Th Better than Uranium
Naturally, I'm speaking of Thorium (Th) here, my perennially favorite nuclear material. In the face of CO2 restrictions, and new energy policies, nuclear power promises to be a sector of our power portfolio. The E.U., North America, China, India and others have left the door wide open for nuclear energy development. However, in the respected journal Nature, there was recently published a little bad news- 'Canned Nuclear Waste Cooks Container' - storing nuclear waste looks more difficult then we expected. But, not all nuclear waste is created equal, and Thorium offers compelling possibilities when it comes to thinking about waste. In every sustainable industry model, it is a good idea to think about living with your waste. Shoving Yucca Mountain full of spent nuclear fuel never really seemed like a great idea to me...sure I couldn't think of anything better either, but it still made me uneasy. When I heard about the new challenges facing containment of nuclear waste, I wondered how Thorium held up against the rest of the crowd.
I'm not an expert in nuclear energy- I can hardly tell Uranium 238 from Uranium 235 - so I turned to someone who has rapidly become the 'go to guy' for Thorium energy- Kirk Sorensen. His blog, Energy from Thorium, does an excellent job of discussing the details, and he furnished me with a rather amusing 45 min podcast from December where he talks about just this issue on 'The Atomic Show'. Kirk further explained to me specific ways we can reduce the waste problem through using thorium reactors.
In today's "once-through" uranium-fueled reactors, we mine uranium, enrich it a little in uranium-235, burn-up some of that U-235, and then throw it away, supposedly in Yucca Mountain. (very much in the model of a 'thow away society') When we start out with pure uranium oxide, (roughly 97% U-238 and 3% U-235) and run it though current methods we end up with three broad categories of "stuff" in the fuel.
First, there's the unburned uranium-238 and uranium-235. This uranium is no more dangerous after being in the reactor than it was before (except that now it's mixed with other products). It has billion-year half-lives, which means it practically never decays (which is why it's still around to dig up five billion years after it formed in a supernova). So the uranium's not a risk.
Then second group of leftovers are the fission products (the actual waste of fission). These fission products are very radioactive, and give off dangerous radiation. We have to keep these fission products away from people and the environment. But because the fission products are so radioactive, they decay quickly. Most decay to stable elements in a few hours, some take days. And a very few take years or decades. But, if we leave the fission products alone for a few hundred years, they will decay to normal background levels of radiation (Safe enough we don't need to worry about them as much).
Finally, there are the transuranic isotopes. These are formed when uranium absorbs a neutron and doesn't fission, and include some nasty elements like neptunium, plutonium, americium, curium. The transuranics are radioactive for hundreds to tens of thousands of years, and as they decay they give off different kinds of radiation. It's the transuranic waste that is the reason why you have to build a place like Yucca Mountain that must remain geologically isolated for tens of thousands of years.
The really exciting thing about thorium is that it is possible to build thorium-fueled reactors that don't throw away thorium or uranium, don't produce transuranics, and only generate fission product waste. The fluoride reactor technology that was developed in the United States in the 1950s and 1960s at Oak Ridge National Lab is the key.
These reactors use chemically-stable fluoride salts (similar to the sodium fluoride salt in your toothpaste) that are impervious to radiation damage. They can also be processed while the reactor is operating to scrub out the fission products that are the real nuclear waste. So there's no reason to ever throw away valuable thorium and uranium-233—just keep burning it until it's all gone.
Thorium is better because it has to absorb five neutrons before it will turn into a transuranic isotope, whereas common uranium only has to absorb one- a built in buffer. So by operating a reactor on pure thorium and uranium-233, you can avoid producing the kind of long-lived waste that needs a place like Yucca Mountain.
Thorium is a great possibility- it could be a high density source of clean energy. The fluoride salt thorium reactor can produce nuclear wastes that consist only of fission products, which quickly decay to stable elements - in fact some elements like xenon or rhodium represent valuable commercial products after a few months 'cooling down'. Having waste that only consists of fission products means that the waste only needs to be stored for a few hundred years, not the thousands of years needed for "once-through" uranium waste. The case for Thorium is so convincing, it is almost irresponsible of governments not to pay heed, and establish serious development programs.