Thorium-powered nuclear reactors are highly resistant to nuclear proliferation. During the course of a traditional uranium-powered nuclear reaction, uranium transmutes into plutonium 239, the source material of choice for nuclear weapons. Thorium, on the other hand, transmutes into Uranium 233, to which DBI reactors will add a mixture of isotopes that make it impractical and dangerous for weapons use, even with the required costly isotopic enrichment.
Several features of the DBI thorium reactor program make weapons proliferation extremely difficult. First, using a proliferation resistant start-up fuel, such as the 240Pu-denatured thorium/uranium fuel developed by Thorium Power, Inc., reduces the risks associated with the plutonium bred from the start-up uranium fuel. Additionally, a small percent of natural uranium or depleted 238U can be added to the thorium oxide Breeding/Breeder fuel elements, making the bred 238U useless for weaponization.
Next, after the initial load, in a full breeder reactor no uranium needs to be added and only small amounts of thorium need to be added periodically to a DBI reactor. Also, no nuclear materials need to be removed from the facility, reducing transportation costs and more importantly simplifying the security and international monitoring of fissile material.
One by-product of breeding U-233 is U-232, an unstable uranium isotope that decays with a 70-year half-life into thallium-208. Thallium-208 emits hard gamma rays as it decays. These gamma rays typically penetrate 2 cm of solid lead, are very difficult to shield against and therefore provide a highly recognizable (and nearly impossible to conceal) signal for counter-proliferation agents to detect.
Finally, the fissile material remaining in the spent fuel from a DBI reactor will be at significantly lower concentration than the plutonium in spent conventional nuclear waste, making the DBI waste an even less attractive source for weapons material than existing nuclear waste stockpiles.
Interestingly, its proliferation resistance is the reason why thorium was not selected as the nuclear fuel of choice in the early mid-20th century, when the US and the former Soviet Union developed the first generation nuclear reactors. A 1997 international scientific symposium reported as much when it concluded that the principal reason thorium had not been used more widely to date is that the ore contains no fissile isotope. In other words, because it was harder to make bombs from thorium, thorium technologies were not developed aggressively.