Thorium - the other nuclear fuel

November 27, 2009 10:05 PM

Why haven't thorium nuclear reactors been used much compared to Uranium reactors? It would appear there are sizable reserves of thorium available and that reactors based on thorium would produce considerably less nuclear waste and could be considerably safer than Uranium based ones. Given that thorium reactors would appear to have advantages what is the catch?

One reason that appears is that the Uranium fuel cycle sets up the infrastructure to support nuclear weapons. However, there are a number of countries such as Japan, Canada, Sweden and others that have nuclear reactors but no weapons why didn't they start using Thorium reactors or at least start researching?

Or is it just the cost of an alternative system to Uranium based reactors? But given that Canadian CANDU reactors can use Thorium why haven't they?

Note, answers about how bad nuclear power is in general are off topic. Please don't put them here. That is a separate issue.

posted by sien to technology (3 comments total)

Copied and pasted from this link.

Despite the thorium fuel cycle having a number of attractive features, development has always run into difficulties.

The main attractive features are:

*The possibility of utilising a very abundant resource which has hitherto been of so little interest that it has never been quantified properly.
*The production of power with few long-lived transuranic elements in the waste.
* Reduced radioactive wastes generally.

The problems include:

* The high cost of fuel fabrication, due partly to the high radioactivity of U-233 chemically separated from the irradiated thorium fuel. Separated U-233 is always contaminated with traces of U-232 (69 year half-life but whose daughter products such as thallium-208 are strong gamma emitters with very short half-lives). Although this confers proliferation resistance to the fuel cycle, it results in increased costs.
* The similar problems in recycling thorium itself due to highly radioactive Th-228 (an alpha emitter with two-year half life) present.
* Some concern over weapons proliferation risk of U-233 (if it could be separated on its own), although many designs such as the Radkowsky Thorium Reactor address this concern.
* The technical problems (not yet satisfactorily solved) in reprocessing solid fuels. However, with some designs, in particular the molten salt reactor (MSR), these problems are likely to largely disappear.

Much development work is still required before the thorium fuel cycle can be commercialised, and the effort required seems unlikely while (or where) abundant uranium is available. In this respect, recent international moves to bring India into the ambit of international trade might result in the country ceasing to persist with the thorium cycle, as it now has ready access to traded uranium and conventional reactor designs.

Nevertheless, the thorium fuel cycle, with its potential for breeding fuel without the need for fast neutron reactors, holds considerable potential in the long-term. It is a significant factor in the long-term sustainability of nuclear energy.
posted by water bear at 10:16 PM on November 27, 2009

The magical thing about uranium is that it goes critical all by itself. Natural uranium, sitting in the ground, has some fraction that spontaneously fissions and makes a few free neutrons. All you have to do is put enough uranium together that the best place for the neutrons to go is onto another uranium nucleus. A uranium nucleus eating a neutron breaks up, freeing two or three more neutrons; they are already in your pile, and so the best place for them to go is onto another uranium. It used to be that you didn't even need people.

Reactor folks talk about the "neutron multiplication factor," usually called k or k_effective, which is the number of fission neutrons that go on to produce another fission. A heavy element like uranium or thorium has half again as many neutrons as protons; two fragments from the middle of the periodic table only want to have about 20% more neutrons than protons; this is why you get a few free neutrons from a fission. An "average" fission in U-235 makes about 2.4 neutrons. It took one neutron to induce the fission, so if all the fission neutrons made other fissions you would have k = 1.4 and a runaway reaction. Your reactor must contain some material that eats 0.4 neutrons per fission, and it should be possible to add or remove this "neutron poison" as different parts of the fuel burn faster or slower.

To induce fission on Th-232 takes two neutrons: one to initiate the sequence Th-233 → Pa-233 → U-233, and a second to induce fission in U-233. If thorium reactors exist, U-233 must produce more than three neutrons per fission. But now there is a much bigger variation in the amount of control that's needed. Early in the fuel cycle you need a neutron source, which is usually enriched U-235. The protactinium takes weeks to decay to U-233; during this time you'd like it to be out of the neutron flux, since it apparently likes to send off an extra neutron and turn into the nastier U-232. Finally your U-233 has to be dense enough to sustain a reaction, and you need some neutron absorber to control its rate. If the protactinium decay were faster this would be a nice little system. As it is, it sounds pretty touchy.

Thorium reactors are probably doable in the long term. But sustaining a reaction is much more complicated than simply assembling a pile of enriched uranium, and politically the "simple" reactors are hard enough to build.
posted by fantabulous timewaster at 4:00 AM on November 28, 2009 [3 favorites]

This youtube is a good review of thorium reactors and why we're not using them yet.
posted by anadem at 7:20 AM on November 28, 2009