Whatever Became of Thorium?

February 6, 2007 | By | 2 Replies More

And why should we care about this material rarely mentioned outside of science-fiction? Well, it involves recycling, our energy future, and a chance to de-proliferate nuclear weaponry and reduce the threat of “dirty” bombs. First, a brief bit of history:

Back in the 1930’s, it was discovered that isotopes of 3 elements could sustain nuclear fission chain reactions in a reactor. They are:

  • A rare and hard-to-isolate isotope of Element 92, Uranium 235,
  • Two hazardously manufactured isotopes of Element 94, Plutonium 239 and Plutonium 240
  • The cheap and common isotope of Element 90, Thorium 232

Although all three could be used to make a nuclear reactor, in the 1940’s it was important to make a bomb. The amount of the common Thorium isotope needed just to get a reaction going was too heavy to airlift, so they focused on refining Uranium, and producing Plutonium. Both of these 2 tracks bore hideous fruit.

By 1950 we already had big facilities to make these 2 rare and expensive fissile substances, so why not just use them for power stations? (Answer: It’s just too darn easy to make wayward Plutonium into portable bombs.) Our present generation of slow-neutron reactors is reasonably efficient at getting energy out of the original isotopes, and almost as safe as the reactors now being built in Europe. But what about the 95% of the original fuel that we call radioactive waste?

The knee-jerk reation is to do with it what one does with garbage. Bury it. Originally, 10,000 years (about the time between the oldest discovered human city and the present) was considered a long enough storage time to be safe. But the no-nukes crowd managed to get Congress to up the certified unattended time-to-failure requirement for Yucca Mountain storage to a million years.

I’m not here to argue that this is a silly requirement. Just note that highly-radioactive isotopes are mutually exclusive from long-lived isotopes. I’m here to tout thorium, and the fast-neutron fission reactor that can “burn” thorium and also “burn” 90% of the radioactive “waste” that we are burying or planning to bury. Burn it to produce usable energy.

What? Look up fast neutron reactors. Here’s one wiki about them. But the point in this case, is not to produce more fuel, but to use a big reactor to produce energy from slow-neutron-reactor waste isotopes, medical waste isotopes, and from thorium. Sure, a fast-neutron reactor produces plutonium. I’m suggesting that we leave it in there to keep burning until we get useless isotopes of iron, lead, and the relatively safe and/or useful lanthanide isotopes in between those and thorium.

I’d advocate building this reactor right on Yucca mountain, where the 10% leftover waste from burning the original “waste” can still be buried. We already are shipping the waste (fuel!) there. Sure, the energy produced would have to be transported out, and long stretches of wire are expensive, both in terms of installation and parasitic losses. However, we are talking about a reactor that essentially burns dirt and garbage. Okay, thorium is not quite as cheap as dirt, but it is much cheaper than 92U235.

We could set up long lines of power towers, or build a superconducting power conduit cooled by hydrogen produced at the reactor, or just package the energy by producing methane or hydrogen or some other convenient energy storage medium that can be piped or trucked out.

One caveat is that a thorium reactor has to be big. The core needs to be twice as big as for a normal Uranium fast-neutron reactor in order to keep the reaction going. Fine. What is the real-estate on top of a nuclear disposal site going for? Out in the middle of nowhere, the only objection to “big” is that it costs more to build. Once it is going, it should run for about the same operating costs as the reactor near your town. One factor that helps is that the waste used as a co-fuel reduces the size requirement compared to a thorium-only reactor.

One other issue: If we don’t get a reactor like that built before we run out of fossil fuels, we may not have the wherewithal to build it later. Fusion might bear fruit; it’s been predicted “within ten years” since the 1960’s. The best bet for commercial fusion still requires a reactor bigger than what I’m suggesting here.

This is not an original idea, but I’d like to get it out before another audience. Just fuel for thought.


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Category: Economy, Energy, Environment, global warming, Health, History, Military, Science, Technology

About the Author ()

A convoluted mind behind a curly face. A regular traveler, a science buff, and first generation American. Graying of hair, yet still verdant of mind. Lives in South St. Louis City. See his personal website for (too much) more.

Comments (2)

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  1. Erich Vieth says:

    Dan, thanks for the post. I didn't realize there was an alternative nuclear fuel to plutonium and uranium.

  2. Dan Klarmann says:

    One detail I neglected: The "depleted" uranium that we're busily spreading all over the middle-east landscape, in bodies, lungs, etc, is also fuel for this type of reactor.

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