There has been a lot of noise in the alternative energy camp these last few years about a wonderful new source of energy that some claim will enable us to thrive in a post-carbon world. A couple of pegs down on the periodic table from that classical atomic age element known as uranium, this super fuel is much more abundant, and will arguably be much safer to work with in the nuclear plants of the future. Christened after the Norwegian Thunder God without a hint of irony in a time long before its potential in sparking an energy revolution was recognized, to say nothing of the concept of radioactivity itself, thorium (element 90) has rekindled the imaginations of atomic scientists worldwide.
By far, the most common form of thorium has 142 neutrons, giving it an atomic weight of 232. In this form, thorium is extremely stable, with a half life approximating the age of the universe. But by flinging a free neutron at its core, scientists can transform the thorium atom, however briefly, to Th-233, a highly unstable isotope. Thus conceived, the germ beta-decays in a matter of minutes to protactinium-233, and four weeks later, to uranium-233, a highly fissile form of uranium. (In beta decay, a neutron turns into a proton, while spitting out an electron and an anti-neutrino.) Fissile uranium-233 splits into multiple elements lower down on the periodic scale, and releases two or three neutrons that continue the reaction with other thorium atoms. The process is ultimately self-sustaining, and will, in theory, continue until the thorium and uranium supply has completely run dry. If so, the problem of waste disposal that so plagues the nuclear industry may vanish; even current nuclear wastes may be cleaned up if used properly in a thorium reaction.
Uranium-238 is the most common form of that element, and in that form is almost as stable as thorium itself. U-235, a highly fissile form of uranium, is also present in any given sample of U-238, and various techniques can be used to enrich the substance to bring its ratio of U-235 to the 3-5% necessary for sustained reaction. The most common nuclear power plant designs in the USA require enriched uranium rods, where reactions are moderated by light (normal) water. Free neutrons must be slowed down in order to glom onto a U-235 atom to trigger fission, and these reactors are thus dubbed “slow reactors”. By contrast, the “fast” CANDU reactors in Canada, while a much more challenging design, work with natural U-238 rather than its enriched form, and require fast free neutrons to trigger a reaction with that element. Heavy water (deuterium, which is packed with surplus neutrons) is necessary in these designs as it does not absorb free neutrons.
The present-day ubiquity of uranium-fueled nuclear plants in North America is in many ways a historical artefact of World War II. Scientists of that time trained explicitly to use their knowledge of atomic characteristics towards destructive ends. The Manhattan Project was a massive undertaking, and one of the most expensive the world has ever known, with sites across the USA designated for specific aspects in the design of the bombs ultimately used against two Japanese cities. Post-war nuclear scientists were justifiably uncertain about their career futures; at this point, little discussion centred on the peaceful use of nuclear technology, though there were coffee-table chats and late-night dreams about the energy potential of uranium, plutonium and, yes, thorium. The question of whether or not the secrets of nuclear technology should be made available for civilian use was a hot debate topic until the military stepped in and demanded designs that could power next-generation submarines.
Progenitor of early light-water uranium reactor designs, Alvin Weinberg (1915-2006) began his career working on methods to produce the plutonium for the Fat Man bomb that obliterated Nagasaki. By 1948 he had become research director at Tennessee’s Oak Ridge National Laboratory, and facility director come 1955. An outspoken critic of the nuclear industry while remaining a strong supporter of the technology, Weinberg is an important figure among today’s thorium enthusiasts because he initiated the plans for liquid-core, thorium-based reactor designs. His 1946 paper “High Pressure Water as a Heat Transfer Medium in Nuclear Power Plants” described a thorium reactor design, and by the end of the decade, he’d shifted away from the inherent challenges faced by pressurized-water reactors to molten salt reactors (MSRs). Only once, in Pennsylvania in 1959, was a molten salt reactor experiment run. For 105 days, then a record for uninterrupted operation of any kind of nuclear reactor, a thorium reactor hinted at a clean future for energy generation. Then, for reasons political, military and traditional, thorium was quietly removed from the stage even as uranium reactors expanded their reach.
Today, the prospective panacea for our energy woes lies in Liquid Fluoride Thorium Reactors (LFTRs or “lifters”), a design that has yet to be realized anywhere outside the fevered imaginations of nuclear engineers. The majority of the kinks of reactor design have been ironed out by a small, loosely connected, but ferociously dedicated group of thorium advocates, working largely independently and free of government interference (or funding). In LFTR design, there is no solid core at all, but a homogenous liquid solution that is home to the nuclear reactions. Because of this, proponents claim that LFTRs are inherently safe. The liquid itself is a mixture of fluoride and U-233, encased in a Teflon-style blanket of solid thorium that requires replacement far less frequently than its exclusively uranium-based competition. As a liquid, the potential for nuclear meltdown is reduced to almost nothing; should a failure occur, the liquid is passively flushed out of the core into a separate tank, and the reaction ceases.
The US seems to have little interest in developing this technology, but the rest of the world has been excitedly shifting towards thorium reactors in recent years. China and India have both jumped on it, and Norway began testing their own thorium reactor at the beginning of this year. Design approaches differ from country to country, and it will be interesting to see which ones ultimately win out.
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