Molten Thorium Salts: Nuclear Power For The Future?
by Donald Goldsmith
For sixty years, the use of nuclear energy generation to provide clean, safe, and inexpensive electrical energy has remained a dream deferred. Conventional nuclear reactors have generated dangerous nuclear waste; they have suffered meltdowns that have come close to devastating nearby regions and scattering nuclear debris over large distances; and have cost far more than originally projected. The reactor accidents at Three Mile Island in 1979 in Pennsylvania, in 1986 near Chernobyl in the Ukraine, and in 2001 at Fukushima in Japan raised fears around the world, even as hundreds of other nuclear-power plants have continued to function without serious incident. Thirty countries now rely on nuclear reactors to produce electricity, most notably in Belgium and Slovakia, where they provide more than half of the country’s electrical power, and in France, where they supply fully 75 percent of the total.
Decades of scientific and technological investigation demonstrate an opportunity for generating electricity with nuclear reactors that would run more safely, more manageably, less expensively, and with less radioactive waste than those now in operation. How is it that you have almost certainly never heard of this alternative, and that none of these reactors has been constructed on anything approaching a commercial scale?
One part of the explanation resides in the mammoth investment in constructing the nuclear-power plants already in existence, facilities so expensive that their construction costs more than the far-from-cheap uranium fuel required for several decades of their operation. Another lies in the fact that despite their hazards, and despite the difficulty of disposing of their nuclear waste safely, these facilities produce electrical energy without generating the carbon-dioxide and other waste that coal- or oil-fueled power plants do. However, the radioactive spent fuel from uranium power plants decays over time scales measured in tens or hundreds of thousands of years, far longer than can be safely contained with conventional shielding. To deal with this problem, the United States investigated, sited, designed, and built the Waste Isolation Pilot Plant (WIPP) in Eddy County, Nevada. WIPP hollowed out a deep underground cavern within a geological formation judged stable over long eras of time. However, concerns of safety (just how stable is the formation?) and nimby (not in my backyard) have led to the government’s placing a hold on any disposal of nuclear waste in the WIPP facility. In late August of this year, the Nuclear Regulatory Commission gave permission for power-plant nuclear waste to be stored indefinitely above ground, and for new nuclear-power plants to be constructed, and older plants expanded, on this basis. In France, an underground waste-disposal facility near the German border seems nearly complete, though there too environmental and safety concerns remain high. Not one of the thirty nuclear-power-using countries has a functioning underground storage site—quite a remarkable feat after five decades of power-plant operation.
Thus despite their advantages, nuclear-power plants understandably pose enormous technical, political, and operational problems. But what if—marvelous in the telling—we had another avenue to nuclear power, free from most of the dangerous short- and long-term problems arising from the nuclear facilities now in operation? Without going completely overboard, we may recognize that such an avenue exists: molten salt nuclear reactors that use thorium as their fuel.
All nuclear-power generating plants draw energy from radioactive isotopes, specific varieties of atomic nuclei that change from one form into other (“decay”) with the release of fast-moving fragments plus gamma rays (the highest-energy cousins of the photons that form visible light). The fragments and the gamma rays impart their energy to whatever they encounter. In almost all nuclear-reactor designs, this newly-released energy heats molecules of water contained within a circulating stream that passes its own heat by close proximity to another vessel of circulating water, which produces steam that can turn a turbine, as in a conventional coal- or gas-fueled power plant. The chief immediate danger in using nuclear decay to generate heat (as opposed to the long-term problem of disposing of nuclear waste) lies in the details of how the isotopic decays are managed. A secondary danger resides in the high-pressure vessels carrying the water that brings heat from the radioactive-decay sites to the streams of water that turn to steam to run the turbines. The first of these problems rightly draws the most attention: Provoking and managing the release of energy through isotopic decay is a risky business.
Conventional nuclear-power plants are fueled with the rare isotope of the element uranium denoted as uranium-235, more than a hundred times scarcer in nature than uranium’s non-radioactive isotope, uranium-238. (Like all nuclei, uranium-235 and uranium-238 nuclei consist of protons and neutrons. The number of protons determines the element (uranium isotopes all have 92 protons) and the number of neutrons specifies the particular isotope (143 or 146 for these two uranium isotopes). Unlike uranium-238, which is stable, uranium-235 isotopes will decay, taking on average 700 million years to do so. However, a neutron that strikes uranium-235 will split that nucleus (we call this “nuclear fission”) into isotopes of barium and krypton, and will also produce more than one neutron. Hence any sufficiently large cluster of uranium-235 isotopes stands ready for a chain reaction, in which each isotopic splitting provokes the nearby nuclei to split, releasing more energy and more neutrons that in turn induce additional nuclear fission. With a sufficiently rapid chain reaction, you have a bomb; keep the reaction under strict control, and you have a uranium-powered nuclear reactor. There!—nuclear physics in a nutshell.
To deal with the neutrons that appear when the uranium-235 isotopes split, a nuclear-power plant intersperses “control rods,” packed with neutron-absorbing isotopes of boron or cadmium, among the “fuel rods” containing uranium-235. These control rods can be withdrawn partially or fully from their positions among the uranium fuel rods. Keep all the control rods at their neutron-absorbing best, and nothing will happen; take them entirely out, and you will have something close to a uranium bomb. Maintain them in just the right position, and the plant will hum along perfectly, generating a constant amount of heat from the neutron-induced fission of its uranium isotopes.
Two problems emerge here. The first, already discussed, is the waste-product dilemma: After a time, the uranium fuel rods can no longer generate much energy, even though they still contain large amounts of uranium-235, because the waste products that they have produced absorb neutrons and damp the chain reaction. At this point, the fuel rods are loaded with radioactive (and toxic) isotopes of the elements uranium and plutonium, which require extreme care for removal and for storage over stretches of time that span many millennia. The second, “Homer Simpson problem,” resides in the danger that something will go wrong with the positioning of the control rods, leading to a runaway chain reaction and an explosion that scatters radioactive waste over many kilometers of surrounding countryside and, through air- and water-borne contamination, over thousands of kilometers worldwide.
How can molten-salt nuclear reactions solve these twin problems? As their name implies, molten-salt nuclear reactors keep their isotopic fuel in a near-liquid state, which, as we shall see, proves much safer to handle. In addition, the reactors’ basic fuel consists of isotopes of thorium-232. Thorium-232 (the only variety of thorium naturally in existence) qualifies as barely radioactive, but when neutrons strike thorium-232 nuclei, they turn them into nuclei of uranium-233, which undergo fission similar to that of uranium-235. A molten-salt reactor qualifies as a “breeder”: Neutrons generated by a small amount of radioactive uranium or plutonium isotopes will turn the thorium into more uranium isotopes, which produce more neutrons as they split apart. One again, the reactor achieves a controlled chain reaction that releases energy.
So far this design seems simply a roundabout way to mimic what happens within a conventional nuclear reactor. But a molten-salt reactor has at least three crucial advantages. First, the near-liquid state of molten salt provides built-in safety from the laws of physics. If the temperature of the salt rises, it will expand. This will reduce the rate at which the chain reaction proceeds, because the uranium-233 isotopes will now be farther apart. Therefore, if the thorium- and uranium-rich salt happens to generate more energy than desired, the extra energy will make the salt expand and decrease its rate of energy production. This sort of self-regulation allows the sun and other stars to maintain a constant rate of nuclear-energy generation: If a star produces too much energy at its center, the extra energy will expand the stellar core, reduce the rate of energy generation, and restore the previous balance.
Second, as a backup on the Homer Simpson front, a molten-salt reactor can be designed with a plug of solid salt at the bottom of its container. If the control mechanism for the reactor should lose all power, with the result that the molten salt begins to overheat, it will melt the plug and drain into an underground holding tank. The energy-generating process will cease, and the molten salt will solidify quickly. Such a reactor cannot experience a “meltdown” analogous to what can occur in a conventional nuclear reactor’s fuel rods, since the working fluid has already melted.
Finally, the fact that the nuclear fuel exists in near-liquid form allows much easier removal of old fuel and adding new fuel to the mixture. In a conventional nuclear reactor, operation must cease at intervals so that new fuel rods can replace the old ones, which must be carefully transported for eventual safe storage—where? In contrast, the liquid within a molten salt reactor can be circulated in and out of the main site of activity without disrupting the reactor’s operation. This allows the reactor to function with little down time, and to generate almost no nuclear waste, because the fuel can be recycled into the reactor until it achieves near-zero activity. (Furthermore, a reactor based on uranium-233 produces far less plutonium than one that uses uranium-235, basically because uranium-233 isotopes lie farther from plutonium (mostly plutonium-239) along the chain of reactions that produce these highly radioactive and toxic isotopes.)
To these advantages we should add the additional facts that thorium is about four times more abundant than uranium—you can find it in beach salts called monazite all over the world—and all this thorium consists of thorium-232 isotopes. In comparison, only 0.7 percent of uranium consists of uranium-235 isotopes. Fueling a reactor with thorium-232 therefore requires far less mining, processing, and waste disposal than a conventional reactor does to obtain its uranium-235.
Why, then, haven’t we heard more about molten-salt reactors, or seen a single one beyond some small test facilities? Here history provides some intriguing answers.
The molten-salt reactor concept originated with the United States’ military forces’ desire to design a nuclear reactor that could power a bomber aircraft, the counterpart to the nuclear-powered submarines already being developed. (Today this seems rather ridiculous from the start, given the impressive weight of any nuclear reactor). During the 1960s, research in this effort shifted to the Oak Ridge Laboratory in Tennessee, where the uranium-235 isotopes for the first atomic bombs had been separated from uranium-238 during the Second World War. Seeking to produce a safer nuclear reactor, scientists and engineers at Oak Ridge hit on the idea of keeping its fuel—radioactive isotopes of uranium or thorium—in molten salts, made primarily of isotopes of the light elements lithium, beryllium, and fluorine. Carrying the fuel within a molten salt would allow radioactive isotopes to be added or removed with relative ease, along with the other advantages discussed above.
During the 1960s, Alvin Weinberg, the director the Oak Ridge National Laboratory, strongly advocated for molten-salt reactors. In 1973, after 18 years as director, he suddenly retired. According to Weinberg’s memoir, “we were being troublesome over the question of reactor safety” to the point that he was told that perhaps he should resign—and he did. Research on molten-salt reactors within the United States, and indeed throughout the world, essentially vanished for three decades. Only in the new millennium did researchers seriously begin to investigate whether commercially-scaled, molten-salt reactors could someday replace the solid-uranium plants that utterly dominate the nuclear-power industry.
Scientists and engineers are now considering more effective ways to use the power that a molten-salt reactor can produce. For example, heat from the reactor can be applied to biomass from recently grown plants or trees, converting it into “biochar” (charcoal used for a particular purpose), which offers a highly effective means of storing and transporting energy. (The growth of the biomass removes the same amount of carbon dioxide that the fuel consumption releases.) Alternatively, the heat can separate water molecules into oxygen and hydrogen, with the hydrogen atoms providing an excellent way to store energy.
What stands between our current situation and a world in which much safer, molten-salt reactors provide most of the electrical power for our civilization? Neither a lack of thorium, nor an understanding of how such reactors would function, nor a refusal to appreciate the far greater safety that these reactors would provide. We face a much larger problem: The enormous expenses involved in designing and constructing new types of nuclear reactors, and in decommissioning the old ones (though this must happen in any case—they have operational lifetimes measured in decades, not centuries). What would it take, for instance, to persuade France, where nuclear reactors provide three-quarters of all electrical power, or the United States, where the relevant fraction remains just under one-fifth, to spend billions of dollars to reexamine and to rebuild their nuclear energy-generating installations? Inertia remains an enormous drag on our ability to bring ourselves into the bright future suggested by the discovery of nuclear fission. Can we get there? Do we have the will to try?