Comment Re:What about Thorium, Molten Salt Reactors (Score 1) 560
Rebuttal from Physicians for Social Responsibility
Weapons-grade fissionable material (U-233) is harder to retrieve safely and clandestinely from a thorium reactor
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 uranium235 (U235) or plutonium239 (which is made in reactors from uranium238), is required to kickstart 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 uranium233 (U 233) to take over much or most of the job. An advantage of thorium is that it absorbs slow neutrons relatively efficiently (compared to uranium238) to produce fissile uranium233. The use of enriched uranium or plutonium in thorium fuel has proliferation implications. Although U235 is found in nature, it is only 0.7 percent of natural uranium, so the proportion of U235 must be industrially increased to make “enriched uranium” for use in reactors. Highly enriched uranium and separated plutonium are nuclear weapons materials.
Use of U-235 or Pu-239 is only required as "start up" charge if U-233 is unavailable (regarding the proliferation risk of Thorium-derived U-233, see below). Also, this is only true for molten salt reactors, Accelerator-driven systems aka "subcritical reactors" may even work without any fissile material present from the get-go (though they have their own problems)
In addition, U233 is as effective as plutonium239 for making nuclear bombs. In most proposed thorium fuel cycles, reprocessing is required to separate out the U233 for use in fresh fuel. This means that, like uranium fuel with reprocessing, bombmaking 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 weaponsusable uranium, but very little additional work is needed to move from 20% enrichment to 90% enrichment. Most of the separative work is needed to go from natural uranium, which ahs 0.7% uranium235 to 20% U235.
Reactors don't have to be 100% proliferation resistant, it just has to be harder to use them to make a bomb than the old graphite/uranium pile + plutonium extraction process. In other words, if someone can do the former, they could do the latter much more easily. U-233, like any fissile material, can be used to make bombs. However, if U-233 is bread from Thorium, it is invariably contaminated with U-232 which has a massive gamma emitter in its decay chain. This makes handling this material hard, requiring shielding both when making the bomb. Even worse, it would make an inferior bomb since you would have to shield the bomb itself to make it safe for the operator as well as shield the electronics of the bomb. Finally, the gamma emission would make the presence and location of such a device easy to detect. If you are a bad actor with the appropriate resources, it's much easier to just build one of those World War 2 piles and extract the plutonium from it.
Furthermore, note that commercial power reactors tend to be poor sources for bomb material in general unless they are specifically designed to make it easy, you require regular fuel changes in matter of months to avoid spoiling the material with elements which will ruin your bomb-making effort. This will interrupt operation and raise red flags if the reactor is shut down on every three months, which ruins your effort to be secretive which is likely your reason to use a commercial plant in the first place. This is why all the bomb making efforts in countries either use straight enrichment (rare - South Africa is the only example I'm aware of) or special-purpose bomb-making reactors (all the rest).
Thorium produces 10 to 10,000 times less long-lived radioactive waste;
Proponents claim that thorium fuel significantly reduces the volume, weight and longterm radiotoxicity of spent fuel. Using thorium in a nuclear reactor creates radioactive waste that proponents claim would only have to be isolated from the environment for 500 years, as opposed to the irradiated uraniumonly fuel that remains dangerous for hundreds of thousands of years. This claim is wrong. The fission of thorium creates longlived fission products like technetium99 (halflife over 200,000 years). While the mix of fission products is somewhat different than with uranium fuel, the same range of fission products is created. With or without reprocessing, these fission products have to be disposed of in a geologic repository.
It's the composition that makes the danger. If you have a reactor with a high burn-up like a thorium molten salt breeder, you don't get large amounts of transuranic actinides which are the main problem in spent fuel, because they are more within the critical range regarding half-lives, short enough to be dangerous, long enough to be a long-term problem (e.g. 24,000 years for Pu-239). With a thorium breeder with high burn-up you avoid that range of half-lives. Not that this argument also applies to any reactor with high burn-up, including uranium breeders. In any case, the amounts generated combined with the long half-lives place severe restrictions on their radiation danger.
The claim that fission products have to be disposed in geologic repository is wrong. Many of the fission products are in strong demand and could, with the right incentives, be useful. In particular, the quoted Tc-99 is in high demand as a beta radiation source and as a catalyst. Furthermore, if the neutron economics of the reactor is good, it is conceivable to re-inject fission products that are deemed problematic into a blanket which will convert them to something less challenging.
Thorium comes out of the ground as a 100% pure, usable isotope, which does not require enrichment, whereas natural uranium contains only 0.7% fissionable U-235
Compared to uranium, thorium fuel cycle is likely to be even more costly. In a oncethrough mode, it will need both uranium enrichment (or plutonium separation) and thorium target rod production. In a breeder configuration, it will need reprocessing, which is costly. In addition, as noted, inhalation of thorium232 produces a higher dose than the same amount of uranium238 (either by radioactivity or by weight). Reprocessed thorium creates even more risks due to the highly radioactive U232 created in the reactor. This makes worker protection more difficult and expensive for a given level of annual dose.
(The article goes into a bit more detail. One does have to keep in mind that PSR is generally quite anti nuclear - but I think these are fairly reasonable counterarguments)
The cost issue only applies for traditional solid-core reactors. Creating thorium-oxide fuel rods would, indeed, be more challenging than traditional uranium-oxide fuel rods, mainly due to the high melting point of the former. In a molten core reactor, thorium could be directly dissolved into the blanket. This is so simple, it actually reduces fuel cost and is actually used as an economic counterargument to thorium molten salt breeders since production of duel assemblies is a major income source for nuclear manufacturers.
A molten core reactor would drastically reduce reprocessing cost since the molten salt can be directly used to reprocess the fuel while the reactor is running (online reprocessing). It is also quite telling that the U-232 is now mentioned as a counterargument to reprocessing but not as a hindrance to proliferation. Indeed, online reprocessing would not require human interaction since it involves dealing with a liquid which can be used in an automated, continuous chemical process similar to an oil refinery.
The argument regarding Th-232/U-238 is laughable. While it is certainly undesirable to inhale any radioactive substance, both Th-232 (half-life 14 billion years) and U-238 (half-life 4 billion years) are among the least radioactive elements that are not stable. The danger of (chemical) heavy-metal poisoning is likely higher than the radiological danger. The usual handling caveats in that regard apply, but they also apply to other elements routinely used, e.g. lead.
Lastly, no one has actually made a commercial level thorium cycle reactor despite decades of trying. It MIGHT have some advantages and engineering and research efforts should continue, but it's hardly a viable solution as of yet.
This argument hinges on the word "commercial" and your definition of "trying". There has been successful experimental reactors using the molten salt breeder concept. There have been some weak attempts to produce commercial solid-core thorium reactors, which have been unsuccessful but not necessarily for technical reasons. There has been no attempt to produce a commercial molten salt breeder until recently. R&D effort is definitely required, but given the results of the experimental reactors of the 60s, it should be modest.
