At Last – A Good Idea in Nuclear Reactors?
by Cheryl Rofer
The Guardian reports on a small nuclear reactor that can provide electricity to 20,000 homes. The reactor would be installed underground and refuelled every 5-10 years. Hyperion Power Generation, located in New Mexico, plans to build the reactors. According to the Guardian, Hyperion now has more than 100 firm orders for reactors.
As usual, there isn’t as much information as I’d like to see, but there are some tantalizing hints. This website (from an organization I’ve never heard of) claims that the reactor design is based on the TRIGA reactor, which has been installed in many universities around the world. The original TRIGA design required highly enriched uranium fuel. That fuel, because it might be made into nuclear bombs, is now being replaced with fuel enriched only to reactor grade, not capable of being made into bombs.
Question: How does that work? For fission to take place, there must be a critical mass of uranium-235. If the fuel is less enriched, there must be more total uranium in the core. This problem has apparently been overcome, however, in the replacement fuel that is being loaded into university training reactors.
Installing the reactor underground is a good idea for many reasons: it is safe from having an airplane flown into it, human access can be limited, and, if anything goes wrong with the reactor, soil and concrete will attenuate radiation. However, there have to be two kinds of access: for heat removal and for refueling. In this drawing (too many pyramids – never mind), you can see two tubes going to the surface for heat removal. 
A nuclear reactor produces heat, not electricity. The heat, just like the heat from a coal plant or solar thermal, runs a turbine that produces the electricity (or, in the drawing, runs a desalination plant). Turbines need maintenance and therefore cannot be sealed underground. So the heat from the underground reactor must be brought to the turbine. Typically reactors have a fluid that cools the fuel elements and a working fluid. The fluid that cools the fuel elements becomes slightly radioactive, so it cannot be blown through a turbine and exhausted to the atmosphere. It is run through a heat exchanger, where it heats the secondary coolant. Heat exchangers need maintenance, too, so they would likely be located at the surface, with the turbine. The two tubes in the drawing allow the primary coolant to cycle from the reactor to the desalination plant and back.
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