tough going, page-67

Hokay. This is likely to be fairly long and detailed, but here's what I know about thorium. (Me? Bored at work? Naaaaaaaaah.) But first, a slight diversion - this next paragraph is fairly basic stuff, but it sets the scene.

When generating electricity, you're pretty much talking about generating heat, boiling water into steam, and using that steam to drive a turbine. There are exceptions (wind power drives the turbine directly, and photovoltaic solar cells use the photoelectric effect to generate electricity without needing a turbine), but by and large, that's the rule, especially at the gigawatt end of town. That heat can be generated by several approaches: burning something (coal, oil, natural gas, biogas); focusing sunlight (thermal solar); and nuclear power (fusion, which has not yet reached consistent break-even, and fission.)

So let's talk about fission. That means splitting one atom into two smaller pieces. Nothing more, nothing less. Some fission reactions release energy; others require energy. Naturally, power generation focuses on those reactions that release energy, which means you're using a fissile substance to generate the heat. In nature, only one fissile substance is found in any quantity: uranium-235. U-235 makes up a small percentage (about 0.72%) of natural uranium; most of the balance is uranium-238 (U-238). So uranium is normally enriched, to give a mix that's around 5% U-235 (give or take), for use in a nuclear reactor.

It's possible to design reactors that turn U-238 (via neutron capture) into U-239, which rapidly decays into neptunium-239 (Np-239), which in its turn rapidly decays into plutonium-239 (Pu-239). Pu-239 is also fissile. This is called breeding; U-238 is called a fertile material: although it's not particularly useful as a fission fuel on its own, it can be turned into one relatively easily. Because of the chemical differences between plutonium and uranium, it's easier to extract highly concentrated plutonium, which is then useful as a fuel (provided it's not contaminated with significant quantities of Pu-240; most reactors used for breeding weapons material are designed to pull the fuel out before that happens.)

So. On to thorium. Thorium is also a fertile material, and it's far more abundant than uranium. Most (>99.999%) natural thorium is Th-232. This can be turned (via neutron capture) into Th-233, which decays to protactinium-233, which in turn decays to U-233. U-233 is fissile.

Without going into the full gory details (of which I only understand a small part), U-233 is less useful for weapons than U-235 or Pu-239. That makes thorium reactors less of a proliferation risk. But it also means that they're not useful from a military point of view, which is why the USA and the USSR opted for uranium reactors: it's much easier to produce weapons-grade material with a uranium-based fuel cycle than with a thorium-based fuel cycle (see above comments about chemical differences.)

If the world moves to a thorium fuel cycle, a lot of the proliferation risks become less of a concern, which makes waste disposal much easier: over 95% of the "waste" from current reactors is actually unused fuel, but removing the useful fuel from that waste stream poses a proliferation risk (handwave, handwave, ignoring radiation issues.) Because thorium reactors can be designed to reduce the amount of useful weapons-grade material, their waste can be reprocessed to recover unused thorium, leaving behind only relatively high-level waste that decays quickly, hence doesn't need to be stored for extended periods of time (more than a couple of hundred years.)

So the major advantage of the thorium fuel cycle: much more base fuel; much lower proliferation risk. The major disadvantage: research isn't as advanced as uranium research, because it isn't as useful as a weapons base, and it's military applications that drove the development of nuclear power.

The other thing: because thorium (and uranium) have a long half life (and hence stick around for a long time), they're actually relatively safe to handle. I'd rather have a kilo of thorium in my hand than a kilo of raw sewage, for example. Just don't eat it; it's an alpha emitter, and they're really nasty if they get into the body. Or, in other words, the concern about radiation is not entirely misplaced, but it should be about the possibility of the material entering the food chain, more than anything else (I think.)

I am not a nuclear physicist. Take all the above with appropriate shakers of salt, and feel free to correct me if I'm wrong.