WMD Junction: Thorium reactors are often promoted as being an alternative form of nuclear power that limits nuclear weapon proliferation risks. Unfortunately, that isn’t true. Thorium reactors begin with thorium-232, a nonfissionable material. Irradiating the thorium with neutrons (typically from a “seed” source such as uranium 235) creates protactinium-233, a highly radioactive isotope with a half-life of 27 days, which decays into uranium-233. In molten salt thorium reactors, highly pure uranium-233 is obtained by removing the protactinium-233 while it decays in order to prevent it from absorbing further neutrons (and producing protactinium-234 as a result). That makes obtaining fissionable material a relatively easy process. Although uranium-235 is the preferred source of fissionable material for weapons, enriching it is an intensely industrial process that is easily detectable. And a US test in 1955 showed that uranium-233 can be used in a nuclear weapon, albeit with a lower-than-expected yield. However, advances in industry and technology suggest that the yield could easily be adjusted upward.