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.