I think Makhijani might be referring to this paper on Uranium-232:
http://www.princeton.edu/sgs/publications/sgs/pdf/1_3-4FetterA.pdfUranium-232 is produced as a parasitic byproduct of the breeding process in the thorium fuel cycle. It is also a decay product of Uranium-233. What really hurts is that very minute quantities of it can make the whole pile of Uranium-233 very hazardous to people and electronics. Because the quantities are tiny, centrifuging out all the U-232 takes a considerable amount of Separative Work Units and time, during which more U-232 is produced from decay. You could potentially ignore the U-232 and still make a bomb, but you'd be killing the reactor's fuel cycle in doing so because the amount of uranium-233 produced by the thorium cycle is only barely enough to cover the initial fissile fuel used up in the process. Furthermore, forget about using any unshielded electronics or guidance systems in your bomb.
Time and time again, nuclear opponents will downplay how difficult it is to actually build a nuclear weapon. There are many reasons why nuclear bombs are built with uranium-235 from very powerful enrichment systems and plutonium-239 from specially-designed breeder reactors incapable of producing electricity. We've known about thorium for a long time and it was killed in favor of plutonium.
