Thorium and uranium are the two natural-occurring elements on Earth that can release nuclear energy through fission. Thorium is about three times more common than uranium, because it has a longer half life and decays more slowly. Most of the uranium and all of the thorium has to be converted to fissile material before it can release its nuclear energy. It is called “fertile material”, because absorbing a neutron will cause the thorium or uranium to become “fissile material”. A tiny fraction of natural uranium is already fissile, and this is the tiny amount of material that powers all of today’s nuclear reactors.
The long-term viability of nuclear energy will come about when we are able to use nuclear fuels with far greater efficiency than we can today.
Both thorium and uranium-238 require two neutrons to release their energies. One neutron converts them into a fissile form and the other neutron actually causes the fission. Thorium absorbs a neutron and becomes uranium-233, which will fission when struck by another neutron. Uranium-238 absorbs a neutron and becomes plutonium-239, which will also fission. But there is an important difference between these two options. The fission of uranium-233, when one accounts for non-fission absorptions, will produce 2.3 neutrons. This is enough to continue the conversion of more thorium to uranium-233 fuel, even when accounting for various losses. But the fission of plutonium-239 will produce less than two neutrons. It is not sustainable in today’s thermal-spectrum reactors.
The only way to sustainably use uranium-238 and plutonium is to go to fast-spectrum reactors, which intentionally attempt to keep neutrons at as high a velocity as possible. In these reactors, non-productive neutron absorption in plutonium is suppressed and plutonium fission will produce more than two neutrons, enabling sustained consumption of uranium.
The fundamental disadvantage of fast reactors is that the probability of neutron reactions, typically represented by a cross-sectional area, is much lower when the reactions are caused by fast neutrons than by slowed-down, thermal neutrons. This picture depicts the “size” of a plutonium-239 nucleus to a thermal neutron on the left. The blue region represents the probability that the nucleus will fission and the red area represents the probability that the plutonium will simply absorb the neutron. As you can see, plutonium-239 will absorb thermal neutrons roughly one out of three times. On the right, to the same scale, you can see the size of the cross-sections of plutonium-239 for fast neutrons. They are much, much smaller; so much so that it requires hundreds of plutonium atoms to achieve the same probability of fission as a single plutonium atom to a thermal neutron. The implication of this difference is that fast reactors require much larger inventories of fissile fuel for a given power rating.
Using the thorium fuel cycle efficiently can nearly eliminate the production of transuranic materials such as plutonium, which are a major concern in the disposal of nuclear fuel. Because the thorium fuel cycle begins roughly six mass units before the uranium approach, it requires more neutron absorptions before it reaches its first transuranic nuclide, in this case neptunium-237. Because there are two opportunities for fission along this path, in the form of uranium-233 and uranium-235, the theoretical maximum production of transuranics is only 1.5% and plutonium generation has the potential to be completely eliminated. By contrast, most of uranium fuel is only a single neutron absorption away from plutonium production.