The two heaviest naturally occurring radioactive elements (by atomic weight) on the earth are Uranium and Thorium.  Uranium is used as a fuel in modern commercial nuclear power reactors for electricity generation.  A lesser known fact is that thorium could also be used as a nuclear fuel .  Naturally occurring thorium is not fissile and so not able to undergo nuclear fission (separation) and as such it takes an initial nuclear reaction to enable this process.

Thorium only occurs naturally in the form of the isotope Thorium-232 (232Th).  This means that along with 90 protons, the nucleus of these atoms have 142 neutrons (90+142=232).  Most atoms have some ability to absorb an additional neutron although this often makes the atom radioactive. 

When 232Th absorbs a neutron, it becomes 233Th and within an hour or two will decay with the emission of an electron from its nucleus. 

 When the electron is emitted from the 233Th nucleus, one of the neutrons also gives off a neutrino and a proton.  In essence, the neutron becomes a proton an electron and a neutrino with only the proton being held in the nucleus through the strong nuclear force.  What is left is known as protactinium or 233Pa. 

Image: dymidziuk.janusz, CC BY-NC-SA

After around a month or more, the 233Pa which itself is radioactive will also decay through the same process of giving off an electron while converting a neutron into a proton.   Why this is significant is that the final product from these two successive radioactive decay processes is a special isotope of uranium known as 233U.  This is unique in that 233U is fissile, radioactive (although with a long half life) and not naturally occurring.

   This is a rather large bit of trouble to go through to create a fissile uranium species but there are some attractive features to this technology.  Perhaps the main attractive feature is that thorium is around 4 times more abundant than naturally occurring uranium in the earth's crust.  Uranium is more abundant in seawater than is Thorium due to solubility chemistry.  Overall, there is estimated to be much more thorium than uranium in the earth. 

To date, only the nation of India is actively pursuing a thorium based nuclear fuel cycle for electricity production.  This is based largely on the fact that their country does not have many native uranium resources but it does have substantial thorium ores.

Thorium does show promise for an economically viable fuel source someday but the potential use of it in the US does not appear to be likely in the near term.