Reactor type 9 of 10
Microreactors
by Joe Miller
Microreactors are tiny reactors of about 1 to 20 megawatts that fit on a truck. Many use heat pipes, sealed tubes with liquid sodium inside, that carry heat out of the core with no pumps at all.
Plain-English explainer by Joe Miller, a nuclear engineer. Not investment advice. Figures are the latest public numbers we could find and change fast; sources are at the bottom of each page.
What it looks like
How the coolant circulates
- Fuel heats the core block.
- Heat pipes carry the heat passively to the heat exchanger.
- Air or CO₂ is heated and spins a gas turbine.
- Waste heat goes to the air (no cooling water needed in many designs).
The main components
| Component | What it does |
|---|---|
| Core block | A solid steel or graphite block holding fuel, often TRISO. |
| Heat pipes | Sealed tubes; sodium inside boils at the hot end, travels to the cool end, condenses and wicks back. |
| Heat exchanger | Transfers heat to air, CO₂ or helium. |
| Power conversion | A small gas turbine (Brayton cycle) or steam; some designs just supply heat. |
| Shielding and transport cask | Lets the reactor be shipped and returned to the factory for refueling. |
Key details
| Item | Detail |
|---|---|
| Coolant | Heat pipes, gas or sodium |
| Size | About 1 to 20 MWe |
| Fuel | TRISO or metal, HALEU |
| Refueling | Years between refueling; often swapped as a whole unit |
| Cooling | Often air-cooled, no water source needed |
| Uses | Bases, mines, remote towns, data centers, disaster relief |
How it is maintained
- Few moving parts in heat-pipe designs: no reactor pumps.
- Remote monitoring with a small on-site crew.
- Return-to-factory refueling: the whole reactor is shipped back after several years.
- Security for small, remote sites is a key design issue.
Where it runs
In 2026, four test microreactors reached criticality under the Energy Department's Reactor Pilot Program, including Antares (sodium heat-pipe) and Valar Atomics (helium-cooled). Westinghouse eVinci, the Defense Department's Project Pele and Oklo's Aurora are in development.
Who's writing this? I'm Joe Miller, a nuclear engineer since 1974 and an ASME Fellow. I've run safety analyses for PWRs and BWRs, directed nuclear engineering at a working BWR, and reviewed new reactor designs for the NRC. This site explains every kind of reactor in plain English. My background and papers.