To what extent is Thorium proliferation resistant? We consider three cases of weapons: 'dirty bombs', bombs from isotope enrichment, and bombs from man-made fissile isotopes.
There is no equivalent of the 235U device. This is what makes the process suitable for nations that genuinely want to develop peaceful uses of nuclear power without military applications. Conversely, if a nation insists on developing uranium based nuclear power when offered the possibility of thorium, it lays itself open to suspicion.
So one of the two routes to nuclear weapns is completely closed. The relevance of this is highlighted by the current status in Iran. Are their uranum enrichment plants producung low-enrichement reactor fuel or high-enrichment weapons material?
Man-made Fissile IsotopesEdit
233U is produced as fuel, and it is theoretically possible, if some of this were misapporopriated, to construct a nuclear device from it. Indeed its low rate of spontaneous fission means that a simple gun can be used rather than the more sophisticated implosion technique.
However the reactions that produce 233U also produce 232 U. It is produced by (n,2n) reactions on 232 Th, 233 Pa, or 233U. The isotope has a half life of 70 years, and its decay produces two high-energy gamma rays.
This means that anyone working with 233U, which contains even a small fraction of 232U, rapidly receives a lethal dose of gamma radiation. It is also bad for electronics. Working with this material is not impossible, but it requires very sophisticated handling equipment, out of reach of a back-street terrorist, and also the 2.6 MeV gamma rays are readily detected so such a device is hard to hide.
This does have an important consequence: after a few hundred years the 232U in a fuel rod will have decayed away, and the safety factor will be lost. This is a reason for using reprocessing in the thorium fuel cycle, and not following a once-through system. The stored spent fuel would become a proliferation hazard for future generations.
A possible loophole - and the Ionium solutionEdit
It has been proposed that 232U production could be avoided by chemically removing the protactinium (233Pa), a technique which is very important for the molten salt reactor concept. This avoids the 233U(n,2n) reaction which is the main source of 232U and your extracted Pa sample gives you (after a few times 27 days) 233U which is pretty much free of 232U. This separation technique is an significant part of most Molten Salt Reactor designs, and avoids the undesirable consequences (loss of neutrons, loss of fissile material) of exposing the Pa to neutrons.
Adding 238U to the fuel has been proposed. Then if the uranium is extracted chemically it would only contain a small fraction of 233U. This could be extracted by centrifuge but that is very much mofr difficult. This proposal has the problem that the 238U would absorb neutrons, creating undesirably long lived actiindes.
A better strategy is to 'spike' the Thorium with Ionium. This 'element' is not to be found in the periodic table: it is in fact 230Th: it is found in Uranium deposits and was historically found there without it being realised that it was chemically the same as the known element Thorium. It will capture neutrons to form 231Pa, so a chemically separated Pa sample will also include this isotope. This then absorbs another neutron to give 232Pa which decays (1.8 days) to 232U. So precautions can be taken to stop exploitation of this possible
method of extracting fissile material that is safe to handle.
There is some Plutonium produced in an ADSR, but the amounts are small and it is hard to see this as a major proliferation hazard.
Certainly a thorium reactor will produce radioactive fission product waste, which is dangerous. If the contents of a spent or partly-used fuel rod were to be dispersed in a crowded city the effect would be considerable.
Yet this effect would be more on the public perception, and it is unlikely that even a successful attack would bring many real casualties. A recent study (?) concluded that a terrorist group would get more effect by replacing the radioactive material with conventional explosive. While there is a danger here, and the radioactive materials produced must be protected not only from accidental mishap but from deliberate action, this is not a world-threatening problem