Another semi-recent story concerns the use of thorium as a “fuel” in typical, light-water reactors. I put “fuel” in quotations because the only way to truly utilize the thorium resources of the world is to follow the three-step process I have outlined. First, convert the thorium to protactinium-233, isolate the protactinium until it decays to uranium-233, then introduce the uranium-233 back into the reactor to fission and produce the neutrons to convert additional thorium.
As has been mentioned, this process is not only possible in thermal-neutron reactors, but attractive. However, there are more and less attractive ways to do this, and a recent article Thorium Fuel for Nuclear Energy certainly falls in what I would call the latter category.
Thorium as a Solid fuel
Like all other plans that involve using Thorium in a solid form, this plan misses the crucial step where protactinium is isolated from neutrons. Protactinium-233 has a large “cross-section” for thermal neutrons, meaning that it really wants to gobble one up. And if it does, it becomes protactinium-234 and then decays to uranium-234, which is not fissile and can’t continue the process.
Some solid-fueled reactors have attempted to beat this problem by reducing the neutron flux levels in the reactor to the point where Pa-233 has a chance to decay to U-233, rather than gobble up a neutron. But by reducing neutron flux, you’re also reducing the fission rate and the amount of thermal power that the reactor can generate. This means less electricity and less reason to have the reactor in the first place.
But the proposal discussed in this article doesn’t even get that far. The proposers say they want to burn weapons-grade nuclear fuel in thorium, so that as the plutonium burns, additional plutonium isn’t formed. Seemingly admirable, until you realize that again the goal isn’t sustained nuclear energy production but just the destruction of this material that we labored so hard to produce in the first place–material that could be utilized in the right reactor to produce much much more energy.
So rather than taking the short-sighted view and mixing a little thorium with your plutonium, let’s get to work on the liquid-fluoride and chloride reactors, so we can destroy the plutonium, make the U-233, and get our thorium reactors running as soon as possible.