Nuclear power, from a nuclear engineer

Every kind of nuclear reactor, explained

by Joe Miller

Nuclear power is back: Big Tech is signing reactor deals, old plants are restarting, and a dozen new designs are racing to market. Here are the 10 kinds of reactors, how they work, what's real, and when each one will actually make power.

Watch: the narrated video (6 minutes)

Every nuclear power station in the world

A map and list of every major nuclear power station, how many reactors each country runs, and how well each fleet actually performs. See the world map and list

The 10 kinds at a glance

Thermal reactors slow neutrons down; fast reactors don't; SMRs and microreactors are sizes; fusion is a different process.

#	Type	Coolant	Neutrons	Status
1	Pressurized water (PWR)	Water, high pressure	Slow	Most of the world's reactors
2	Boiling water (BWR)	Boiling water	Slow	About a third of U.S. reactors
3	Heavy water (CANDU)	Heavy water	Slow	Canada, India and others
4	High-temperature gas	Helium	Slow	Running in China; U.S. units planned
5	Molten salt	Liquid salt	Slow (most)	Test reactors
6	Sodium-cooled fast	Liquid sodium	Fast	Running in Russia; Wyoming plant permitted
7	Lead-cooled fast	Liquid lead	Fast	Under construction in Russia
8	Small modular (SMR)	Usually water	Usually slow	First units under construction
9	Microreactors	Gas, sodium or heat pipes	Varies	Test units reached criticality in 2026
10	Fusion	Not fission	Fusion	Experiments; no net electricity yet

Read the full guide to all 10

How every fission plant works

Six steps from splitting atoms to electricity. The 10 types differ mostly in the coolant, moderator, fuel and size.

When each type will deliver power

Today's water reactors carry the load for at least the next decade.

Why this matters now: AI and Big Tech

AI data centers need enormous amounts of power around the clock. Microsoft, Google, Amazon and Meta have signed close to 10 gigawatts of nuclear deals, from restarting Three Mile Island to backing sodium, gas-cooled, salt-cooled and fusion startups. Which reactor each company is betting on, and my plain-English guides at datacentercritical.com.

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.

Nuclear power at a glance

Core strengths

Extreme energy density. A uranium fuel pellet the size of a fingertip makes about as much energy as a ton of coal or 17,000 cubic feet of natural gas. Reactors need very little fuel and very little land for the power they make.
High reliability. U.S. plants run about 90 to 93% of the time; the world average was about 84% in 2025, far higher than coal, gas, wind or solar. They make steady power around the clock and refuel only every 18 to 24 months.
Very low greenhouse gas emissions. Running a plant emits essentially no CO₂ or air pollution. Lifecycle emissions are among the lowest of any source (median about 12 grams of CO₂ per kilowatt-hour), like wind and far below gas or coal.
Excellent safety record per unit of energy. About 0.03 deaths per terawatt-hour, including Chernobyl and Fukushima: similar to wind and solar, and hundreds of times lower than coal (about 25). Modern designs have multiple passive and active safety systems.

Main challenges

High up-front cost and long build times. Big reactors can take a decade to build, which raises financing risk. Once running, operating costs are low.
Radioactive waste. Spent fuel is highly radioactive and long-lived, but small in volume: all U.S. commercial used fuel from decades of operation would fit on one football field stacked less than about 10 yards high. It is stored in pools and dry casks today. Finland's Onkalo deep repository received the world's first operating licence for permanent spent-fuel disposal in October 2026.
Rare, high-consequence accidents and public perception. Three Mile Island (1979), Chernobyl (1986) and Fukushima (2011) shaped lasting opposition, even though the overall safety record is strong and Western reactors differ greatly from Chernobyl's design.
Proliferation and fuel-cycle issues. The technology overlaps with weapons materials and requires strict international safeguards. Mining and enrichment have environmental footprints.

Scale and outlook

In 2025 nuclear made a record of about 2,700 terawatt-hours, about 9% of the world's electricity, from about 400 gigawatts in some 30 countries. The U.S. is the largest producer (about 18 to 20% of its electricity from 94 reactors), France has the highest share (about 68%), and China is building the most.

In short: nuclear is a proven, dense, low-carbon, highly reliable source with an outstanding safety record per unit of electricity. Its main barriers are cost and build time, waste politics and public acceptance, not technical limits.

Sources: World Nuclear Association (2025 figures), Our World in Data (deaths and emissions), U.S. DOE (fuel and waste facts), Finland's government and Posiva (Onkalo).

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.