Reactor type 5 of 10
Molten salt reactor
by Joe Miller
In a molten salt reactor the fuel is dissolved in a hot liquid salt that flows through the core, or, in salt-cooled designs like Kairos, the salt is the coolant for solid pebble fuel. The salt runs at near-normal pressure and very high temperature.
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
- The fuel pump circulates fuel salt up through the graphite core, where it becomes critical and heats up.
- Hot fuel salt goes to the primary heat exchanger.
- It heats a clean, non-radioactive intermediate salt.
- The intermediate salt boils water in the steam generator.
- Cooled fuel salt returns to the core.
The main components
| Component | What it does |
|---|---|
| Fuel salt | A fluoride salt mixture with uranium (or thorium) dissolved in it, flowing at about 650 to 700°C. |
| Graphite core | Graphite channels slow the neutrons; the salt only becomes critical inside the core. |
| Primary pump and heat exchanger | Move the fuel salt and pass its heat to a clean intermediate salt loop. |
| Freeze plug and drain tanks | A plug of frozen salt kept solid by a fan. If power is lost it melts and the fuel drains by gravity into tanks shaped so it cannot stay critical. |
| Off-gas system | Removes xenon and krypton fission gases from the salt as the reactor runs. |
| Steam generator | The clean salt boils water for the turbine. |
Key details
| Item | Detail |
|---|---|
| Coolant | Molten fluoride salt |
| Moderator | Graphite (some designs are fast-spectrum) |
| Fuel | Uranium or thorium dissolved in salt |
| Pressure | Near atmospheric |
| Temperature | About 650 to 700°C |
| Efficiency | About 40 to 45% |
| Refueling | Add fuel to the salt while running |
How it is maintained
- Remote maintenance: the pump and heat exchanger become highly radioactive, so they must be replaced by remote tools; the MSRE proved this in the 1960s.
- Corrosion monitoring of nickel alloys exposed to hot salt.
- Salt chemistry control and removal of fission products.
- Graphite replacement every several years in some designs.
Where it runs
China's 2-MW thorium test reactor at Wuwei (thorium-to-uranium conversion in 2025, a 10-MW demonstration planned by about 2029); Kairos Power's Hermes salt-cooled test reactor at Oak Ridge (for Google); designs from Terrestrial Energy, Copenhagen Atomics and others.
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.