# How do Thorium reactors work? **URL:** \ **Category:** Factual Questions\ **Created:** [February 24, 2022, 11:18am UTC](https://boards.straightdope.com/t/how-do-thorium-reactors-work/960045 "2022-02-24T11:18:29Z")\ **Posts on this page:** 7\ **Page:** 1
**Author:** ![AK84](https://avatars.discourse-cdn.com/v4/letter/a/85e7bf/32.png) [@AK84](https://boards.straightdope.com/u/AK84)\ **Post date:** [February 24, 2022, 11:18am UTC](https://boards.straightdope.com/t/how-do-thorium-reactors-work/960045/1 "2022-02-24T11:18:29Z")
[Thorium based fuel cycle.](https://en.wikipedia.org/wiki/Thorium-based_nuclear_power). Ok, I understand the cycle involves the transmutation of Thorium into U-233. My question is, how does that _start_? Take Thorium and put it in a reactor…and? ---
**Author:** ![Whack-a-Mole](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/whack-a-mole/32/141_2.png) [@Whack-a-Mole](https://boards.straightdope.com/u/Whack-a-Mole)\ **Post date:** [February 24, 2022, 11:37am UTC](https://boards.straightdope.com/t/how-do-thorium-reactors-work/960045/2 "2022-02-24T11:37:17Z")
I found this. I only quoted a bit. The article goes on to describe different mechanisms. > Thorium (Th-232) is not itself fissile and so is not directly usable in a thermal neutron reactor. However, it is ‘fertile’ and upon absorbing a neutron will transmute to uranium-233 (U-233)[a](https://world-nuclear.org/information-library/current-and-future-generation/thorium.aspx#Notes), which is an excellent fissile fuel material[b](https://world-nuclear.org/information-library/current-and-future-generation/thorium.aspx#Notes). In this regard it is similar to uranium-238 (which transmutes to plutonium-239). All thorium fuel concepts therefore require that Th-232 is first irradiated in a reactor to provide the necessary neutron dosing to produce protactinium-233. The Pa-233 that is produced can either be chemically separated from the parent thorium fuel and the decay product U-233 then recycled into new fuel, or the U-233 may be usable ‘in-situ’ in the same fuel form, especially in molten salt reactors (MSRs). > > Thorium fuels therefore need a fissile material as a ‘driver’ so that a chain reaction (and thus supply of surplus neutrons) can be maintained. The only fissile driver options are U-233, U-235 or Pu-239. (None of these is easy to supply) [SOURCE](https://world-nuclear.org/information-library/current-and-future-generation/thorium.aspx) ---
**Author:** ![AK84](https://avatars.discourse-cdn.com/v4/letter/a/85e7bf/32.png) [@AK84](https://boards.straightdope.com/u/AK84)\ **Post date:** [February 24, 2022, 11:39am UTC](https://boards.straightdope.com/t/how-do-thorium-reactors-work/960045/3 "2022-02-24T11:39:04Z")
So, you put Thorium into a normal breeder reactor? Paging @Stranger_On_A_Train ---
**Author:** ![Francis\_Vaughan](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/francis_vaughan/32/3093_2.png) [@Francis\_Vaughan](https://boards.straightdope.com/u/Francis_Vaughan)\ **Post date:** [February 24, 2022, 11:47am UTC](https://boards.straightdope.com/t/how-do-thorium-reactors-work/960045/4 "2022-02-24T11:47:38Z")
It doesn’t need to be what we currently term a breeder reactor. Those are specifically designed to create Plutonium. You just need a source of neutrons. Any reactor has those buzzing about. So in principle you could breed Thorium into fissile Uranium in most reactors. It seems either fast or thermal neutrons are both good for use with Thorium. So it is a wide set of possibilities. If you wanted a reactor that then burns the transmuted Thorium directly you would probably need to design that reactor from the ground up. I would imagine that the geometry, moderation, fuel densities, and all manner of things would need optimising. Molten salt reactors seem to be a favoured design. But if you just wanted to externally extract the Uranium chemically and manufacture conventional fuel rods for use in conventional reactors, you could breed from Thorium in many existing reactor designs. ---
**Author:** ![AK84](https://avatars.discourse-cdn.com/v4/letter/a/85e7bf/32.png) [@AK84](https://boards.straightdope.com/u/AK84)\ **Post date:** [February 24, 2022, 11:58am UTC](https://boards.straightdope.com/t/how-do-thorium-reactors-work/960045/5 "2022-02-24T11:58:10Z")
Isn’t U-232 contamination a big problem in using U-233? ---
**Author:** ![Horatius](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/horatius/32/3572_2.png) [@Horatius](https://boards.straightdope.com/u/Horatius)\ **Post date:** [February 24, 2022, 12:19pm UTC](https://boards.straightdope.com/t/how-do-thorium-reactors-work/960045/6 "2022-02-24T12:19:16Z")
An alternative system being investigated is [accelerator driven reactors](https://world-nuclear.org/information-library/current-and-future-generation/accelerator-driven-nuclear-energy.aspx). > The accelerator supplies a beam of high-energy protons down a beam pipe to the spallation target inside the core, and the neutrons produced enter the fuel and transmute the thorium into protactinium, which soon decays to U-233 which is fissile. The neutrons also cause fission in uranium, plutonium and possibly transuranics present, releasing energy. A 10 MW proton beam might thus produce 1500 MW of heat (and thus 600 MWe of electricity, some 30 MWe of which drives the accelerator). With a different, more subcritical, core a 25 MW proton beam would be required for the same result. Today’s accelerators are capable of only 1 MW beams. Still needs some work to get a good enough beam, but the physics behind it is well-understood. ---
**Author:** ![Francis\_Vaughan](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/francis_vaughan/32/3093_2.png) [@Francis\_Vaughan](https://boards.straightdope.com/u/Francis_Vaughan)\ **Post date:** [February 24, 2022, 12:35pm UTC](https://boards.straightdope.com/t/how-do-thorium-reactors-work/960045/7 "2022-02-24T12:35:15Z")
> [@AK84](#): > > Isn’t U-232 contamination a big problem in using U-233? ~~I think you can avoid ever having the two in the same physical mixture. So there shouldn’t be any contamination. They may both be present in the reactor, but there is no need for them to be in the same container. For native Thorium powered reactors it probably doesn’t matter, or you can set things up so that the system can be kick started with an external source of neutrons, perhaps from a mini-reactor that contains the other fissile material and contaminants.~~ EDIT: The above is just plain wrong. Summary here: > **[Thorium fuel cycle](https://en.wikipedia.org/wiki/Thorium_fuel_cycle)** > > The thorium fuel cycle is a nuclear fuel cycle that uses an isotope of thorium, 232Th, as the fertile material. In the reactor, 232Th is transmuted into the fissile artificial uranium isotope 233U which is the nuclear fuel. Unlike natural uranium, natural thorium contains only trace amounts of fissile material (such as 231Th), which are insufficient to initiate a nuclear chain reaction. Additional fissile material or another neutron source is necessary to initiate the fuel cycle. In a thorium-fue...