Certainly terrorists could not steal U-233 dissolved in a molten salt solution along with lethally radioactive fission products inside a sealed reactor. Diverting any would stop the reactor, alerting authorities to a breach. 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.Ī commercial reactor will make just enough uranium to sustain power generation. 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. It is a fairly copious alpha decayer, a property that can lead to premature detonation if the core is contaminated by light elements. 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. This can be made significantly smaller-perhaps halved-by use of a lightweight beryllium tamper. It has a considerably smaller bare critical mass than U-235, about 15 kilograms versus 45 kilograms. U-233 is an excellent fuel for a fission weapon. As only a small fraction of neutrons generated in fissions are this energetic, the production rate of U-232 is very low. The first reaction in the production of U-232 has an extremely small cross-section for neutrons below about 6 mega-electron volts. Neither article quantified how much U-232 is produced, making the claim difficult to judge. 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. However, their nonproliferation potential may not be quite as promising as the article implies. 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. Robert Hargraves and Ralph Moir’s article “ Liquid Fluoride Thorium Reactors” (July–August 2010) presents a strong case for developing such plants.
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