-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.
-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.
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.
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.
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.
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.
-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.
-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.
I wouldn't pick this nit if my link wasn't so awesome [1].
Nuclear-powered aircraft are workable if you don't care about shielding. Who in their right mind would design a flying, unshielded nuclear reactor? A Cold War weapons designer who wanted to build:
... a locomotive-size missile that would travel at near-treetop level at three times the speed of sound, tossing out hydrogen bombs as it roared overhead.
The article at the other end of the link is the most tooth-curling engineer porn I've ever read. It's horrible. But also strangely awesome:
Pluto's designers calculated that its shock wave alone might kill people on the ground. Then there was the problem of fallout. In addition to gamma and neutron radiation from the unshielded reactor, Pluto's nuclear ramjet would spew fission fragments out in its exhaust as it flew by. (One enterprising weaponeer had a plan to turn an obvious peace-time liability into a wartime asset: he suggested flying the radioactive rocket back and forth over the Soviet Union after it had dropped its bombs.)
Wow, I was totally unaware of this project, or how far along they got. The folks at Oak Ridge had hypothesized a horse and carriage type device where the fuselage would trail the unshielded reactor engine. The part about the company that is now CoorsTek is also interesting. CoorsTek still makes lots of important high tech ceramic bits.
--Providing HN the details about Thorium that the Forbes article lacks--
tl;dr - While the technology is solid, thorium reactor technology still has some practical kinks to work through, the kind that have already been addressed in large scale pwr/bwr commercial reactors.
-Drawbacks-
1. Thorium fueled reactors can still contribute towards the production of atomic devices, though not as readily, and most likely not the "classic" PU-239 based weapons.
2. The Oak Ridge reactor was built for a very specific purpose, with very high outlet temperatures and as little weight as feasible. It literally ran red hot. This is not the operating conditions under which a commercial power generating facility would operate. See the wikipedia article on the Oak Ridge reactors here: http://en.wikipedia.org/wiki/Aircraft_Reactor_Experimenthttp://en.wikipedia.org/wiki/Molten-Salt_Reactor_Experiment
3. The handling of liquid fuels requires a complete paradigm shift in the regulation and methods of the existing nuclear industry
4. There were some hot corrosion issues of the Inconel alloys used in the piping of the MSRE. Evidently the Japanese solved these problems in the 1990s, but I have not been able to find the papers that talk about how.
5. In the US, at least, we will have to overturn Jimmy Carter's ban on nuclear fuel reprocessing to get the maximum benefit of the technology. This needs to happen independent of LFRs.
There are probably other downsides on the tip of my tongue, but I cannot tease them out at the moment. Folks who are interested in more can contact me off HN at the email address in my profile.
tl;dr - Read Dr. Alvin Weinberg's book from the late 1990s. Amazon link here: http://www.amazon.com/First-Nuclear-Era-Times-Technological/...
-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.
-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.
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.
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.
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.
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.
-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.
-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.
-phil