Talk of a large-scale U.S. nuclear renaissance in the post-Three Mile Island era has long been stalled by the high cost of new nuclear power plants, the challenges of safeguarding weapons-grade nuclear material, and the radioactive lifespan of much nuclear waste, which can extend far beyond 10,000 years. But a growing contingent of scientists believe an alternative nuclear reactor fuel—the radioactive metal called thorium—could help address these problems, paving the way for cheaper, safer nuclear power generation.
Three to four times more plentiful than uranium, today's most common nuclear fuel, thorium packs a serious energetic punch: A single ton of it can generate as much energy as 200 tons of uranium, according to Nobel Prize-winning physicist Carlo Rubbia. In the mid-twentieth century, some U.S. physicists considered building the nuclear power landscape around thorium. But uranium-fueled reactors produced plutonium as a byproduct, a necessary ingredient for nuclear weapons production, and uranium ended up dominating through the Cold War and beyond.
Thorium could recapture the lead if a Virginia-based company called Lightbridge (formerly Thorium Power) fulfills its promise. Lightbridge was founded on the vision that the existing fleet of nuclear reactors would continue to function for decades to come, so its proprietary nuclear fuel assembly—which features a small amount of uranium surrounded by a blanket of thorium—is designed to work in light water reactors, the most common variety in service worldwide. The company is also developing an all-metal fuel capable of incorporating thorium. "This is like going from leaded to unleaded fuel for your car—the operation [of the reactors] is the same," says Seth Grae, Lightbridge's CEO.
Examining the reactions inside a thorium-fueled reactor, however, reveals some important differences. In a traditional light water reactor, uranium-235 interacts with uranium-238 to produce plutonium-239 as a byproduct—a radioactive isotope that can be used for weapons. But when thorium is used instead of uranium-238 as a fertile material to kickstart nuclear fission, the thorium eventually "becomes uranium-233, which fissions almost instantaneously in the reactor, generating other isotopes that make power," Grae says. That means usable weapons-grade nuclear material is not produced, which would theoretically eliminate some security issues now associated with nuclear plants. Grae also claims thorium-powered light water reactors produce a much smaller volume of waste products that decay to relatively safe levels in just six to seven hundred years. Lightbridge has completed test runs of its thorium-based fuels in Russia and hopes to conduct tests at Idaho National Labs next year, meaning its thorium-fueled reactors could be up and running here in the U.S. as early as 2015.
Even as Lightbridge hurtles toward commercialization, other researchers are hatching longer-range plans to squeeze even more efficiency out of thorium. "Thorium is a great fuel in light water reactors, but it really excels in molten salt reactors," says David LeBlanc, a nuclear physicist at Carleton University in Ottawa, Canada. The fuel rods used in light water reactors tend to succumb to radiation damage within a few years. In proposed molten salt reactors, by contrast, thorium is dissolved in a mixture of damage-resistant liquid salts, allowing for more plant uptime. Radioactive fission products generated in a thorium-fueled molten salt reactor can also be re-added to the reactor for many successive rounds of power generation, enabling utilities to extract more power from small amounts of fuel. Japanese company IThEMS, which is working on a thorium-fueled molten salt reactor, estimates power generated by such a reactor would cost at least 30 percent less than power from today's light water reactors. In addition, molten salt reactors could potentially burn through hazardous waste stockpiles produced by previous generations of nuclear reactors.
Still, LeBlanc says new test reactors need to be built so that scientists can confirm the practicality of the thorium-molten salt approach (some observers think the corrosion salt inflicts on reactor surfaces could cause problems), so it may not take off commercially for more than a decade. In the meantime, Grae relishes the prospect of being the first to capitalize on thorium's efficiency and waste-management advantages. "We've already gotten results in the reactor in Russia, so what we're doing is very different from a 'paper reactor,'" he says. "We're very confident that this will prove out."