It sounds like something from the distant future or a 1950s pipe dream: clean, safe nuclear power with little waste and zero proliferation threat. In fact, while uranium-fueled light water reactors were sprouting up around the world, molten salt reactors fueled by thorium were being tested; research suggested that the thorium cycle might offer safer nuclear power.

Thorium is a heavy element that is far more abundant than uranium. It is up to 200 times more energy-dense than uranium -- and it's also cheap. Thorium is a major waste product of rare-earth metal mining, and aside from nuclear fuel, it has few commercial uses.

Oak Ridge National Laboratory's work with molten salt reactors began in the 1960s. Small test reactors ran successfully for a decade. In 1976, however, the program was shut down. Theories as to why abound: political issues with the Nixon administration; Honeywell and GE pushing to stick with the uranium light water reactor designs they'd already invested in; and even the fact that the thorium cycle couldn't be used to produce enriched uranium plutonium by-products.

Brooklyn-based Motherboard recently released a documentary covering the history of the thorium reactor. Thanks in part to the Fukushima disaster and the public's increased interest in lessening fossil fuel use, thorium has received new interest in recent years. China, India, Japan, France, Russia and the U.S. are all currently developing thorium-based reactors, with various degrees of commitment.

India is already well into its thorium fuel development. The country's three-stage nuclear power plan laid out in the '50s was designed specifically to take advantage of India's vast thorium reserves. India has taken a more conventional route, utilizing uranium-catalyzed pressurized heavy water reactors that use thorium compounds as breeder fuel to produce more uranium.

A few startups have appeared to attempt to take advantage of one aspect of the thorium reactor: modularity. One, Flibe Energy, is developing liquid-fluoride thorium reactors. A refinement of the original molten salt design, LFTRs are fueled by thorium within a molten fluoride coolant. They operate at much lower pressures than light water reactors, which eliminates a number of packaging constraints. Flibe claims that the liquid-fluoride cycle will allow them to build small modular reactors  to compete with commercial diesel generators.

A more advanced thorium cycle was conceived by Carlo Rubbia, a Nobel laureate and former director of CERN. Rubbia's accelerator-driven system uses a particle accelerator to smash heavy metals with proton beams, releasing energy in the form of excess neutrons. Norwegian firm Aker Solutions bought Rubbia's patent for the cycle. The company's 600-megawatt station won the Energy Award at the 2010 Institution of Chemical Engineers' Innovations and Excellence Awards.

As far as the industry is concerned, the renewed interest in thorium highlights a shift towards smaller, simpler nuclear power plants (see NuScale's small modular reactors). Smaller plants require less capital costs and offer more flexibility in deployment and grid integration. Because the thorium cycle produces only small amounts of plutonium, thorium reactors may be able to skirt nuclear proliferation regulation.

Will the U.S. commitment to uranium light water reactors and the uranium fuel cycle prevent the nation from keeping up with India and China and their pursuit of a better nuclear technology?

Tags: light water reactor, light water reactors, lwr, lwrs, nuclear power, nuclear power plant, nukes, small modular reactors, smr, smrs, thorium