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.

Questions: How is the reactor accessed for refueling? What are the safeguards against unauthorized access? Some details cannot be made available, obviously, but the broad outlines of these procedures should be.

Hyperion says

Water not used as coolant; cannot go “supercritical” or get too hot

This seems to be related to another claim,

Uniquely safe, self-moderating using a natural chemical reaction discovered 50 years ago

Question: What is the coolant?

There are multiple things wrong with those claims. The composition of the coolant has little to do with going “supercritical” or getting too hot. Moderation of a nuclear reactor has to do with physics, not chemistry. Getting too hot and moderation have to do with the rate of nuclear fission.

That other website points out that the fuel in TRIGA reactors, uranium-zirconium hydride, has a negative thermal coefficient of reactivity, which means that as it heats up, fission slows down. The RBMK reactors at Chernobyl were designed the opposite way, which had a lot to do with how they got out of control. So the Hyperion reactors (referred to as “hydride,” which suggests that this is indeed the type of fuel they use) should inherently keep from overheating.

Moderation means slowing down the neutrons to speeds at which they cause fission most efficiently. Carbon and hydrogen are frequently used as moderators, and the TRIGA fuel contains hydrogen. It is possible that the chemical reaction Hyperion refers to is the decomposition of the fuel at high temperatures into hydrogen gas, which could decrease the moderation and thereby damp down the fission reactions, another safety feature to prevent overheating of the reactor. Come to think of it, this may be what provides the negative thermal coefficient of reactivity, so perhaps Hyperion is taking credit for that twice.

I disapprove, also, of trying to get away with using words that people aren’t going to understand by putting those words in quotes. Supercritical refers to the state in which more neutrons are produced than are needed to maintain the fission reaction. A reactor should chug along at pretty close to critical, balancing neutrons produced with neutrons absorbed. Supercritical, in my experience, is most often used to describe nuclear explosions. So, good heavens, I should hope not.

Another of Hyperion’s claims is

Think: Large Battery!

A nuclear reactor is a heat generator; a battery generates electricity directly. I suspect that an overenthusiastic publicity person, aided and abetted by a scientist who didn’t realize that accuracy is important in developing public trust, came up with this one.

My biggest concern about the nuclear industry has been less technical than temperamental. Too many people associated with that industry have been too ready to believe that the public doesn’t need to know all the details; heck, they don’t need to know any of the details. It has been this attitude that has destroyed public confidence in the industry and inflated the industry’s confidence in itself to where it has made mistakes like those at Three Mile Island.

It is that “baffle ‘em with bullshit” approach that I sense in those Hyperion claims. If Hyperion's attitude is the sameold sameold of the nuclear industry, this enterprise will fail too. That would be too bad, because there appear to be many positives for little reactors like this one.

Drawings from Hyperion's website.