Although not fissile itself, Th-232 will absorb slow neutrons to produce uranium-233 (U-233)a, which is fissile (and long-lived).

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That work, by a group of researchers at the Institut Laue-Langevin (ILL) and published in a paper in Nature, used slow-moving neutrons falling due to gravity.

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Professor Egil Lillestol of Bergen University has been pushing thorium for some years now, and thinks that Norway should set the trend in building a prototype accelerator-driven reactor in which a massive particle accelerator converts thorium-232 to uranium-233 by irradiating it with slow (spallation) neutrons generated by the impact of a 1.6 GeV proton beam on a lead target.

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Given a start with some other fissile material (U-233, U-235 or Pu-239) as a driver, a breeding cycle similar to but more efficient than that with U-238 and plutonium (in normal, slow neutron reactors) can be set up. (The driver fuels provide all the neutrons initially, but are progressively supplemented by U-233 as it forms from the thorium.) However, there are also features of the neutron economy which counter this advantage.

FORBES: Is Thorium the Biggest Energy Breakthrough Since Fire? Possibly.