Fri Jul 8th, 2005 at 03:16:45 PM EST
(tossed over the wall from Booman)
For decades, scientists have worked toward the dream of harnessing a fusion reaction. Fusion, which involves the joining together of two light atomic nuclei, should produce much more energy and much less radioactive waste than the current nuclear reactors which are based on fission (splitting) of uranium. Unfortunately, making a controlled fusion reaction has proved a lot harder than many people would have thought. For years, the joke has circulated through energy circles: fusion is the power source of the future, and it always will be. Things are not completely bleak on the fusion front. Just last week, France was selected as the site for a massive international effort to build the first practical fusion reactor. If all goes as plans, the reactor could be operational by the end of the decade. Maybe.
But this diary isn't about fusion. It's about that other kind of nuclear reaction, the kind in use around the world today -- fission. It's about how we might be able to make fission not only cleaner, but 100% safe.
Commercial reactors run on uranium
The essence of a conventional nuclear reactor is the controlled fission chain reaction of U-235 and Pu-239. This produces heat which is used to make steam which drives a turbine. The chain reaction depends on having a surplus of neutrons to keep it going (a U-235 fission requires one neutron input and produces on average 2.43 neutrons).
That's the way it's been since the first commercial reactors were started up in the 1950's. Put in uranium, extract an admix of uranium, plutonium, and assorted byproducts.
Only the reasons for using uranium don't all have to do with producing power. In fact, reactors were designed around uranium for a very different reason.
... design challenges and a Cold War-era interest in using nuclear waste byproducts in atomic bombs pushed the industry to use uranium as its primary fuel.
Uranium was picked, not because it was the cleanest or safest possible fuel, but because it was both dirty and dangerous
. Its waste products include everything you need to start up your own Cold War era bomb assembly line - which is exactly why we're so antsy about Iraq having nuclear reactors based on this fuel. Building uranium reactors was easy, since the reaction is self-sustaining, which is exactly why it's also easy to have a reaction run out of control.
Traditional power plants, which involve fuel rods stuffed with pellets of uranium regulated by various means, had some degree of inherent instability. Some of these designs are worse than others (i.e. Chernobyl). Newer power plant designs, like the Pebble Bed Reactor enclose the uranium fuel in small "pebbles" that make it nearly impossible for the reactor to ever run out of control.
But what if there was another choice? What if you didn't run your fission reactors on uranium at all? What if you ran them on another element, one that's much more common, produces less overall waste, and whose use creates much less of the plutonium suitable for making nuclear weapons? What if you ran reactors on thorium?
Why throrium? First of all, it's a lot more abundant than uranium.
For many years there has been interest in utilizing thorium (Th-232) as a nuclear fuel since it is three times as abundant in the earth's crust as uranium. Also, all of the mined thorium is potentially useable in a reactor, compared with the 0.7% of natural uranium, so some 40 times the amount of energy per unit mass might be available.
40x as much energy available from thorium, and thorium is not only available in the United States (which has a decent supply of uranium), but also in countries where uranium is much more scarce. Further, since thorium can't be as easily refined to make nuclear weapons, it can be shipped around the world with less concern.
If we were running our reactors on thorium, we would produce much less waste. If Iraq was running its reactors on thorium, we'd be a lot less concerned about them turning spent fuel into weapons. So why don't we run on thorium?
Well, there's a problem.
A thorium reactor would work by having Th-232 capture a neutron to become Th-233 which decays to uranium-233, which fissions. The problem is that insufficient neutrons are generated to keep the reaction going.
Oh. That sounds pretty bad, huh? A reaction that can't be sustained is back into fusion land - looks good, but not very practical. However, what looks like a problem on the surface, is actually another benefit of using thorium as a fuel. It is
possible to sustain a reaction in a thorium-based reactor, but to do so you have to stimulate it using an "Accelerator Driven System," or ADS. In an ADS, a high energy accelerator is used to spawn additional neutrons through a process called "spallation"
. This does make sustaining the reaction more complicated, but it has a big advantage: turn off the ADS, and the reaction stops. A "subcritical," ADS-based reactor can never run out of control.
So thorium is more abundant than uranium, a reactor based on thorium makes it much tougher to make weapons, and a thorium reactor can never "go Chernobyl." How about nuclear waste?
Fueling nuclear reactors with the element thorium instead of uranium could produce half as much radioactive waste and reduce the availability of weapons-grade plutonium by as much as 80 percent.
There's one other big advantage of using an ADS system. With it, you can "burn" some of the waste products (in this case, the actinides) down to stable isotopes, eliminating a good percentage of the waste. It's not an absolute positive, as the short-term result is even more energetic radioactive isotopes, but over a longer period, the resulting waste should be less radioactive than uranium ore.
With all these advantages, thorium has finally started to get some attention over the last few years. There's been some significant research in both the United States and Russia, and for nearly a decade, India has been running a research reactor on uranium-233 created from thorium fuel. Now India is getting ready to make the next step. They're going to test their own ADS-based, thorium reactor. They have high hopes, and so does the thorium mining industry.
And in January, India -- which has the world's second largest reserve of thorium behind Australia --announced it would begin testing the safety of a design of its own.
The anticipated surge in demand for thorium has led at least one mining company to begin buying as many thorium deposits and stockpiles as it can.
"We feel that it's inevitable that the U.S. and other countries in the world -- India of course -- will exclusively use thorium in the future," said Novastar Director of Strategic Planning Seth Shaw.
How likely is Shaw's "we all move to thorium" scenario? Unfortunately, none too likely -- at least, not in the short term. The current nuclear industry is entrenched in the uranium business. From mining to refining to the reactor, uranium is what they know. Market prices currently put the cost of thorium only slightly below uranium, not enough to cause any major company to consider making the switch.
However, there's one factor overlooked in the current pricing - the cost of waste disposal.
As an interim solution, the United States could change the way it charges power plants for the nuclear waste that they produce, said Kazimi.
Currently, waste fees are calculated as a fraction of the cost of the electricity that is produced by the fuel. Kazimi proposes charging by the volume of plutonium instead, so as to discourage its creation.
Current waste fees are calculated only on the output of the plant, with no regard to the toxicity of the waste. If waste fees were calculated so that the creation of plutonium was discouraged, thorium would suddenly look a lot more attractive to the US industry.
Changing this fee structure is just one of the many items that should be involved in a effective national energy policy. It's one of those minor things, easy to overlook in the glut of new tax breaks for oil companies, that might just lead us to safer power plants.