Over a year ago I wrote a post on Thorium Reactors, noting the “miniscule” amount of radioactive waste produced by such reactors. A recent article by Robert Hargraves and Ralph Moir, Liquid Fuel Nuclear Reactors, in the Physics and Society forum of the American Physical Society, contains an excellent discussion of the safety advantages of this technology. Since I had been thinking about the situation at Fukushima-Daiichi, I was especially struck by their description of the early testing of liquid reactors: “the intrinsic reactivity control was so effective that shutdown was accomplished simply by turning off the steam turbine generator.”
Liquid flouride thorium reactors (LFTRs) operate at atmospheric pressures, providing immunity against the risks of explosion in pressurized designs (and enabling simpler construction and a smaller footprint). But the increased margin of safety for LFTRs is primarily due to the underlying physics:
A molten salt reactor cannot melt down because the normal operating state of the core is already molten. The salts are solid at room temperature, so if a reactor vessel, pump, or pipe ruptured they would spill out and solidify. If the temperature rises, stability is intrinsic due to salt expansion. In an emergency an actively cooled solid plug of salt in a drain pipe melts and the fuel flows to a critically safe dump tank. The Oak Ridge MSRE researchers turned the reactor off this way on weekends.
Hargraves and Moir also explore the cost advantages of LFTRs and the difficulties that LFTRs would pose to proliferation and weaponization.
So what is not to like? Well, there are still technical challenges. Hargraves and Moir, for instance, describe the need to develop corrosion resistant materials for containment, and to discover ways to separate the fission products. But the main problems, in my view, are social and political. We have a nuclear industry with engineers who are specifically trained for pressurized water reactors, regulators and regulations that are heavily invested in this technology, and an educational system which supports the entire superstructure. Overcoming this inertia might be the biggest challenge.
Here is an interesting talk on thorium technology by Kirk Sorensen:
For more information see Kirk’s blog, Energy from Thorium, which also has some interesting discussion of what is happening at Fukushima-Daiichi.
P.S. Here is a nice graphic from WellHome: