A nuclear pioneer’s work on safer, cheaper, inexhaustible nuclear power is still inspiring nuclear environmentalists.
by Robert Hargraves
Physicist Alvin Weinberg worked on the Manhattan Project and later co-invented the pressurized water nuclear reactor. As Director of Oak Ridge National Laboratory he led development of liquid fuel reactors, including walk-away-safe liquid fluoride thorium reactors with inexhaustible fuel. Today such cheap, safe, clean energy has the potential to economically displace worldwide coal burning, inspiring many efforts to implement Weinberg’s achievements.
Alvin Weinberg was a Chicago product, born there in 1915, educated in Chicago schools, attending the University of Chicago, earning BS, MS, and PhD degrees in physics. Ironically, his master’s thesis dealt with the infrared absorption spectrum of CO2, presaging his later efforts to warn of global warming. His PhD work in cell metabolism taught him about diffusion, which turned out to be applicable to neutron diffusion, of interest to the Manhattan Project.
Obtaining his PhD in 1939 he joined the University Chicago Metallurgical Laboratory, conducting work for the nascent Manhattan Project. There he rubbed shoulders with physicists Edward Teller, Leo Szilard, and Nobel-prize-winners Arthur Compton, Eugene Wigner, and Enrico Fermi.
Fermi led the project to demonstrate the first nuclear chain reaction. Neutrons from fission of uranium-235 needed to be slowed down to have a good chance of fissioning more uranium-235 before escaping. Collisions with carbon atoms slowed neutrons, so Fermi designed a lattice of graphite and uranium, separated so that the slowing took place outside the uranium, where the neutrons might be absorbed by uranium-238.
After many experiments Fermi developed a design in which the criticality constant k, the ratio of the number of neutrons produced to those absorbed, was 1.007 – enough to sustain a chain reaction. Weinberg didn’t get to see the December 1942 criticality demonstration of Chicago Pile 1 under the football stadium at Stagg Field; his priority number was 54 out of 50. Fermi’s first pile produced 0.5 watts so needed no cooling.
Meanwhile Wigner had been leading a team including Weinberg to design a large reactor for making plutonium for an atomic bomb, a surprise to Weinberg. Extra neutrons from a uranium-235 chain reaction would be absorbed by uranium-238, which became fissile plutonium-239. This reactor, which would generate 100 megawatts of heat, did require cooling. Helium gas, molten bismuth, and water coolants were studied and water was selected even though it absorbed some precious neutrons. The design was done 5 months before Fermi’s criticality demonstration. The final reactor, producing 500 grams of plutonium per day, requiring cooling of 500 megawatts, was built at Hanford, Washington, in 1944.
Before the Hanford reactor was completed, the Army wanted a pilot plant to create small quantities of plutonium and to test the chemical reprocessing to extract it. Wigner assigned Weinberg to design this one-megawatt, air-cooled, graphite-uranium-lattice reactor. It became operational 9 months later in November 1943, hosting the world’s second man-made nuclear chain reaction, successfully demonstrated isolation of the plutonium.
Deuterium is a form of hydrogen that has both a proton and a neutron. Heavy water, D2O, is much less likely to absorb precious neutrons. While DuPont was building the Hanford reactor, Wigner and Weinberg designed a backup alternative – a heavy-water-moderated, light-water-cooled reactor. In 1944 they discovered that the extra neutrons saved from absorption by H2O meant the design could dispense with the elaborate graphite-uranium lattice used to prevent loss of too many neutrons to the uranium-238. Rather the uranium could be mixed homogeneously with the heavy water, with k = 1.08. “Gone would be the thousands of carefully machined uranium slugs; gone, too, would be the intricate system of piping…”. This sparked Weinberg’s lifelong interest in fluid fuel reactors, temporarily deferred.
Breeder reactors and thorium
Although the Hanford reactor made plutonium-239 for the atomic bomb, some of it was undesirably converted to plutonium-240 by neutrons in the reactor. The contaminating plutonium-240 spontaneously fissioned, emitting neutrons that would pre-detonate the bomb before two plutonium metal chunks were explosively forced together sufficiently. Alternatively, in 1944 Wigner proposed the idea of a reactor that would fission the plutonium, using the generated neutrons to breed thorium-232 to fissile uranium-233, possibly suitable for a bomb. This was not done, for Robert Oppenheimer built a spherical implosion device that did compress the plutonium together fast enough.
But Weinberg had learned about uranium-238/plutonium-239 and thorium-232/uranium-233 breeding and understood the potential of thorium as a nuclear fuel. Later Wigner designed a uranium/plutonium fast breeder reactor with unslowed, fast neutrons, while Weinberg designed a intermediate breeder, with partially slowed neutrons. Wigner never liked his fast breeder design with its huge plutonium core and multiple critical masses. He preferred the thermal breeder with a slurry of thorium and uranium-233 particles in heavy water, actually built in the Netherlands in 1974. The liquid-fuel thorium thermal breeder idea dominated Weinberg’s thinking for many years.
Pressurized water reactor
Weinberg returned to the idea of a light-water reactor, discovering that although k = 0.96, it was tantalizingly close to 1.0 needed for a chain reaction. A tank of ordinary water with fuel rods containing uranium slightly enriched beyond the natural 0.7% uranium-235 isotopic composition could sustain a chain reaction.
Thus Weinberg invented the idea that water could be both moderator and coolant, if uranium were slightly enriched. He wrote, “such a system…would probably be much more compact and consequently simpler to build”. In his September 18, 1944 memo he also invented the idea of harnessing nuclear energy for power, “…it may be possible to run such a system under pressure and obtain high-pressure steam which could be used for power production.” This pressurized water reactor design first harnessed the explosive force of atomic power. In a 1946 paper Weinberg wrote, “We actually described the thorium power breeder that was built at Shippingport, Pa, some twenty-five year later.”
Rickover, the Nautilus, Shippingport, and commercial nuclear power.
In 1944 Hyman Rickover’s team came to Oak Ridge to learn of the potential of nuclear power for the US Navy. Rickover favored sodium-cooled reactors, but Weinberg convinced the Navy that the simpler, more compact pressurized water reactor (PWR) would fit better in a submarine. In 1955 Rickover’s atomic-powered Nautilus was launched.
“Thus was born the pressurized-water reactor—not as a commercial power plant, and not because it was cheap or inherently safer than other reactors, but rather because it was compact and simple and lent itself to naval propulsion.” Weinberg went on, “It was chosen for Shippingport after President Eisenhower had vetoed the Navy’s proposal to build a nuclear aircraft carrier powered by a larger version of the Nautilus power plant. A demonstration of a power plant that would operate as part of an electrical utility was being urged by the Atomic Energy Commission. The only reactor that was on hand was the one designed for the canceled aircraft carrier.” A hundred commercial PWR-style electric power plants were consequently built by US utilities and staffed largely by veterans of the Navy’s nuclear submarine corps. Weinberg was long astonished at the resulting 100% US market dominance.
Although “Rickover’s thorium-based U-233 seed-blanket light water breeder” at Shippingport also demonstrated a 1.01 breeding ratio, producing more fissile fuel than it consumed, Weinberg was disappointed that the public hardly noticed this proof that the world had an inexhaustible energy source – thorium.
Oak Ridge National Laboratories’ liquid fuel reactors
Wigner returned to Princeton and in 1948 Weinberg became associate director, research director, then laboratory director of Oak Ridge National Laboratories (ORNL). Weinberg felt that the liquid fuel reactors they had conceived were much simpler than the Swiss-watch-like sodium-cooled fast breeder reactor technology advanced at Argonne National Laboratories back in Chicago. Furthering development of the thorium-uranium liquid-fuel breeder required chemical expertise, fitting the talents at ORNL, so he made this the lab’s goal.
Rather than removing, reprocessing, and replacing solid fuel rods multiple times, ORNL pursued the idea of putting the thorium and uranium compounds in solution in heavy water. The fission products could be removed continuously; noble gas xenon could bubble out; solid fission products could be removed by centrifugal separation. Wigner and Weinberg had come to this idea while conceiving the uranium-thorium breeder reactor, but ORNL first implemented it as a uranium-only power reactor. ORNL scientists learned that uranium sulfate would be stable dissolved in water at the 250°C operating temperature, pressurized to 67 atmospheres.
This first liquid fuel reactor began operation in 1953. This aqueous reactor at ORNL fed 140 kW into the electric grid for 1000 hours. In operation it successfully removed xenon fission products. The intrinsic reactivity control was so effective that the reactor was idled simply by turning off the steam turbine generator. Weinberg called this the “forerunner of a true thermal breeder”.
Aircraft Reactor Experiment
The Atomic Energy Commission (AEC) put ORNL out of the reactor development business in 1948, only to be promptly returned to it because the Air Force wanted a nuclear-powered airplane. Powering a jet engine requires red-hot, 860°C heat. A PWR achieves only about 315°C temperature. Weinberg’s team hit on the idea of a molten mixture of zirconium and sodium fluoride salts, in which would be dissolved the fissile uranium fluoride fuel. The stable ionic fluoride salts did not corrode stainless steel, and the salt would stay liquid at atmospheric pressure even at 1400°C. An ORNL team of chemists tested and studied various molten salt compositions, the solubility of uranium fluorides, and many alloys of nickel, chromium, iron, and molybdenum that could be pipes, vessels, and pumps. In 1954 this Aircraft Reactor Experiment produced up to 2.5 MW of thermal power at red-hot 860°C for 100 hours. It demonstrated intrinsic reactivity stability, automatically adjusting power with no control rods, as the heat exchanger airflow varied.
This ARE success led to the design of the compact, 200 MW Fireball reactor to power jet engines of an airplane, however President Kennedy cancelled the aircraft nuclear propulsion project after his 1960 election.
Molten Salt Reactor Experiment
Weinberg continued pursuit of the thorium-uranium breeder goal. ORNL designed a 10 MW molten salt reactor with uranium fluoride dissolved in molten fluoride salts of lithium and beryllium. By 1966 the 7.5 MW Molten Salt Reactor Experiment (MSRE) began operation, continuing until 1969. This prototype did not include thorium-uranium breeding. It was tested with uranium-235 and then uranium-233 bred from thorium in other reactors. No turbine generator was attached; the fission energy heat was dissipated with a salt-to-air radiator.
MSRE was a success. Fission product xenon gas was continually removed to prevent unwanted neutron absorptions. Online fuel addition was demonstrated. Minor inter-grain boundary corrosion of the Hastelloy vessel, piping, and heat exchanger was later addressed. Oak Ridge also developed chemistry for separation of thorium, uranium, and fission products in the fluid fluoride salts. Fluorination and distillation processes could separate fission products from the salt.
Weinberg was thrilled with the success that would lead to inexhaustible energy. ORNL then developed a conceptual design for the Molten Salt Breeder Reactor for sustainable commercial power generation. Considering the burgeoning global population demand for resources, he wrote, “humankind’s whole future depended on the breeder”. His Energy as the Ultimate Raw Material described applications of cheap energy: desalination, gasoline from coal, ammonia, iron by electrolytic reduction, chlorine, steel, and aluminum production. Burning the Rocks pointed out that ordinary dirt contains thorium with energy content far exceeding that of the same amount of oil.
But Weinberg’s dream was not to be achieved in his lifetime. The Oak Ridge work was stopped when President Nixon decided instead to fund work on the solid-fuel liquid-metal fast breeder reactor in California. Weinberg wrote to NRC Commissioner Glenn Seaborg, “Our problem is not that our idea is a poor one, rather it is different from the main line and has too chemical a flavor to be fully appreciated by non-chemists.” Later Weinberg said “It was a successful technology that was dropped because it was too different from the main lines of reactor development.” Colleague Herbert MacPherson explained, “Political and technical support is too thin geographically. Oak Ridge is the only stakeholder.”
Weinberg and Wigner knew that loss of water cooling of the graphite-moderated, war-essential plutonium-production reactors at Hanford might cause them to blow up and spread radioactive materials, later evidenced at Chernobyl. In 1947 these reactors were replaced by ones with temperature-reactivity stability. Back in 1942 Weinberg and Teller were concerned with and had computed possible radioactivity releases for a air-cooled graphite reactor. Edward Teller made reactor safety a central element of reactor engineering, leading in 1948 to the Advisory Committee on Reactor Safeguards, still operating today.
Weinberg directed ORNL to become engaged in nuclear-safety research, by 1959 establishing the journal Nuclear Safety. A hundred scientists and engineers were engaged in safety research at ORNL. Before that time the pressurized water reactor had been deemed safe because there were three barriers between the public and radioactive materials: (1) the zirconium solid fuel cladding, (2) the pressure vessel, and (3) the containment vessel or building.
But as reactors became larger “million-kilowatt monsters”, ORNL expressed concerns that in a loss-of-cooling accident, shutdown residual afterheat might breach all three barriers. Weinberg wrote, “we had to argue that, yes, a severe accident was possible, but the probability of its happening was so small that reactors must still be regarded as safe. Otherwise put, reactor safety became probabilistic, not deterministic.” Analyses of common-source failure modes of supposedly-independent safety features forced extremely expensive back-fitting and emergency core cooling systems. The lack of ECCS forced the closure of Indian Point 1 outside New York. By 1972 reactor safety became a primary source of contention among the industry, the AEC, interveners like Union of Concerned Scientists, ORNL, and Weinberg.
Hyman Rickover’s protégé Milt Shaw was director of the AEC Division of Reactor Development and Technology. Shaw had the confidence of California representative and committee chair Chet Holifield. In 1970 Holifield had blown his stack at Weinberg’s efforts with Senators Howard Baker and Edmund Muskie to establish a National Environmental Laboratory at ORNL because Holifield “didn’t want nuclear labs tainted with the environmentalist brush”. Shaw had also stopped MSR development in favor of the LMFBR. Weinberg’s pursuit of nuclear safety led to a 1973 meeting where Holifield told him, “Alvin, if you are concerned about the safety of reactors, then I think it may be time for you to leave nuclear energy.” Weinberg was fired shortly thereafter. The Three Mile Island accident occurred six years later.
Aside from editor: The Three Mile Island accident proved to many experts that Weinberg’s concerns about reactor safety were exaggerated. Despite numerous equipment failures, minor design faults and incorrect actions, the multiple barriers did their job and protected the pubic from harm.
The recently completed State of the Art Reactor Consequences Analysis (SOARCA) indicates that light water reactors built to US licensing standards are safe. They will not harm the pubic even in the case of numerous failures in safety systems. End Aside.
After a stint as Director US Office of Energy Research and Development in 1974 Weinberg had managed to found the Institute for Energy Analysis (IEA) at Oak Ridge Associated Universities, concerned with the future of energy. IEA invented today’s energy-return-on-investment (EROI) analysis concept.
In 1976 at IEA Weinberg rather accurately predicted the 21st century climate crisis, “….atmospheric concentration of 375-390 ppm may well be a threshold range at which climate change from CO2 effects will be separable from natural climate fluctuations … The consequences of an increase of this magnitude in atmospheric CO2 make it prudent to proceed cautiously in the large-scale use of fossil fuels.”
“So I went from office to office in Washington, curves of the carbon dioxide buildup in hand… I reminded them that nuclear energy was on the verge of dying. Something must be done. I almost screamed.” For eight years IEA was the center for CO2 and climate matters, summarized in the 1982 Carbon Dioxide Review.
Inherently safe reactors
Instead of festooning reactors with safety systems, Weinberg pursued research into reactors with intrinsic or passive safety systems. “Can we develop nuclear reactors whose safety is deterministic, not probabilistic, and which, if developed, would meet the public’s yearning for assurance of safety, not simply assurance of the probability of safety?” Even after Three Mile Island the Department of Energy was not interested, but the Mellon Foundation funded the work.
One result was the PIUS (Process Inherent Ultimately Safe) reactor from Sweden. It had a gigantic concrete vessel with no failure-sensitive cooling rods. Cooling water circulated atop a pool of dense water containing boron, which would absorb neutrons and quench the chain reaction if coolant circulation stopped, causing the borated water to mix. Safety depended on the fundamental laws of thermodynamics, not control systems.
Another design endorsed by Weinberg and cohorts was the high temperature gas-cooled pebble-bed reactor designs by General Atomics, Germany’s Siemens-KWU, and later by Adams Atomic Engines, PBMR Pty Ltd, and Tsinghua University. The overheat safety has been demonstrated for loss-of-coolant accidents in Germany and China. Similar safety demonstrations with loss-of-cooling have been conducted with the US EBR-2 liquid-sodium fast-breeder reactor.
Today’s new PWRs incorporate passive safety features. The Westinghouse AP1000 overhead water reservoir can cool a powerless reactor for three days after shutdown. The B&W mPower continues passive cooling for fourteen days on battery power and large volumes of stored water. The smaller NuScale reactor continues with air cooling indefinitely after its water reservoir boils away.
Weinberg colleague Edward Teller, who had headed the Advisory Committee on Reactor Safety, became interested in reactors that were inherently so safe that schoolchildren could use them. Weinberg taught nuclear reactor physics to Princeton’s Freeman Dyson and others at General Atomics, where Dyson and Teller led a project that developed such a reactor. The 10 MW TRIGA reactor used uranium zirconium hydride (UZrH) metal fuel. Moderation of neutrons takes place with the hydrogen in the metal as well as the hydrogen in the water. If the metal fuel overheats, the neutrons do not slow down effectively and are captured before they cause fission, stopping the chain reaction. Rapid removal of the reactor control rods will peak the power to 22,000 MW, but the intrinsic reactivity control turns off the reactor in milliseconds. Today at Reed College the TRIGA operators are undergraduate students.
Weinberg’s molten salt reactors at ORNL had also demonstrated temperature stability when cooling was cut. Additionally a molten-salt freeze plug would melt if overheated and dump molten salt into a drain tank where the reaction stopped. Both reactivity-temperature stability and the freeze plug relied on immutable physical properties, not control systems.
Working with Ralph Moir at Lawrence Livermore National Laboratory, Edward Teller gained interest in walk-away-safe molten salt reactors. Together they proposed an underground version of a thorium-fueled MSR. Teller died in 2003 just before the paper was published.
Weinberg’s nuclear environmentalists
Weinberg’s work on a totally different sort of safer, cheaper nuclear reactor continues to inspire many people to pursue this alternative to the PWR.
Ralph Moir continues studies of these fluid fuel reactors, advising three start-up ventures who seek to bring the technology to commercial success.
Kirk Sorensen was formerly a NASA employee researching nuclear power plant designs for a moon base. He discovered molten salt reactor R&D document dormant in ORNL’s records, published them on the Internet, and later founded Flibe Energy to commercialize the technology here on earth.
Robert Hargraves and Ralph Moir published Liquid Fluoride Thorium Reactors in American Scientist in 2012, sparking China’s Academy of Science to undertake a $350 million development project.
Canadian David Leblanc writes and presents articles as well as starting up Terrestrial Energy, which may first harness process heat from a commercial MSR.
Recent MIT PhD Leslie Dewan and student Mark Massie founded Transatomic Power to promote an MSR design moderated with zirconium hydride.
Other, quiet private ventures continue in Florida, New York, and South Africa.
The French government has funded MSR R&D for several years and the group at Grenoble has published much leading research.
The US DOE has provided funding for university nuclear science programs at MIT, UC Berkeley, and U WI Madison, which along with ORNL research the use of molten salts in reactors, primarily as a high temperature coolant.
Seeking “safe, clean and affordable nuclear energy technologies to combat climate change and underpin sustainable development for the world”, the London-based Weinberg Foundation’s “core objective is to rapidly re-catalyse the research, development and deployment of MSRs first designed, built and proven by Alvin Weinberg”.
All these people and organizations have similar goals. Cheaper nuclear power can discourage construction of CO2-emitting coal and natural-gas electric power plants. Ending particulates from burning fossil fuels can save lives of millions of people. Affordable electric power can end energy poverty and develop lifestyles that include diminishing birthrates, reducing contention for natural resources. As petroleum-sourced fuels become more expensive, low-cost, high-temperature energy sources may be used to fabricate economic, carbon-neutral, synthetic vehicle fuels.
Inexhaustible nuclear power emits no CO2, disturbs a tiny fraction of the land of coal mining, and has the smallest number of deaths per unit of energy produced of any energy source. When the cheaper, safer power from fluid fuel reactors enables these benefits, we will thank Alvin Weinberg.
About the author
Robert Hargraves, has lived a life of achievement including writing a well received book titled Thorium: Energy Cheaper than Coal, founding a business, serving as Chief Information Officer for Boston Scientific, serving as an assistant professor and associate director of the computation center at Dartmouth College, publishing numerous peer-reviewed articles on a variety of topics and earning a PhD in Physics from Brown University. He is the author of “Radiation: The Facts.”
Much of this material comes from Alvin Weinberg’s 1994 memoir, The First Nuclear Era. It’s newly available in a Chinese translation. http://www.amazon.com/The-First-Nuclear-Era-Technological/dp/1563963582/
Syd Ball and Richard Engel, who worked for Weinberg, discuss the MSR at dinner. https://www.youtube.com/watch?v=ENH-jd6NhRc Syd Ball Richard Engle
Ralph Moir and Edward Teller’s proposed underground thorium molten salt reactor http://www.ralphmoir.com/wp-content/uploads/2012/10/moir_teller.pdf
Robert Hargraves’ book on the world climate/energy/poverty crises and the liquid fluoride thorium reactor, http://www.thoriumenergycheaperthancoal.com.
Oak Ridge Associated Universities, Remembering a Nuclear Pioneer: Alvin Weinberg, http:// http://www.orau.org/weinberg/default.html