from the thorium energy blog, using thorium as the fertile material and U-233 as the fissile material has a secret advantage in the "proliferation" department--the inevitable formation of uranium-232. The "4n" decay chain, of which U-232 and Th-232 both follow has a decay product (thallium-208) that emits a strong and penetrating gamma ray during its decay that makes it very unattractive in weapons use. Weapons, especially those that want to be launched on ICBMs, can't afford thick heavy gamma shielding around their fissile cores to protect the sensitive electronics that trigger detonation.
In a two-fluid thorium reactor, thorium nuclei in the blanket will intercept neutrons and breed--first to protactinium-233, then to uranium-233. If the protactinium is isolated chemically after it is formed, then the U-233 in the protactinium decay tank will have little U-232 contamination.
The real question I've had is: how can we generate U-232 contaminated material, even in the Pa decay tank? Ionium is thorium-230, which is part of the natural decay chain of uranium-238, which is rather abundant. If the thorium in the blanket was "spiked" with ionium, it would be impossible to chemically separate the two forms of thorium (since they are chemically identical) but the ionium would preferentially absorb neutrons and form Pa-231.