Nearly two decades ago I took a short class on nuclear criticality safety – basically a class on how to work safely with things like enriched uranium. When we were reviewing the nuclear properties of U-235 (the isotope of uranium that fissions) I happened to notice that U-233 seemed to have a lot of advantages over U-235, and that U-233 could be fairly easily produced from natural thorium, virtually all of which is Th-232. Why this is significant is that, in a nuclear reactor, Th-232 will capture a neutron and transmute into U-233. From my geochemistry classes I knew that there is four times as much thorium on Earth as there is uranium, and that less than 1% of uranium consists of U-235 to begin with – this is why we need to go to all the bother of uranium enrichment. Given that it seemed possible to use the abundant thorium to breed fissionable U-233, I asked our instructor, why weren’t we using thorium for fuel? His answer was that the radioactive waste from U-233 fission was much more dangerous and harder to work with than that from “conventional” U-235 fission, and I was content to leave it at that.
Fast forward to two weeks ago when I saw a book called “Superfuel” in which the author, journalist Richard Martin asks fundamentally the same question I raised, but actually goes into the details. According to Martin, the US dabbled with thorium power in the 1950s before giving up on it in the 1970s. Not only that, but there is a thorium movement of sorts in the US, but even more so in India and China. The International Atomic Energy Agency (IAEA) has written about thorium power, and there are a number of papers about it in the scientific and engineering literature.
I’ve been going through a lot of this literature for the last few weeks, trying to condense it down into a single ScienceWonk posting. What I’ve found is that the pros and cons of thorium power are not as simple as one might be led to believe by those arrayed in favor or against thorium power. On balance it seems that there is a lot in its favor, but the case is not as simple as the pro-thorium advocates might have us believe. In any event, I’ve come to realize that the issues surrounding thorium power are far too much to summarize in a single posting – Martin struggled to address them all in his book – and doing justice to the topic really calls for several postings. So that’s what I’m going to do – in this posting I will briefly discuss the fundamental science and technology without a great deal of editorial commentary and in future postings I’ll try to address some of the advantages and drawbacks of thorium power compared to nuclear and other sources of energy. Since thorium power touches on issues of radioactive waste management, greenhouse gas emissions, proliferation, and more, a series of postings seems justified. So – on with the basic science, and we’ll take on the other issues in coming weeks!
The fundamental idea here is fairly straight-forward – if we put thorium into a neutron-rich environment then some of those neutrons will be captured by thorium atoms, transforming the natural Th-232 into Th-233. With a half-life of a bit over 22 minutes, Th-233 quickly decays to form protactinium (Pa-233, to be precise), and the Pa-233 decays with a half-life of just a tad under four weeks to form U-233, which fissions quite nicely. Or, put another way, load a reactor with Th-232 and within several months you’ll have a reactor full of U-233, which can continue to power the fission reaction. A thorium reactor is a breeder reactor, producing as much or more fuel as it consumes.
One of the nicest things about thorium fuel is that every atom of Th-232 can be turned into U-233 fuel. Not only that, but there is four times as much thorium on Earth as there is uranium. So think about these two facts – and remember that, of uranium, less than 1% is the fissionable U-235. This means that one gram of thorium has more than 100 times as many potentially fissionable atoms as a gram of natural uranium – and does not require a costly enrichment process – and Earth contains 400 times as much potential fuel from the thorium cycle as from uranium.
The way a thorium reactor would work in practice is that the whole process would have to be initiated by a sort of “starter core” fueled with uranium and the uranium core would be surrounded by a thorium blanket. Neutrons from uranium fission would be captured by the thorium blanket, breeding the U-233 that would fuel the reactor from there. If the thorium is in solid pellets it’ll be necessary to do some chemical processing to separate the uranium, but chemical processing is a lot easier than uranium enrichment. But an intriguing idea is to use liquid fuel – dissolve the thorium and the uranium into a liquid salt – and the liquid can simply be run through chemical processing on a recurring basis to not only separate out the uranium, but also to remove the fission products. Liquid fuel has both pluses and minuses – I’ll go over them in a later posting on this topic.
With this as a starter, here’s a little bit of a roadmap to future postings on this topic:
- Thorium reactors can be designed to be inherently safer than the current crop of water-cooled nuclear reactors
- The waste from thorium-cycle reactors is shorter-lived than that from U-235-fueled reactors, reducing the problem of radioactive waste disposal
- Thorium-cycle reactors have some advantages when it comes to proliferation resistance
- And liquid fuel reactors have some advantages over reactors with solid fuel rods.
There’s more, but we’ll leave it at this for the moment. Stay tuned for more postings!