American Scientist

To the Editors:

Robert Hargraves and Ralph Moir’s article “Liquid Fluoride Thorium Reactors” (July–August 2010) presents a strong case for developing such plants. The relative simplicity of their construction and operation, their inherent safety and their lack of plutonium (Pu) production are powerful advantages that should be carefully considered. However, their nonproliferation potential may not be quite as promising as the article implies. The authors—just as Mujid Kazimi did previously (see “Thorium Fuel for Nuclear Energy, American Scientist, September–October 2003)—suggest that Uranium-232 in the Uranium-233 produced by thorium reactors makes proliferation unlikely due to the former’s prolific and high-energy gamma-ray emissions. Neither article quantified how much U-232 is produced, making the claim difficult to judge. The first reaction in the production of U-232 has an extremely small cross-section for neutrons below about 6 mega-electron volts. As only a small fraction of neutrons generated in fissions are this energetic, the production rate of U-232 is very low.

U-233 is an excellent fuel for a fission weapon. It has a considerably smaller bare critical mass than U-235, about 15 kilograms versus 45 kilograms. This can be made significantly smaller—perhaps halved—by use of a lightweight beryllium tamper. Unlike the plutonium present in spent fuel, U-233 is immune to predetonation problems in even a crude gun-type bomb due to its low rate of spontaneous fission. It is a fairly copious alpha decayer, a property that can lead to premature detonation if the core is contaminated by light elements. But because the rate of alpha decay is only about one-sixth of that of Pu-239, this might not represent an insurmountable purification problem for would-be bomb makers. Perhaps liquid-fluoride thorium reactors could be engineered to enhance production of U-232 as a nonproliferation measure even if that produced a performance penalty.

Cameron Reed
Alma College

Drs. Hargraves and Moir respond:

A commercial reactor will make just enough uranium to sustain power generation. Diverting any would stop the reactor, alerting authorities to a breach. Certainly terrorists could not steal U-233 dissolved in a molten salt solution along with lethally radioactive fission products inside a sealed reactor. International Atomic Energy Agency (IAEA) safeguards would require security, accounting of all nuclear materials, surveillance and intrusive inspections. It is conceivable that a nation or revolutionary group might expel IAEA observers, stop a liquid-fluoride thorium reactor (LFTR) and attempt to remove U-233. Skilled engineers would need to modify the radioactive reactor’s fluorination equipment to separate uranium from the fuel salt. U-233 produced in a LFTR is a poor choice for nuclear weapons because the neutrons that produce U-233 also produce 0.13 percent contaminating U-232, whose decay products emit 2.6 mega-electron volt, penetrating gamma radiation. That would be hazardous to weapons builders and obvious to detection monitors. The U-232 decays via a cascade of elements to thallium, which emits the radiation. A year after U-233 separation, a weapons worker one meter from a subcritical 5-kilogram sphere would receive a radiation dose of 4,200 millirems per hour, compared to 0.3 millirems per hour from plutonium. Death becomes probable after 72 hours of exposure. After 10 years, this radiation triples. U-232 cannot be removed chemically; centrifuge separation would make the equipment too radioactive to maintain. Conceivably, nuclear experts might try to devise chemistry to remove the intermediate elements of the U-232 decay chain before thallium is formed. But at-risk nations could be limited to using a LFTR variant with no chemical-processing capability. Deploying LFTRs will decrease, not increase, risks of nuclear weapons proliferation. Kickstarting LFTRs with plutonium can consume existing stocks of that weapons-capable material. Using thorium fuel reduces the need for U-235 enrichment plants, which can make weapons material as well as power reactor fuel. This energy source is cheaper than coal, can increase prosperity and can reduce the potential for wars over resource competition.