Project Pele: Why the DoD is using tiny nuclear reactors to solve its energy problems

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In 2019 the government signed a declaration mandating us to develop a tiny nuclear reactor by 2027. In accordance with this arrangement, the US Air Force is launching a “microreactor” pilot project at Eielson AFB in Alaska.

As usual, the Air Force plays its cards pretty close to the chest. As of October 27, the Office of Energy Assurance (OEA) hadn’t even announced that it had decided on a particular reactor technology. But all evidence suggests that this new installation is part of an energy resilience effort known as Project Pele. The aim of the Pele project, according to the Department of Defense Research and Engineering Office, is “to design, build and demonstrate a prototype of a mobile nuclear reactor within five years”. Three separate development contracts were awarded, with the final “mature” design submissions being TBA.

Project Pele has two main themes: the reactor must be 1) small and 2) safe. What we learned from Chernobyl and Fukushima is that a failure in the cooling system can have dire consequences, and in both cases, the failure of the cooling system to power the fuel caused the fuel to get so hot that it melted. Failure is just unacceptable. With nuclear energy, we also need to consider decay heat and the disposal of spent fuel. The inability to dispose of hazardous by-products is considered unsafe. Worse, the same stuff we use to make the Force can be used to make weapons too. But the new Generation IV reactors can fuel the conversation.

Without getting completely breathless, I’d like to talk about one of the three designs that are likely to be particularly brought out. One of the commercial contractors selected to submit a design is a household business called X-Energy, whose managers come from NASA and the US Department of Energy. Its CEO, Jeffrey Sells, was previously Deputy Secretary of Energy, and founder Kam Ghaffarian ran a NASA service provider supporting the former Mission Operations Data Systems at Goddard. The X-Energy model is a Generation IV gas-cooled, high-temperature pebble bed reactor. It uses TRISO fuel pellets or “pebbles” (TRISO stands for TRi-structural ISOtropic Particle Fuel), which are loaded into a column which is then loaded with a heavy, non-reactive gas is flooded. And the whole thing is absolutely tiny: The X-energy website does not describe its reactors as construction sites, but as modular products that can be shipped on the existing road and rail.

The pebble bed model used by X-Energy is clearly intended to specifically address many known flaws in nuclear power generation. Whether it actually keeps this promise remains to be seen, because all of this is still in the planning stage, but the design principles are there. The first and worst is the core meltdown, which X-Energy mitigates through the composition of the fuel itself. The TRISO pebbles consist of a poppy-sized uranium oxycarbide granulate coated with pyrolytic graphite and embedded in a firebreak made of silicon carbide. The whole thing is the size of a game ball.

Silicon carbide is what NASA uses in the heat shielding of many spacecraft. It’s tough stuff, very pressurized, and very difficult to melt. Carbides are not melted and poured like normal metals because their melting points are higher than any other metal. Instead, uranium oxycarbide is made by spark plasma sintering. TRISO pebbles are also passively controlled by a negative feedback mechanism that starves the fuel with neutrons when the temperature rises, regardless of active or mechanical control. Higher temperatures mean a decreasing reaction force, which is forced by the nature of the material itself. It’s hard to have a meltdown when your fuel just … doesn’t melt.

Explosions also present their own hazards, including particles of burning fissile material or graphite shields. With this construction, the reaction is maintained at temperatures well above the glow point of graphite. This prevents potential stray energy from being “stuck” in the crystal lattice of the graphite due to neutron bombardment and ultimately escaping in an uncontrolled outbreak, which happened with the windshed fire. Pyrolytic carbon can burn in air when it is in the presence of enough water to catalyze the reaction, but there is no water cooling circuit to prevent a steam explosion.

The use of uranium oxycarbide instead of uranium oxide or carbide is said to reduce oxygen stoichiometry; Carbides are strong under pressure but not under expansion, so the oxycarbide should produce less gas as it decomposes. This means that even if one of the carbide pebbles should burst, asphyxiated in the heavier than air gas, it will not catch fire. The coolant never leaves the gas phase. The design is based on simply bringing a critical mass of fissile material into a gas-cooled reaction vessel, where it becomes critical on its own. They just sit a bunch of angry jaw breakers on the bottom of a tank where they make each other produce energy. Instead of shutting down to replace fuel rods, ball bed reactors periodically collect a pebble from the bottom of the container by gravity, test it, and return it to the top of the column.

Look at it. It’s the worst gobstopper.

When fully commissioned, the reactor will produce between one and five megawatts. That’s pretty small for any power plant, and even more so for a nuclear power plant – nuclear power plants are often rated in the hundreds of megawatts or even the gigawatts range. With five megawatts, it still covers almost a third of the gross energy budget of the Eielson base. But the microreactor is not installed in such a way that it can handle the power consumption of the base. This is a proof of concept for both a reactor design that fails towards safety and a portable radiant energy source that does not require a constant external supply of material.

One serious vulnerability this reactor could fix is ​​the way the armed forces come to power in the field. In Iraq and Afghanistan, for example, the military used fuel convoys to transport diesel to their facilities that ran on diesel generators. But generators are noisy, dirty, expensive and prone to failure. They also pose a threat to human health: generators that burn fuel produce dangerous fumes and fine particles. In addition, the convoys themselves were the low-hanging fruit of insurgent attacks. All of this requires maintenance and a lot of security. Much of the reasons Eielson was chosen to alternate is because of its reliance on fossil fuels that need to be transported, such as coal and diesel. The armed forces have a direct strategic interest in weaning their operations off of petroleum fuels as much as possible.

However, what benefits the military often ends up improving the lives of civilians as well. Eielson AFB is only a hundred miles south of the Arctic Circle. During the heating season, the base can burn 800 tons of coal a day. Like much of Alaska, it is committed to failure-prone power lines exactly when they are needed most. Most of the state uses coal or diesel to supply electricity and heat. Much of Alaska can only be reached by boat or plane. Juneau doesn’t even have a road connecting it to the outside world because the terrain is so uncooperative. One point of failure can easily coincide with another. Eielson’s northern location and inexhaustible fuel requirements make it an excellent sandpit (snow bank?) For field testing of the microreactor. Greater Alaska is also showing great interest: According to the Anchorage Daily News, “an inexpensive 1-5 MW generator that does not require refueling could mean a fundamental change for rural energy in our state, as this range meets the needs of dozens of villages off the road network, which currently have one of the most expensive power grids in the state – and which are prone to generator failures in the dead of winter, when the consequences can be life-threatening.

The question of waste disposal remains unsolved. As shiny and chrome as these pebbles may be, they still represent roughly the same radioactivity per kilowatt-hour as conventional spent fuel – it is just distributed over a larger volume. While this hypothetically makes dealing with generated waste less awful, there is more of it and it complicates the already multiple problems of waste treatment and storage.

The final drafts are to be selected in the 2022 financial year. From there, the DOD plans to commission a reactor by 2027.

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