--Providing HN the details about Thorium that the Forbes article lacks--<p>tl;dr - Read Dr. Alvin Weinberg's book from the late 1990s. Amazon link here: <a href="http://www.amazon.com/First-Nuclear-Era-Times-Technological/dp/1563963582/ref=sr_1_1?ie=UTF8&qid=1315794920&sr=8-1" rel="nofollow">http://www.amazon.com/First-Nuclear-Era-Times-Technological/...</a><p>-Background-
I discovered liquid fueled nuclear reactors, and the thorium subset thereof, as a consequence of the chemistry minor I undertook in grad school at Georgia Tech (my background is materials engineering). One of the classes I took was taught by Dr. Jiri Janata, and it was functionally a class in analytical radiochemistry. Dr. Janata's expertise is in chemical sensors, and he worked for an number of years at Pacific Northwest National Lab (PNNL) on methods to detect the spread of radiation in the environment. Dr. Janata exposed our class to the liquid fueled reactors.<p>-LFR-
To read in Dr. Weinberg's book, Oak Ridge was left out in the cold when it came to reactor design. This despite the fact that Weinberg and Eugene Wigner wrote "The Physical Theory of Neutron Reactors," as the definitive first text on reactor physics. Wigner and Weinberg dreamed up many dozens of potential power reactor design concepts in the 40s.<p>Oak Ridge National Lab managed to procure funding to pursue reactors that might power airplanes. Weinberg is candid about how the concept of nuclear powered flight was nearly fiction, but any grant in a storm! Any grant in a storm is still alive and well, btw.<p>Out of that work came the liquid fueled reactors, of which thorium could be one of the fuels. The liquids were composed of multi-component molten halide salt solutions that had some partial solubility for certain radionuclide salts. Much of the molten salt chemistry details we have today come as consequence of the research into their behavior from Oak Ridge.<p>Liquid fuels for reactors have many advantages:
1. They operate at atmospheric pressure (1 atm), so there's no pressure vessel to worry about bursting in an accident.
2. Molten salts have very little vapor pressure, and therefore don't volatilize as readily.
3. The molten salts allow very high operational temperatures for better Carnot efficiency, in part because of 2.
4. The systems is single phase, liquid only. This is in contrast to 2-phase behavior of something like BWR reactors
5. Waste fission products (e.g. iodine) can be scrubbed from the molten fuel during operation. The fuel composition can be monitored and changed as needed during operation.
6. Neutron reflectors are needed to obtain criticality in the system. The molten salt with nuclear material in it is subcritical by nature.<p>For a liquid fuel reactor, there is no loss of coolant accident, as the fuel is in the working fluid. The amount of decay heat from fission products remaining in the fuel can be lower if there would be scrubbing in place. As the world sees now, decay heat is the tiger in the room for reactor safety.<p>-Brief Accident Scenario-
If an accident occurs, and power is lost, the molten fuel drains back into a core sump vessel which then is cooled to deal with the decay heat. Because the fuel is dispersed, and there are no high pressures to deal with, passive cooling of the decay heat in the molten fuel sump is greatly simplified. Further, natural convection can be stimulated in the sump to help circulate the fuel and remove heat.<p>-Follow Up(?)-
There are some downsides, of course, but this is already crazy long. If the OP is still around in the morning on the East Coast, I'll discuss some of the negatives in another comment.<p>-phil