While the Los Alamos lab was suffering from a mismatch of funds-to-ambitions, the Oak Ridge National Laboratory was working on a very interesting mutation of the aircraft reactor. The nuclear bomber had been euthanized as soon as John F. Kennedy was sworn in as president, but lessons learned while operating the ARE led to a new, radical design for a civilian power reactor, the Molten Salt Reactor Experiment (MSRE). It was possibly the most important advance in nuclear-reactor design in the 20th century.
At the end of the nuclear airplane era, the ARE was dismantled, and Oak Ridge secured funding to construct the MSRE in the same building. One fluid, a mixture of salts composed of fluorine compounds of fuel plus neutron moderator, was pumped into the round reactor chamber, where the fuel would fission, and out into a salt-to-sodium heat exchanger. The cooled but molten salt would then re-introduce into the reactor in a loop configuration.[289] Operating temperature was 1,300° Fahrenheit. Steam was then made in a liquid-sodium-to-water heat exchanger. Neither the primary fuel loop nor the secondary sodium loop inflicted any pressure on the reactor structure.[290]
The innovation of this reactor was the fuel. Instead of using uranium or plutonium, it used thorium-232. It turns out that thorium is four or five times more common than uranium, and there is a large reserve concentration of it in the United States. Mined uranium is over 99 % uranium-238, and less than 1 % is the usable isotope, uranium-235. All of the thorium available is thorium-232. There is no unusable thorium in nature. No isotope separation or enrichment is necessary.
Thorium-232 is not a reactor fuel. It will not fission, but upon capture of a neutron, it develops into uranium-233, which is as fissile as uranium-235. The conversion of thorium into uranium can take place in the reactor core, using surplus neutrons produced in the fission process.
When uranium-233 fissions, it produces 10 times less radioactive fission product than does uranium-235. Moreover, the cumulative fission product that it does produce has a half-life 100 times shorter than that produced by uranium-235. The danger of the radioactive waste is gone after 300 years, whereas the waste from uranium-235 fission remains dangerous for 30,000 years.
There was no fuel cladding, no zirconium egg-crates holding the fuel in a rigid matrix, no steam in the reactor vessel to float away with fission products into the atmosphere, and, of course, there was no danger of the fuel melting. In the power-plant embodiment of the MSRE, the fuel would be continuously reprocessed by running the primary loop through a chemical scrubber to extract fission products and inject fresh thorium. An atomic bomb cannot be made from thorium-232, because it does not fission, and it becomes uranium-233 only in the reactor core under neutron bombardment.[291]
The advantages of this reactor design seem overwhelming, particularly after the turn of the century, when we have seen an entire power plant go down because of melted core structures and broken steam systems spreading fission products. These weaknesses that destroyed light-water reactors would not exist in the molten-salt reactor.
The MSRE ran for four years. The program was shut down in 1970, and by 1976 any trace of the reactor was gone. There was no need for an improved way of making power by fission, and all the eggs by now were in one basket. An entire industry, from fuel-pellet manufacture to steam-generator fabrication, had been built around the water-cooled reactor designs used in Rickover’s nuclear navy, and there would be no turning back. We effectively had to dance with the reactor we came with.
It could have been worse. The water reactors were well designed, and they have given us 40 years of reliable electrical power in the United States. A good thing that you can say about water is that it is not sodium, which is an isolated drawback to any molten-fuel reactor concept. The high operating temperature of sodium, which is over 1,000° Fahrenheit, is an advantage for heat-to-electricity conversion, and it makes sodium or sodium-potassium coolant a necessity for reactors using molten metal or salt fuel. If we grudgingly acknowledge the relative dependability of the water reactors, there is one more flaw in the system: where do we put the fission product wastes?
Our system of waste disposal for water reactors is horrifically wasteful and inefficient. We have decided to simply bury everything that comes out of a nuclear reactor core. This amounts to wasting a lot of unburned fuel, along with valuable medical and industrial isotopes. When the fuel comes out of a power reactor and is stored away, 95.6 % of it is uranium, and most of it is harmless U-238. Radioactive nuclides are 0.5 %, 0.9 % is derived plutonium, and 2.9 % is non-radioactive fission products. A tiny fraction of the spent fuel should be buried, but we are set to bury the entire load, without having chemically processed the fuel and separated out what needs to be buried. If we could process the fuel, as nearly every other nation with nuclear power does, it would drastically reduce the volume and weight of the buried material and thus simplify the disposal process.
The problem of waste disposal, while solved, has not been implemented. Although it has been paid for by a coalition of the commercial nuclear power utilities in the United States, the spent fuel repository built under Yucca Mountain in Nevada is currently having trouble accepting fuel deliveries. The state of Nevada has changed its welcoming position to the facility after spending $12 billion of the power companies’ money to study the site and dig the tunnels. Although a federal law designating the Yucca Mountain facility as the nation’s nuclear-waste repository is still in effect, usage of the facility is being blocked. As this controversy continues, nuclear waste builds up in dry storage casks at every light-water reactor in the United States.
The option of processing the waste down into extremely small parcels and easing the burden of burying it was once a goal in the United States. Learning from all previous attempts to process spent reactor fuel as a commercial venture, the Allied Corporation, the Gulf Oil Company, and Royal Dutch Shell combined resources and began construction of a sleek, very sophisticated chemical plant in South Carolina, named the Barnwell Nuclear Fuels Plant. It was fully automated using computer controls and gravity-driven processes through seamless stainless steel pipes and tanks. In 1977 it was nearly completed, and with the permission of the Nuclear Regulatory Commission, it ran a test load of spent fuel through the plant. It performed perfectly, and a license for its operation was assumed to be on the way.
On April 7, 1977, United States President James Earl Carter announced at a special press conference, “We will defer indefinitely the commercial reprocessing and recycling of plutonium produced in the U.S. nuclear power programs. The plant at Barnwell, South Carolina, will receive neither federal encouragement nor funding for its completion as a reprocessing facility.”
289
In the Oak Ridge MSRE, criticality was established in the reactor vessel by sending the fuel through a perforated moderating core made of pyrolytic graphite. A cleaner design would make use of the good moderating qualities of the lithium in the salt and do away with any core structure, but this was the first attempt to build a molten salt reactor, and all advantages could not be accomplished in the initial experiment.
290
The plumbing and reactor vessel did require a special, high-performance nickel alloy to withstand the molten salt. Haynes International developed Hastelloy-N for the MSRE project, and it has found use in other nuclear power applications worldwide.
291
The problem with using U-233 in a bomb is that the bridge nuclide between Th-232 and U-233 is protactinium-233. The Pa-233 is very active, with a half-life of 27 days, and it beta-minus decays into U-233. Unfortunately, its decay also involves a 317 kev gamma ray, and while the half-life is days, all of the protactinium never really goes away. The energetic gamma activity makes it very dangerous to work with, and a lot of shielding is necessary. Inside the reactor primary loop, there is nothing to worry about. The fuel is processed by machinery, with no human interaction; but to make a bomb, a lot of fabrication and machining is necessary. Plutonium, for all its faults, is easier to work with than U-233.