Transatomic is recent nuclear startup by a couple of MIT nuclear engineers which has so far gathered a few million dollars in funding. Their particular design approach uses molten salt for a fuel as does the LFTR proposal, but eschews thorium for a uranium only design. The Transatomic http://transatomicpower.com/white_papers/TAP_White_Paper.pdf [Broken] thoughtfully anticipates questions from thorium advocates in a Chapter titled "Why Not Thorium First?". Excerpt here:
The TAP reactor’s primary innovations – a novel combination of moderator and fuel salt – can also be adapted for use with thorium. Transatomic Power believes that the thorium fuel cycle holds theoretical advantages over uranium in the long run because of its generally shorter half-life waste, its minimization of plutonium from the fuel cycle, and its greater natural supply. However, we chose to start with uranium for several reasons: (1) there is a great deal of spent nuclear fuel, and we want to harness its energy while reducing the risk of onsite SNF storage; (2) the industry already has a commercial fuel cycle developed around uranium, which makes it cheaper to use uranium as fuel is this design; (3) we already greatly eliminate waste; and (4) we already greatly expand the energy potential of existing uranium supplies.
Thorium reactors do not contain plutonium, but they do have a potential proliferation vulnerability because of the protactinium in their fuel salt. Protactinium has a high neutron capture cross section and therefore, in most liquid thorium reactor designs, it must be removed continuously from the reactor. The process for doing this yields relatively pure protactinium, which then decays into pure U-233. By design, the pure U-233 is sent back into the reactor where it is burned as its primary fuel. The drawback, however, is that U-233 is a weapons-grade isotope that is much easier to trigger than plutonium. We believe that the proliferation objection to liquid thorium is actually related to protactinium-233 in the thorium portion of the reactor. If this can be extracted chemically, it decays quickly into pure U-233.
It is possible to denature the U-233 by mixing it with other uranium isotopes, or modify the design to further reduce diversion risk, but additional research is required to implement these anti-proliferation measures in thorium molten salt reactors. Some may discount the proliferation risk of the thorium fuel cycle because the U-233 in the reactor would be mixed with U-232, rendering it a poor source for proliferation purposes. However, it is not the presence of U-232 that decreases proliferation risk, it is the decay products of U-232 that produce high energy gamma radiation that renders it difficult to handle. Therefore, even with U-232 mixed in with U-233, it may be possible to chemically extract any decay products produced from U-232 before they become gamma emitters, thereby leaving weaponsgrade uranium that is not protected by high energy gamma radiation.
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