Thorium can be used as a nuclear fuel through breeding to fissile uranium-233. For those technically-inclined readers, here is a geek-friendly explanation of what that means:
Although not fissile itself, Th-232 will absorb slow neutrons to produce uranium-233 (U-233)a, which is fissile (and long-lived). The irradiated fuel can then be unloaded from the reactor, the U-233 separated from the thorium, and fed back into another reactor as part of a closed fuel cycle. Alternatively, U-233 can be bred from thorium in a blanket, the U-233 separated, and then fed into the core.
In one significant respect U-233 is better than uranium-235 and plutonium-239, because of its higher neutron yield per neutron absorbed. 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. In particular the intermediate product protactinium-233 (Pa-233) is a neutron absorber which diminishes U-233 yield.
I have no idea whether thorium is the panacea many people claims it is likely to be, but I believe we’ll be hearing more about it in the years to come.
UPDATE: If you want to know more about Thorium, the person to ask is my fellow Forbes contributor – and resident nuclear energy expert – Kirk Sorensen. Check out Sorensen’s work here:
http://blogs.forbes.com/kirksorensen/
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