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
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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