David,you stated, From a non-proliferation standpoint, thorium-fueled reactors present a unique set of challenges because they convert thorium-232 into uranium-233 which is nearly as efficient as plutonium-239 as a weapons material
I have consulted "THORIUM FUEL CYCLE — POTENTIAL BENEFITS AND CHALLENGES", IAEA-TECDOC-1450, which states,
"Thorium produces through a nuclear reaction the fissile isotope 233U. 233U has been determined to be at least as efficient as 235U as a weapon material. Therefore, a relatively small amount of natural (or enriched) uranium can be added to thorium in order to dilute the generated 233U below the proliferation level of 12%, thus creating an effective barrier to diversion of 233U. . . ."
"The objective of thorium fuel cycle should be to ensure ‘proliferation-resistance’ of ‘fissile’ material and at the same time produce minimum quantities of ‘radiotoxic waste’. The radiotoxicity of the waste can be significantly reduced if the bred 233U is separated and recycled but the disadvantage associated with this strategy is that 233U is ‘fissile’ and constitutes the proliferation problem. The 233U can be rendered proliferation-resistant through mixing with 238U and denaturing. . . . "
"232U is always present in ‘fissile’ 233U and has the daughter product 208Tl, which emits highly penetrating 2.6 MeV gamma photons. . . . .
The gamma activity provides adequate barrier to diversion, particularly when the 232U content is in the higher range. . . .
In contrast to uranium-fuelled reactors (238U+235U), where there is no natural denaturant for plutonium isotopes, 238U is an effective denaturant for the bred 233U in thorium (232Th) cycle. A possible solution to safeguard the reactor grade 233U is to denature with 238U. Denaturing the reactor-grade uranium with a equal quantity of 238U should be regarded as the lower limit for non-proliferation.
"232Th /233U offers potentially significant advantages over 238U/235U/239Pu, in terms of lesser transuranic actinide waste and adequate proliferation-resistance."