Thorium "fuel" has been proposed as an alternative to uranium fuel in nuclear reactors. There are not "thorium reactors", but rather proposals to use thorium as a "fuel" in different types of reactors, including existing light-water reactors and various fast breeder reactor designs.

     Thorium, which refers to thorium-232, is a radioactive metal that is about three times more abundant than uranium in the natural environment. Some of the largest reserves are found in Idaho in the U.S. Large known deposits are in Australia, India, and Norway. The primary U.S. company advocating for thorium fuel is Thorium Power (www.thoriumpower.com). Unlike the claims made or implied by thorium proponents, however, thorium doesn't solve the proliferation, waste, safety, or cost problems of nuclear power, and it still faces major technical hurdles for commercialization.

Not a Proliferation Solution
     Thorium is not actually a "fuel" because it is not fissile and therefore cannot be used to start or sustain a nuclear chain reaction. A fissile material, such as uranium-235 (U-235) or plutonium-239 (which is made in reactors from uranium-238), is required to kick-start the reaction. The enriched uranium fuel or plutonium fuel also maintains the chain reaction until enough of the thorium target material has been converted into fissile uranium-233 (U-233) to take over much or most of the job.
     The use of enriched uranium or plutonium in thorium fuel has proliferation implications. Although U-235 is found in nature, it is only 0.7% of natural uranium, so the proportion of U-235 must be industrially increased to make “enriched uranium” for use in reactors. Highly enriched uranium and separated plutonium are nuclear weapons materials.
     In addition, U-233 is as effective as plutonium-239 for making nuclear bombs. In most proposed thorium fuel cycles, reprocessing is required to separate out the U-233 for use in fresh fuel. This means that, like uranium fuel with reprocessing, bomb-making material is separated out, making it vulnerable to theft or diversion. Some proposed thorium fuel cycles even require 20% enriched uranium in order to get the chain reaction started in existing reactors using thorium fuel. It takes 90% enrichment to make weapons-usable uranium, but very little work is needed to move from 20% enrichment to 90% enrichment.
     It has been claimed that thorium fuel cycles with reprocessing would be much less of a proliferation risk because the thorium can be mixed with uranium-238. In this case, fissile uranium-233 is also mixed with non-fissile uranium-238. The claim is that if the U-238 content is high enough, the mixture cannot be used to make bombs without a complex uranium enrichment plant. This is misleading. More uranium-238 does dilute the uranium-233, but it also results in the production of more plutonium-239 as the reactor operates. So the proliferation problem remains – either bombusable uranium-233 or bomb-usable plutonium is created and can be separated out. Even if the mixture of U-238 and U-233 contains so much U-238 that it cannot be used for making weapons, the U-233 proportion can be increased by enrichment – the same process used to enrich natural uranium in U-235. The enrichment of U-233 is easier than the enrichment of U-235 because U-233 is much lighter than U-235 relative to U-238 (five atomic weight units lighter compared to three).
     There is just no way to avoid proliferation problems associated with thorium fuel cycles that involve reprocessing. Thorium fuel cycles without reprocessing would offer the same temptation to reprocess as today's once-through uranium fuel cycles.

Ongoing Technical Problems
Research and development of thorium fuel has been undertaken in Germany, India, Japan, Russia, the UK and the U.S. for more than half a century. India is often cited as the country that has successfully developed thorium fuel. In fact, India has been trying to develop a thorium breeder fuel cycle for decades but has not yet done so commercially.
     One reason reprocessing thorium fuel cycles haven't been successful is that uranium-232 (U-232) is created along with uranium-233. U-232, which has a half-life of about 70 years, is extremely radioactive and is therefore very dangerous in small quantities: a small particle in the lung would exceed legal radiation standards for the general public. Therefore, fabricating fuel with U-233 is extremely expensive and difficult.

Not an Economic Solution
     Thorium may be abundant, but it does not mean that it is economical. Compared to uranium, the thorium fuel cycle will be costly. In a once-through mode, both thorium target rod production and uranium enrichment (or plutonium separation) are needed. In addition, as noted, inhalation of thorium-232 produces a higher dose than the same amount of uranium-238 (either by radioactivity or by weight). In a breeder configuration, reprocessing is required, which is costly. Reprocessing is made even more costly, because the highly radioactive U-232 created in the reactor from thorium makes worker protection more difficult and expensive for a given level of annual dose.