Reactor type 8 of 10
Small modular reactors (SMRs)
by Joe Miller
"Small modular" is about size and how the plant is built, not the coolant. SMRs are under about 300 megawatts and built mostly in a factory. Most use proven light-water technology, often with the steam generator inside the reactor vessel and passive cooling that needs no pumps or power.
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 water in the core.
- Hot water rises up the riser by natural circulation.
- It passes over helical steam generator tubes at the top of the vessel and cools.
- Cooler, denser water sinks down the outside and returns to the core, with no pumps.
- Steam from the tubes drives the turbine.
The main components
| Component | What it does |
|---|---|
| Integral reactor module | Core, steam generator and pressurizer in one vessel (NuScale), or a small BWR (BWRX-300). |
| Natural circulation | Hot water rises through a central riser and cool water sinks outside it, with no reactor coolant pumps in many designs. |
| Steel containment vessel | Wraps the reactor module tightly. |
| Reactor pool | Modules sit in a large pool that can absorb decay heat for days with no operator action. |
| Shared turbine island | Several modules can feed turbines in one plant. |
Key details
| Item | Detail |
|---|---|
| Coolant | Mostly light water (some advanced SMRs use gas, sodium or salt) |
| Size | 50 to 300 MWe per module |
| Fuel | Standard 4 to 5% uranium oxide for water SMRs |
| Build | Factory-built modules, shipped by truck, rail or barge |
| Safety | Passive cooling for days without power |
| Refueling | Every 18 to 24 months, often one module at a time |
How it is maintained
- Module-at-a-time refueling so the rest of the plant keeps running.
- Fewer components: no reactor coolant pumps or large external piping in integral designs.
- Factory quality control replaces some field inspection.
- New inspection methods for compact, integral vessels are still being proven.
Where it runs
GE Vernova Hitachi BWRX-300 at Darlington, Ontario (target 2029-30) and planned at TVA's Clinch River; NuScale (NRC approval of its 77-MW module in 2025); Holtec SMR-300 at Palisades (early 2030s); China's Linglong One (testing); Russia's floating Akademik Lomonosov.
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.