The concept of this radical design, a reactor that has no metal core structure and no meltable fuel, dates back to World War II, when anything conceived and built at Los Alamos, New Mexico, was top secret and available only to those with a need to know. By the 1950s, the idea was further developed and incorporated into the Aircraft Nuclear Propulsion project, a program that was so far out on the technical limb, anything developed for it was easily dismissed when the project, again secret, folded up in 1961. In the 1960s the usable concepts developed in the defunct atomic airplane quest were built into an experimental power reactor in Oak Ridge. It was the answer to all problems that had to be addressed in the light-water-reactor designs, including the availability of uranium and the expense of enriching it, but it was too late. Westinghouse, General Electric, Babcock and Wilcox, Combustion Engineering, and a host of copying manufacturers in Europe had already thrown their resources into light-water-reactor development. We had fallen too far down into the Rickover trap to escape, and in 1969 the Oak Ridge experimental power reactor was quietly shut down and dismantled. Ten years later, a brand-new B&W power plant at Three Mile Island, Pennsylvania, was a total loss and a clean-up liability because its reactor core had overheated. The bottom-line expense of nuclear power was rightfully called into question.
Reactor designs with liquefied core configurations were started at Los Alamos in 1943. The first was LOPO (LOw POwer), consisting of a stainless steel sphere filled with 14 % enriched uranyl sulfate dissolved in water and surrounded by a reflector made of beryllium. The fuel consisted of the world’s entire stock of enriched uranium at the time. The purpose of the reactor was to measure characteristics of uranium fission, but it was also the first reactor using a single fluid for fuel, neutron moderator, and coolant. It became known as Water Boiler, not because the mixture got hot enough to boil, but because the water broke down into hydrogen and oxygen under the heavy gamma-ray bombardment during fission, and gas would bubble to the surface. It was thus the first observation of radiolysis. Subsequently, any reactor using water as a moderator was equipped with a recombiner, a catalyst screen that encouraged the hydrogen and oxygen to re-form into water.
LOPO begat HYPO, and HYPO begat SUPO, research reactors with increasing mechanical sophistication and power. By 1953, the scientists and engineers working at the Los Alamos Lab began to think about a civilian power source based on nuclear fission. In the early fifties, there was barely enough known reserve of uranium to make bombs for the United States, much less to make electrical power for the world for centuries, but there would be enough plutonium produced artificially to at least power the Western Hemisphere far into the future. Almost 100 % of the world’s stockpile of plutonium happened to be in Los Alamos, New Mexico, and there was no reason not to use some of it to build a prototype power reactor.
The first plunge into the Liquid Metal Fuel Reactors (LMFR) was LAMPRE, the Los Alamos Molten Plutonium Research Reactor.[287] It was the first reactor ever built that used molten metal, a eutectic alloy of plutonium and iron, as the fuel. There was no fear of the core melting down, because the core was melted. The reactor would run at 1,200° Fahrenheit, a temperature that was impossible for any reactor with solid, structured fuel, but it would make very efficient steam for running a turbo-generator. The plutonium-239 fissioned efficiently using fast neutrons, so there was no need for a moderator, and there were plans to add a uranium-238 breeding blanket to the reactor so that it would produce extra plutonium as well as power. Any problem of having the reactor melt was taken care of by making it out of tantalum-tungsten alloy, which would melt somewhere above 6,000° Fahrenheit. It would be possible to slowly draw off the molten plutonium fuel through a pipe at the bottom of the reactor core, filter out fission products, add plutonium to replace that which had fissioned, and pump the fuel back into the reactor.[288]
By the time LAMPRE first achieved hot criticality on March 27, 1961, the concern for uranium scarcity was over, and Rickover’s PWR had become the darling of the civilian nuclear-power industry. LAMPRE-2 was planned for, but funding for exotic reactor projects was tight, and the ambitious follow-on project was dropped. Rickover was pleased. He despised the frontier experiments at Los Alamos, Oak Ridge, and Idaho as a silly and impractical waste of federal money.
The next big step was the Direct Contact Reactor (DCR) at Oak Ridge, where the Aircraft Reactor Experiment (ARE) was underway. The ARE was a hyper-exotic setup, meant to become a jet engine in a strategic bomber, using a molten fuel made of uranium fluoride, sodium fluoride, and zirconium fluoride, moderated with beryllium oxide, with liquid sodium as a coolant. The metal structure was made of Inconel 600 alloy, and the thing ran for 1,000 hours at a temperature of 1,580° Fahrenheit. In the history of the art, nuclear reactors did not get much fancier than the ARE.
The goal of the DCR was a high-efficiency, fast-neutron power reactor using molten plutonium fuel. The fuel was to be a plutonium-cerium-cobalt alloy, and in this innovation the fuel would also be the primary coolant, pumped around in a loop. Using the principle of “critical shape” that was employed in all the fuel-processing plants, the plutonium would be in a critical configuration, generating power by nuclear fission, only when it happened to be in the reactor core, which was a sphere. In the spherical reactor tank, the surface-area-to-volume ratio was at a minimum, and a large percentage of fission neutrons were able to propagate fission. Going around in the coolant pipes, the plutonium was thinned out, and most of any neutrons were lost out the walls of the pipes. There was no need for a moderator, because plutonium-239 was the fuel, and there was no core structure. This was a major idea.
Instead of there being a heat exchanger to pass off the high temperature created by fission, the fuel mixture was mixed with liquid sodium in the loop outside the core by a jet pump. The heat transfer from the direct contact of the hot fuel and the coolant was 10 to 100 times more efficient than using a metallic heat exchanger. The hot sodium then transferred the heat to water in an external steam generator, so that a standard turbo-generator could be used to make electricity.
A gravity-drop separator then took apart the sodium/plutonium mixture, putting the sodium back into the continuous jet pump loop and the cooled plutonium, still melted, back into the reactor. Another great feature of this plan was that the fission products would stick to the sodium and be scrubbed out of the fuel loop. A blanket of depleted uranium around the core would breed new plutonium fuel. With radioactive products continuously processed out of the fuel, there would be no buildup of radioactivity to escape, and there was no chance of the molten metal being able to blow off like steam and spread anything over the environment. The primary loop could, in fact, run at atmospheric pressure. Each DCR in a power plant would be small, only four feet wide, and generating about 227 megawatts of heat. A power plant would consist of 20 reactors, each encased in a 60-foot-long tube, sunken into the ground. It was almost too good to be true. The Atomic Energy Commission applied for the patent in 1961.
By August 1960, a mockup of the DCR had been built at the Los Alamos Science Laboratory and the new subsystems for the reactor were being tested. In May of 1962, a functioning DCR, code-named the Pint Bottle Experiment (PBX), was ready to be built. The budget for experimental reactors, however, was collapsing as the research projects were growing numerous. The nuclear-reactor market was expanding all by itself, without any DCRs, PBXs, or LAMPREs to help it along. The need for anything better than a Westinghouse PWR or a General Electric BWR was vanishing. Not only was there enough uranium to run the world into the far future, but the cost of it was dropping fast. PBX died on the drawing board. A new report predicted that by the year 2000, half of the electricity used in the United States would be generated by nuclear fission.
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A bridge between LOPO and LAMPRE was the Los Alamos Power Reactor Experiment (LAPRE) in 1955. Its liquid fuel was uranium oxide dissolved in phosphoric acid. The fuel was so corrosive, the entire inside of the primary loop, including the reactor vessel, had to be pure gold. The experimental budget of the Los Alamos Scientific Laboratory in the 1950s could cause heart palpitations.
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That was the plan, but there was a lot to learn about molten fuel, and in reality LAMPRE-1 started out smaller and less complicated than was laid out in the proposal. The power was dialed back from 60 megawatts to 1 megawatt, and the core structure was reduced to tantalum tubes containing the molten fuel. Unforeseen problems turned up, as are expected in a leading-edge experimental program, but much was learned in two fuel loadings and three years of experience. The project was terminated in 1964, before the third fuel loading could be tested, and LAMPRE-2 was changed to the Fast Reactor Core Test Facility (FRCTF). The FRCTF project was abandoned, 70 % completed, as was the Molten Plutonium Burnup Experiment (MPBE). The light-water reactors from Westinghouse and GE were succeeding beyond the Atomic Energy Commission’s wildest dreams, and everything else fell by the wayside.