t need to heat the salt pot, a chain reaction of U-235 will do that for you. As the chain reaction heats the pot will do that for you. And as the salt gets hoter, it starts to run over the top of the pot, taking with it, some U-235. Removing U-235 from the pot decreases the chain reaction and thus the heat.

How about Xenon? Well Xenon is an a noble gas. That means it will not form chemical bonds and thus is free to bubble out if the hot salt liquid. Of course it is not quite simple as that, because Xenon is highly radioactive, stuff you would not want floating around your lab. But there are safe ways to get Xenon out of a hot salt fluid. And at any rate the first experimental reactor would not have to solve all of the problems. It could be operated without actually solving the Xenon problem, as long as ORNL reactor designers knew how to solve the problems.

There was an unfolding beauty to the reactor concept Bettis outlined. Consider its negative temperature coefficient of reactivity. The MSR would automatically supply more power to aircraft jet engines when power was needed. As heat was transferred from the reactor to the jet engines, the heat in the reactor dropped. As the heat dropped, more Liquid salts and more U-235 would be drawn into the reactor core, increasing reactor power output. This of course increased the heat available for the engines. As engine power requirements dropped, the engines used less reactor heat. The reactor then heated up and as U-235 was forced out of the core the chain reaction dropped. Thus reactor went to maximum heat while burning very little U-235. But the heat was instantly on tap once power was demanded from the engine.

The negative temperature coefficient of reactivity was a beautiful quality of the MSR, but it was never to be used in flight. Yet it does have potentially valuable uses in electrical generation. First the MSR alone among reactors is a load follower. The MSR is capable of automatically adjusting its power output to follow load demands on electrical systems. This would make the MSR particularly valuable in balancing the ever fluxuating electrical output of windmill generators, and photovoltaic electrical systems. Secondly the MSR would be well suited for a backup generating role. As generating sources suddenly go off line, reserve MSRs, with their hot salt at peak tempreture, can come online at full power as fast as as their generating turbines can be spun up to full power. MSRs would be equally useful as peak power sources, which can be brought online almost instantaneously as electrical demand warrants. These are qualities that would be very useful in a post-fossil fuel age, and qualities that would cannot be obtained from renewable technologies, or from conventional nuclear power plants.

Weinberg agreed that Bettis’s radical reactor design had great promise, and became an enthusiastic backer of the MSR project. In the late spring of 1950 the Y-12 chemistry group headed by Warren Grimes was administratively transferred to ORNL effective on July 1, to begin work on Molten Salt reactor chemistry. They were assigned the task of investigating various Fluoride salt mineral and metal combinations. Thus my father went to work for ORNL on that day. He remained an ORNL employee for the next 27 years.

A Brief History of the Fluid Fuel Reactor: The Molten Salt Reactor Adventure Begins

Eugene Wigner spent a brief period as Research Director of what was then called the Clinton Laboratories. Oak Ride was in 1943 a town that did not exist, so the Laboratory could not be named for it. Instead the assigned name that of Clinton, the old East Tennessee town that was the county seat of Anderson County, where most of the Oak Ridge complex was located. Wigner's stay was not a happy one for him, but is was exceedingly fruitful for the Laboratory. Wigner brought with him a team of brilliant scientists, and attracted more first rate researchers to Oak Ridge. Frederick Seitz, Erich Vogt, and Alvin Weinberg left a brief account of Wigner's stay in Oak Ridge:

"Wigner planned a two-pronged approach. First, he would establish a training program in which some thirty-five young scientists and engineers could learn the principles involved in nuclear reactors. These individuals would become future leaders in reactor development. Second, he would assemble an expert team to design nuclear reactors that could produce useful power efficiently and as safely as possible, placing much emphasis on the so-called "breeder" reactor. A substantial part of his research team in Chicago, including Weinberg and Young, agreed to join him there and spend the next phase of their professional careers promoting the development of nuclear energy for peaceful purposes".

Wigner quickly saw the hand writing on the wall:

"In the meantime, there was a great deal of legislative activity in Washington about the way the national nuclear energy program should be managed in peacetime. The debate was intense and protracted. The final result was the creation of a new civilian agency, the Atomic Energy Commission, which was put in charge of the operation on January 1, 1947. As the year progressed, Wigner eventually decided he was not really suited to serve as manager of a laboratory in such a complex, politicized environment. Many of the most important technical decisions would be made in Washington rather than in the laboratory".

Wigner and Weinberg remained personal friends, and wigner continued to visit the Laboratory on a regular basis. Hence in the Summer of 1971, I was offeed a chance to meet Wigner, along with other ORNL supernumeraries.

Alvin Weinbery was officially the Director oif the Laboratory's Physics Division from 1945 to 1948, when he assumed Eugene Wigner's former position. Weinberg was to become, among other things a custodian of Wigner's legacy, and much of ORNL's work on reactor development overthe next 25 years was to be guided by Weinberg's fidelity to the Wigner vision.

H. G. MacPherson's account of the history of the Molten Salt Reactor states,

"Molten salt reactors were first proposed by Ed Bettis and Ray Briant of ORNL during the post-World War II attempt to design a nuclear-powered aircraft".

Alvin Weinberg stated in 1957,"

At the Oak Ridge National Laboratory we have been investigating another class of fluids which satisfies all three of the requirements for a desirable fluid fuel: large range of uranium and thorium solubility, low pressure, and no radiolytic gas production. These fluids, first suggested by R. C. Briant, are molten mixtures of UF4 and ThF4 with fluorides of the alkali metals, beryllium, or zirconium".

Other sources tell a slightly different story. By M.W. Rosenthal, P.R. Kastin, and R.B. Briggs state "experiments to establish the feasibility of molten- salt fuels were begun in 1947 on

“the initiative of V.P. Calkins, Kermit Anderson, and E.S. Bettis.".

Ray Briant did not come to Clinton Labs until 1948, so it would appear that preliminary MSR research began before his arrival in Oak Ridge.

Rosenthall Kastin, and Briggs add, "At the enthusiastic urging of Bettis and on the recommendation of W.R. Grimes, R.C. Briant adopted molten fluoride salts in 1950 as the main line effort of the Oak Ridge National Laboratory's Aircraft Nuclear Propulsion1 program.”

Here we see a divergence between the collegiate nature of science and the conduits of information. Calkins, Anderson and Bettis appear to have decided on their own to investigate the possibility of a Molten Salt Fuel in 1947, but only Bettis gets credit for their joint invention. Bettis gets credit more for his advocacy than for the uniqueness of his role. Finally Warren Grimes got consulted on the chemistry, because his group was was to be assigned the task of researching MSR chemistry. Now the interesting thing was that in 1950 my father, C.J. Barton, Sr was the expert in Grimes' group on Fluoride Salt Chemistry. That is because my father probably participated in Grimes fluoride salt chemistry literature review that lay behind Grimes recommendation. How much of Grimes' recommendation rested on my father's judgment is probably beyond knowing.

Eugene Wigner was not a politician, not at least a politician in the way that Weinberg was. The giving and taking of credit was an important part of the management system of ORNL in the Weinberg era, and upper level managers were to use the giving and taking of credit to aggrandize themselves, and to reward and punish their subordinates, and not always for the best of reasons.

Bettis, Calkins, and Anderson could not have initiated research without an idea about what they were doing, thus they must jointly be credited with the MSR idea. It would appear that Briant later made the suggestion that thorium could be added to the MSR fuel mix. But note, the idea of converting thorium to U-233 in a fluid fuel reactor goes back to Wigner.

In 1947 a small group of K-25 engineers in Oak Ridge engineers, V.P. Calkins, Kermit Anderson, and Ed Bettis were assigned the task of developing a reactor for the Air Force that could power a bomber. During World War II the Hungarian genius, Eugene Wigner had invented a sodium cooled reactor, an invention which Wigner himself did not like, but in 1947 sodium cooled reactors were all the rage among people who were thinking about advanced nuclear technology. Calkins, Anderson, and Bettis were not working for Eugene Wigner at the X-10 laboratory. Instead they worked for K-25 and someone high up in the management of K-25 had decided that the Air Force needed a sodium cooled reactor to power their bombers. The more the young Oak Ridge engineers looked at the sodium cooled reactor, the less they liked it. It would be, they determined dangerously unstable. The hotter it got, the more power and hence more heat it produced. It could run away in a way similar to the way the Chernobyl reactor did some 39 years later. The young engineers decided that they needed to find a reactor concept that would tend to shut down as soon as it started to over heat. Liquids expand as the become hotter, and the young engineers thought that if the fuel was dissolved in a liquid, the liquid would expand out of the reactor's core as it heated, carrying U-235 out of theu core with it as it expanded, slowing the ongoing nuclear reaction in the core. Wigner was at that time interested in fluid core reactors that used heavy water as a core fluid, but heavy water was not a good candidate for what the K=25 engineers had in mind. K-25 was the world's leading center for fluoride salt chemistry in 1947, and the enginerrs thought that if fluoride salts were heated past their melting point would make an ideal carrier fluid for their reactor. It was a daring and even outrageous concept. In 1950 the project to build a reactor to power the atomic power bomber was turned over to Eugene Wigner's brilliant protoge, Alvin Weinberg, wh had remained in Oak Ridge after Wigner returned to Prinston. Ed Bettis approached scientist who had started thinking about the aircraft reactor project. He quickly convinced a small group of scientists including Ray Briant, Warren Grimes and my father C,J. Barton, Sr., about the liquid salt reactor idea. For the next 25 years, the idea of building a fluid aalt core reactor mesmerized Oak Ridge National Laboratory.