Currently more than 80 percent of the world's energy is obtained from fossil fuels: petroleum, coal, and natural gas. Another 10 percent comes from biomass and waste, leaving only 10 percent from sources other than carbon burning.

According to the World Health Organization, urban outdoor air pollution is estimated to cause 1.3 million deaths worldwide per year, from respiratory infections, heat disease, and lung cancer. Indoor air pollution is estimated to cause approximately 2 million premature deaths mostly in developing countries. Almost half of these deaths are due to pneumonia in children under 5 years of age. The source in both cases is carbon combustion, from fossil fuels or biomass.

The overwhelming majority of climate scientists tell us that the greenhouse gases generated by human carbon combustion are likely to trigger disastrous global climate change in the decades ahead. Even those who deny this surely must admit that the world would be a better place if we could find an economically feasible and safe alternative to the use of carbon combustion as our primary source of energy.

And so, it came as a surprise to me to learn recently that such an alternative has been available to us since World War II, but not pursued because it lacked weapons applications. When the war ended and nuclear reactors were developed to generate electrical power, the designs adopted were based on the same technologies that were used in the nuclear bombs dropped on Japan. These relied on the fission of uranium-235 (U-235) and plutonium-239 (Pu-239).

U-235 constitutes only 0.72 percent of natural uranium, which is mostly U-238, so costly separation is required. Plutonium is not found in nature and must be "bred" by uranium reactors, also a costly process.

It was well known to physicists of the time that another uranium isotope, U-233, is also fissionable. This isotope also does not occur in nature, but can be bred from the element, thorium, which is very common. However, a reactor breeding U-233 also produces U-232, which has a decay chain that generates high-energy gamma rays. This makes U-233 fusion unusable as a weapon, since these gamma rays are very destructive to a bomb's instrumentation and dangerous to the personnel handling it. Furthermore, U-233 is not an efficient breeder of plutonium, since it contains two fewer neutrons than U-235.

Because of its lack of application to weapons, a promising project at the Oak Ridge National Laboratory that was leading toward a thorium reactor was cancelled by the Nixon administration in 1969 in favor of a more efficient plutonium breeder. The Oak Ridge program was advancing the technology of using molten salt as a reactor fuel rather than the solid rods found in existing naval and commercial reactors. It had successfully operated such a reactor for 22,000 hours before being terminated. Liquid fuel offers great advantages in cost and safety over the solid fuel design.

Currently the liquid fluoride thorium reactor (LFTR) is having a resurgence of interest worldwide. Let me list the advantages of an electrical power plant based on LFTR compared to conventional nuclear and fossil-fuel plants:

• Thorium is plentiful and inexpensive. One ton costing $300,000 can power a 1,000-megawatt plant for a year. One pound of thorium yields as much power as 300 pounds of uranium or 3.5 million tons of coal.

• Unlike conventional high-pressure water reactors, LTFR operates at atmospheric pressure, obviating the need for a large, expensive containment dome and having little danger of explosion.

• LFTRs cannot melt down since the normal operating state is already molten.

• LFTRs are stable to rising temperatures since salt expands slowing the reaction. A salt plug kept solid by cooling coils will automatically melt if external power is lost and the fluid drain out to a safe dump tank.

• Salts used are solid below 300 F or higher, so any spilled fuel solidifies instead of escaping into the environment.

• Liquid fuels use almost all the available energy is used, unlike solid fuels that must be removed before they have generated 1-3 percent of the available energy because of damage.

• The radiative waste is much less than from conventional plants and far more manageable.

• Air-cooling possible where water is scarce.

• Should be cheaper than coal, especially if CO2 is sequestered.

• Proliferation resistant. Can't use to build bombs.

• Smaller size and lower cost.

• Could provide the world's energy needs carbon-free for a thousand years.

The gamma rays from U-232 are not a problem for reactors since unlike nuclear weapons they are already sufficiently shielded.

Of course, the disasters at Three-Mile-Island, Chernobyl, and Fukushima has greatly chilled public acceptance of nuclear power. But if these plants had used LFTRs, no radiation would have escaped to the environment.

Although far less of a problem than U-235 and Pu-239, waste from U-233 still has to be stored someplace. Here a comparison can be made with carbon sequestration in which waste CO2 is pumped into the ground, which is being talked about as a solution to problem of coal pollution. The amount of underground space needed to store a year's CO2 output from a single coal power plant is equivalent to 600 football fields filled to a height of ten yards. By comparison, one football field filled to the same height is required for all the waste from the entire civilian nuclear program.

Work on LFTR is going on worldwide, with research being done in China, France, the Czech Republic, Japan, Russia, Canada, and the Netherlands. The only significant U.S. research is on molten salt reactors, but with no emphasis on thorium. The U.S. may end up buying LFTRs from China. Perhaps WalMart will sell them cheap.

Thanks to reactor physicist Bob Zannelli for bringing LFTR to my attention and helping me learn the science.

Further Reading:

Daniel Yergin, The Quest: Energy, Security and the Remaking of the Modern World, (New York: Penguin Press, 2011).

Robert Hargraves, and Ralph Moir, "Liquid Fluoride Thorium Reactors: An Old Idea in Nuclear Power Gets Reexamined," American Scientist 98, no. 4(2010): 304-13.

Robert Hargraves, and Ralph Moir, "Liquid Fuel Nuclear Reactors," Physics & Society 40, no. 1(2011): 6-10.

Also, see the online lecture by Robert Hargraves "Aim High: Using Thorium to Address Environmental Problems".