Reactor explorer

Every reactor is just five parts — fuel, moderator, coolant, control, and a way to turn heat into power. Pick a design to see how it sets those dials and where it stands today.

Work in progress: the reactor diagrams are simplified, schematic illustrations meant to convey the layout of each design — not engineering drawings. We're actively refining them for accuracy and detail.

Pressurized Water Reactor (PWR)

Light Water · Gen II / III / III+

Commercial — dominant

Two loops keep the turbine steam clean.

The most common reactor worldwide. Water kept under high pressure stays liquid and carries heat to a separate steam generator, keeping the turbine loop non-radioactive.

Coolant

Pressurized water

Moderator

Water

Fuel

Low-enriched UO₂ (3–5% U-235)

Spectrum / Temp

Thermal · ~325 °C

Strengths

+ Mature, well-understood
+ Non-radioactive secondary loop
+ Strong operating record

Trade-offs

– High-pressure systems
– Large containment needed
– Capital-intensive

Examples: Vogtle 3 & 4 (AP1000) · Most US & French reactors · Barakah (APR-1400)

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Boiling Water Reactor (BWR)

Light Water · Gen II / III+

Commercial

Direct cycle — steam is made right in the core.

Water boils directly in the core and the steam drives the turbine — simpler than a PWR but the turbine loop is mildly radioactive.

Coolant

Boiling water

Moderator

Water

Fuel

Low-enriched UO₂

Spectrum / Temp

Thermal · ~285 °C

Strengths

+ Simpler, lower pressure than PWR
+ Direct cycle efficiency

Trade-offs

– Radioactive turbine loop
– Complex in-core flow

Examples: Fukushima Daiichi (older BWRs) · GE BWRX-300 (SMR successor)

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Pressurized Heavy Water (CANDU)

Heavy Water · Gen II / III

Commercial

Heavy water lets it run on natural uranium.

Uses heavy water so efficiently that it can run on unenriched natural uranium and refuel while operating.

Coolant

Heavy water

Moderator

Heavy water

Fuel

Natural uranium

Spectrum / Temp

Thermal · ~310 °C

Strengths

+ No enrichment needed
+ Online refueling
+ Fuel flexibility

Trade-offs

– Expensive heavy water
– Larger core
– Tritium production

Examples: CANDU fleet (Canada, India, others)

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High-Temperature Gas-Cooled Reactor

Gas-Cooled · Gen IV

Demonstration

Runs very hot — ideal for industrial heat.

Helium-cooled, graphite-moderated reactors using robust TRISO fuel particles that retain fission products at very high temperatures — enabling process heat and strong passive safety.

Coolant

Helium

Moderator

Graphite

Fuel

TRISO particles

Spectrum / Temp

Thermal · ~750–950 °C

Strengths

+ Very high outlet temperature (industrial heat)
+ TRISO fuel is meltdown-resistant
+ Passive safety

Trade-offs

– Graphite handling
– Lower power density
– Fuel cost

Examples: X-energy Xe-100 · China HTR-PM · Project Pele microreactor

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Sodium Fast Reactor

Liquid Metal · Gen IV

Demonstration

Fast spectrum — can recycle its own fuel.

Liquid-sodium-cooled fast reactors can breed fuel and burn long-lived waste. Decades of test experience (EBR-II) but limited commercial deployment.

Coolant

Liquid sodium

Moderator

None (fast spectrum)

Fuel

Metal or oxide, higher enrichment / Pu

Spectrum / Temp

Fast · ~500–550 °C

Strengths

+ Can breed fuel / burn actinides
+ Low-pressure coolant
+ High temperature

Trade-offs

– Sodium reacts with air and water
– Opaque coolant complicates inspection
– Historically costly

Examples: TerraPower Natrium · EBR-II (historic) · Russia BN-800

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Molten Salt Reactor / LFTR

Molten Salt · Gen IV

Pre-commercial (lab-proven 1960s)

Liquid fuel; a freeze plug drains it if power is lost.

Fuel is dissolved in a liquid salt, so the reactor runs at near-atmospheric pressure and can refuel online. The thorium LFTR is the best-known concept. Proven at lab scale at Oak Ridge; no commercial unit yet.

Coolant

Molten fluoride/chloride salt

Moderator

Graphite (or none)

Fuel

Fuel dissolved in salt (U / Th)

Spectrum / Temp

Thermal or fast · ~650–700 °C

Strengths

+ Low pressure
+ Strong passive safety (freeze plug)
+ Online refueling
+ Thorium fuel option

Trade-offs

– Corrosive salts
– No commercial precedent
– Fuel-salt reprocessing immature

Examples: ORNL MSRE (1965–69, historic) · ThorCon · Terrestrial Energy IMSR · Kairos (salt-cooled)

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Small Modular Reactor (LWR-SMR)

Light Water (small) · Gen III+ / IV

Pre-commercial / first units

A whole PWR shrunk into one factory-built module.

Reactors under ~300 MWe built in a factory and shipped to site. The bet is that series production and lower up-front capital beat the economies of scale of large plants — still unproven commercially.

Coolant

Water

Moderator

Water

Fuel

Low-enriched UO₂ / HALEU

Spectrum / Temp

Thermal · ~300 °C

Strengths

+ Lower up-front capital
+ Factory build quality
+ Sited flexibly
+ Passive safety

Trade-offs

– Loses economy of scale
– First-of-a-kind cost risk
– Unproven supply chain

Examples: NuScale (design certified; lead project cancelled 2023) · GE BWRX-300 · Rolls-Royce SMR

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Microreactor

Micro · Gen IV

Demonstration

Tiny, sealed, transportable — runs for years.

Sub-20 MW units, often transportable, aimed at remote communities, mines, data centers, and defense bases. The US Army's Project Pele/Janus programs are the highest-profile examples.

Coolant

Heat pipe / gas / salt

Moderator

Varies

Fuel

TRISO / HALEU

Spectrum / Temp

Thermal or fast · High

Strengths

+ Transportable
+ Years of operation without refueling
+ Grid-independent

Trade-offs

– Very high $/kW
– Early-stage
– Fuel (HALEU) supply constraints

Examples: Project Pele · Oklo Aurora · Westinghouse eVinci

Full details →