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FusionBenchmark

Fission

A fission reactor releases energy by splitting heavy nuclei, usually uranium. Reactor types are usually grouped by what carries the heat away from the fuel: water, gas, liquid metal, molten salt or sealed heat pipes. Whether the neutrons are slowed, and whether the fuel is solid or dissolved, separate them further.

Regulatory steps for each developer are on licensing. Small modular reactors and microreactors have their own page, modular reactors. Dated construction, criticality, grid connection and the licensing decisions that name a power reactor are on the commercial fission timeline.

Approaches

  • Water-cooled

    Reactors cooled by ordinary or heavy water, which in most designs is also the neutron moderator.

    38 companies · 26 reactor designs · includes Pressurised water reactor, Boiling water reactor, Pressurised heavy-water reactor

  • Salt-based

    Reactors using molten salt as fuel carrier, coolant, or both.

    11 companies · 9 reactor designs · includes Molten salt reactor, Fluoride salt-cooled high-temperature reactor

  • Gas-cooled

    Reactors cooled by gas, usually helium or carbon dioxide, allowing higher outlet temperatures than water.

    11 companies · 14 reactor designs · includes High-temperature gas-cooled reactor

  • Liquid-metal cooled

    Reactors cooled by molten metal at near-atmospheric pressure. Most designs use no moderator, so the neutrons stay fast. A few do slow their neutrons: Aalo Atomics describes its sodium-cooled Aalo-1 as a thermal reactor, and the IAEA lists Idaho National Laboratory's MARVEL microreactor as liquid-metal-cooled with a thermal spectrum.

    13 companies · 9 reactor designs · includes Sodium fast reactor, Lead fast reactor

  • Heat-pipe microreactor

    Heat moved by sealed heat pipes rather than pumped coolant or forced convection. No primary pumps and no coolant loop, which suits very small transportable reactors. Heat-pipe describes how heat is moved, not the neutron spectrum. Moderator and fuel vary by design: some designs are fast and unmoderated, others use a graphite or beryllium oxide moderator, with TRISO or metal fuel, usually HALEU.

    4 companies · 3 reactor designs

The full approach tree

Companies

115 organisations have a commercial fission role. 57 are reactor developers and 45 are utilities.

Grouped by approach. A company working on more than one approach appears under each.

Water-cooled 38

Salt-based 11

Gas-cooled 10

Liquid-metal cooled 13

Heat-pipe microreactor 4

Developers with no approach recorded 4

Reactor developers whose reactor type is not yet in the record.

Utilities, suppliers, investors and others 42

All fission companies, with filters

Reactors

Every fission reactor in the record, by whether it is running, being built or planned, or has stopped.

Operating 108

Commissioning 2

Under construction 11

Planned 8

Shut down 7

  • F-1 Fission · National Research Centre Kurchatov Institute · RU
  • Halden Reactor Water-cooled · Institute for Energy Technology · NO
  • HIFAR Water-cooled · Australian Nuclear Science and Technology Organisation · AU
  • JEEP II Water-cooled · Institute for Energy Technology · NO
  • JMTR Water-cooled · JAEA · JP
  • NRU Water-cooled · Canadian Nuclear Laboratories · CA
  • TRICO I Water-cooled · CREN-K · CD

Decommissioned 13

  • CIRUS Water-cooled · BARC · IN
  • CP-5 Water-cooled · Argonne National Laboratory · US
  • DIORIT Water-cooled · Paul Scherrer Institute · CH
  • FiR 1 Water-cooled · VTT Technical Research Centre of Finland · FI
  • Ford Nuclear Reactor Water-cooled · University of Michigan Nuclear Engineering and Radiological Sciences · US
  • KRR-1 Water-cooled · KAERI · KR
  • KRR-2 Water-cooled · KAERI · KR
  • Omega West Reactor Water-cooled · Los Alamos National Laboratory · US
  • PROTEUS Fission · Paul Scherrer Institute · CH
  • R1 Water-cooled · KTH Royal Institute of Technology · SE
  • RA (Vinča) Water-cooled · Vinča Institute of Nuclear Sciences · RS
  • SAPHIR Water-cooled · Paul Scherrer Institute · CH
  • University of Illinois Advanced TRIGA Water-cooled · University of Illinois · US

Status not recorded 1

  • CAREM-25 Pressurised water reactor · CNEA · AR

Suspended 11

  • ETRR-1 Water-cooled · Egyptian Atomic Energy Authority · EG
  • FRM II Water-cooled · Heinz Maier-Leibnitz Research Neutron Source (FRM II) · DE
  • GRR-1 (Demokritos) Water-cooled · National Centre for Scientific Research Demokritos · GR
  • IRT-1 (Tajoura) Water-cooled · Tajoura Nuclear Research Centre · LY
  • Jōyō Sodium fast reactor · JAEA · JP
  • RA-8 Fission · CNEA · AR
  • RB (Vinča) Water-cooled · Vinča Institute of Nuclear Sciences · RS
  • RECH-2 Water-cooled · Chilean Nuclear Energy Commission · CL
  • TR-2 Water-cooled · Turkish Energy, Nuclear and Mineral Research Agency · TR
  • TRICO II Water-cooled · CREN-K · CD
  • WWR-M (Gatchina) Water-cooled · National Research Centre Kurchatov Institute · RU

Designs by size

Size class is worked out from the electrical output of one unit, as the developer states it (often without saying whether it is net or gross): a microreactor is up to 10 MWe, a small modular reactor up to 300 MWe (the International Atomic Energy Agency describes SMRs as "typically" up to that figure), and anything larger is a large reactor. A design whose output is not stated is not classed.

Microreactor 10

up to 10 MWe per unit

Design Developer Approach Electrical output per unit Licensing stage
4S Super-Safe, Small and Simple Toshiba Sodium fast reactor 10 MWe planned No milestone recorded
Aalo-1 Aalo-1 Reactor Aalo Atomics Liquid-metal cooled 10 MWe planned No milestone recorded
ABV-6E OKBM Afrikantov with Rosatom Pressurised water reactor 6 to 9 MWe planned No milestone recorded
eVinci Westinghouse Electric Company Heat-pipe microreactor 5 MWe planned No milestone recorded
LOKI MMR LOKI NANO Nuclear Energy High-temperature gas-cooled reactor 0.01 to 3 MWe planned No milestone recorded
MoveluX Toshiba Heat-pipe microreactor 3 to 4 MWe planned No milestone recorded
R1 Microreactor Antares R1 Antares Nuclear Heat-pipe microreactor 0.1 to 1 MWe planned No milestone recorded
SHELF-M NIKIET with Rosatom Pressurised water reactor 10 MWe planned No milestone recorded
SOLO SOLO Micro-Modular Reactor Terra Innovatum Gas-cooled 1 MWe planned No milestone recorded
UNITHERM NIKIET with Rosatom Pressurised water reactor 6.6 MWe planned No milestone recorded

Small modular reactor 31

above 10 and up to 300 MWe per unit

Design Developer Approach Electrical output per unit Licensing stage
ACP100 Linglong One CNNC Pressurised water reactor 125 MWe planned No milestone recorded
ACP100S CNNC Pressurised water reactor 125 MWe planned No milestone recorded
Aurora powerhouse Aurora Oklo Sodium fast reactor 15 to 75 MWe planned Application denied for one plant: NRC denies Aurora combined licence application · US Nuclear Regulatory Commission · 2022-01-06
BWRX-300 GE Vernova Hitachi Nuclear Energy Boiling water reactor 300 MWe planned Construction permit issued for one plant: CNSC licence to construct one BWRX-300 at the Darlington New Nuclear Project · Canadian Nuclear Safety Commission · 2025-04-04
CAREM CAREM25 CNEA Pressurised water reactor 30 MWe planned No milestone recorded
Compact Molten Salt Reactor (CMSR) CMSR Saltfoss Energy Molten salt reactor 110 MWe planned No milestone recorded
Energy Multiplier Module (EM2) EM2 General Atomics Gas-cooled 265 MWe planned No milestone recorded
Fast Modular Reactor (FMR) FMR General Atomics Gas-cooled 44 MWe planned No milestone recorded
Gravity Nuclear Reactor Gravity reactor, Deep Fission Borehole Reactor Deep Fission Pressurised water reactor 15 MWe planned No milestone recorded
GT-MHR Gas Turbine Modular Helium Reactor OKBM Afrikantov with Rosatom High-temperature gas-cooled reactor 288 MWe planned No milestone recorded
GTHTR300 Gas Turbine High Temperature Reactor 300 JAEA High-temperature gas-cooled reactor 100 to 300 MWe planned No milestone recorded
HTR50S JAEA High-temperature gas-cooled reactor 17.2 MWe planned No milestone recorded
i-SMR innovative SMR Korea Hydro & Nuclear Power with KAERI Pressurised water reactor 170 MWe planned No milestone recorded
Integral Molten Salt Reactor (IMSR) IMSR, IMSR400 Terrestrial Energy Molten salt reactor 195 MWe planned No milestone recorded
KLT-40S OKBM Afrikantov with Rosatom Pressurised water reactor 35 MWe claimed No milestone recorded
KP-FHR Kairos Power Fluoride Salt-Cooled High-Temperature Reactor, KP-X Kairos Power Fluoride salt-cooled high-temperature reactor 75 MWe planned No milestone recorded
LFR-AS-200 newcleo Lead fast reactor 200 MWe planned No milestone recorded
MHR-100 OKBM Afrikantov with Rosatom High-temperature gas-cooled reactor 25 to 87 MWe planned No milestone recorded
MHR-T OKBM Afrikantov with Rosatom High-temperature gas-cooled reactor 205.5 MWe planned No milestone recorded
NuScale Power Module (US460) US460, NPM, VOYGR NuScale Power Pressurised water reactor 77 MWe planned Design approval or certification for the design · US Nuclear Regulatory Commission · 2025-05-29
PWR-20 Last Energy Pressurised water reactor 20 MWe planned No milestone recorded
RITM-200M OKBM Afrikantov with Rosatom Pressurised water reactor 50 MWe planned No milestone recorded
RITM-200N OKBM Afrikantov with Rosatom Pressurised water reactor 55 MWe planned No milestone recorded
SEALER-55 SEALER Blykalla Lead fast reactor 55 MWe planned No milestone recorded
SMART System-integrated Modular Advanced Reactor, SMART100 KAERI Pressurised water reactor 107 MWe planned No milestone recorded
Stable Salt Reactor - Wasteburner (SSR-W) SSR-W, SSR-W300 Moltex Energy Molten salt reactor 300 MWe planned No milestone recorded
Stellarium Stellaria Molten salt reactor 110 MWe planned No milestone recorded
SVBR-100 Rosatom Lead fast reactor 100 MWe planned No milestone recorded
ThorCon ThorCon 500, TMSR-500 ThorCon International Molten salt reactor 250 MWe planned No milestone recorded
Thorizon One Thorizon Thorizon Molten salt reactor 100 MWe planned No milestone recorded
Xe-100 X-energy High-temperature gas-cooled reactor 80 MWe planned Accepted for review for one plant: Long Mott construction permit application accepted for docketing · US Nuclear Regulatory Commission · 2025-05-12

Large reactor 9

above 300 MWe per unit

Design Developer Approach Electrical output per unit Licensing stage
Advanced Boiling Water Reactor (ABWR) ABWR GE Vernova Hitachi Nuclear Energy Boiling water reactor 1,350 to 1,460 MWe claimed No milestone recorded
AP1000 Westinghouse Electric Company Pressurised water reactor 1,110 MWe claimed No milestone recorded
AP300 Westinghouse Electric Company Pressurised water reactor 330 MWe planned No milestone recorded
Economic Simplified Boiling Water Reactor (ESBWR) ESBWR GE Vernova Hitachi Nuclear Energy Boiling water reactor 1,520 MWe planned No milestone recorded
IPHWR-700 700 MWe PHWR, Indian PHWR-700 NPCIL Pressurised heavy-water reactor 700 MWe claimed No milestone recorded
Natrium TerraPower with GE Vernova Hitachi Nuclear Energy Sodium fast reactor 345 MWe planned Construction permit issued for one plant: NRC construction permit CPAR-1 issued for Kemmerer Power Station Unit 1 (Natrium) · US Nuclear Regulatory Commission · 2026-03-09
Rolls-Royce SMR Rolls-Royce SMR Pressurised water reactor 470 MWe planned Accepted for review for the design · Office for Nuclear Regulation · 2022-04
SMR-300 Holtec International Pressurised water reactor 320 MWe planned Accepted for review for one plant: Pioneer Units 1 and 2: Part 1 of phased construction permit application accepted for docketing · US Nuclear Regulatory Commission · 2026-02-13
VBER-300 OKBM Afrikantov with Rosatom Pressurised water reactor 325 MWe planned No milestone recorded

Output not stated 12

No per-unit electrical rating is recorded, so no size class is given.

Design Developer Approach Electrical output per unit Size class Licensing stage
BWXT Advanced Nuclear Reactor (BANR) BANR BWX Technologies High-temperature gas-cooled reactor Output not stated Not classed No milestone recorded
Copenhagen Atomics Waste Burner CA Waste Burner, Onion Core Copenhagen Atomics Molten salt reactor Output not stated Not classed No milestone recorded
DF-1500 Dual Fluid Energy Liquid-metal cooled Output not stated Not classed No milestone recorded
DF-300 Dual Fluid Energy Liquid-metal cooled Output not stated Not classed No milestone recorded
EPR European Pressurised Reactor, Evolutionary Power Reactor Framatome Pressurised water reactor Output not stated Not classed No milestone recorded
Hualong One HPR1000 CNNC with CGN Pressurised water reactor Output not stated Not classed No milestone recorded
Jimmy Jimmy High-temperature gas-cooled reactor Output not stated Not classed No milestone recorded
Kaleidos Radiant High-temperature gas-cooled reactor Output not stated Not classed No milestone recorded
KRONOS MMR KRONOS, KRONOS MMR Energy System NANO Nuclear Energy High-temperature gas-cooled reactor 3.5 to 15 MWe planned Stated range spans two size classes No milestone recorded
LDR-50 Steady Energy Water-cooled Output not stated Not classed No milestone recorded
MSR-100 Natura Resources Molten salt reactor Output not stated Not classed No milestone recorded
ZEUS NANO Nuclear Energy Fission Output not stated Not classed No milestone recorded

Designs by licensing stage

The furthest regulatory step recorded for each design, furthest first. Each entry says whether the step approved the design itself or one named plant built to it. A design approval is not permission to build: that needs a construction permit for a named site.

Application denied 1

  • Aurora powerhouse Oklo · for one plant: NRC denies Aurora combined licence application · US Nuclear Regulatory Commission · 2022-01-06

Construction permit issued 2

  • BWRX-300 GE Vernova Hitachi Nuclear Energy · for one plant: CNSC licence to construct one BWRX-300 at the Darlington New Nuclear Project · Canadian Nuclear Safety Commission · 2025-04-04
  • Natrium TerraPower · for one plant: NRC construction permit CPAR-1 issued for Kemmerer Power Station Unit 1 (Natrium) · US Nuclear Regulatory Commission · 2026-03-09

Design approval or certification 1

  • NuScale Power Module (US460) NuScale Power · for the design · US Nuclear Regulatory Commission · 2025-05-29

Accepted for review 3

  • Rolls-Royce SMR Rolls-Royce SMR · for the design · Office for Nuclear Regulation · 2022-04
  • SMR-300 Holtec International · for one plant: Pioneer Units 1 and 2: Part 1 of phased construction permit application accepted for docketing · US Nuclear Regulatory Commission · 2026-02-13
  • Xe-100 X-energy · for one plant: Long Mott construction permit application accepted for docketing · US Nuclear Regulatory Commission · 2025-05-12

No licensing milestone recorded for the design 55

Every regulatory milestone, with its docket reference

Guides

  • How fission reactors work How a fission reactor sustains a chain reaction, what the moderator, coolant and control rods do, why delayed neutrons make control possible, where decay heat comes from, and what passive safety does and does not mean.
  • How nuclear reactor licensing works in the US, Canada and the UK The separate permissions a fission reactor needs before it can be built and run, what each one allows, how fusion is regulated differently, and how to read the licensing page.
  • Reactor types explained The three questions that classify any fission reactor, and how pressurised water, boiling water, heavy water, gas-cooled, salt-cooled, molten-salt, sodium, lead and heat-pipe reactors each answer them.
  • Nuclear fuel explained Where reactor fuel comes from, what enrichment does, where the lines between LEU, LEU+, HALEU and HEU fall, how TRISO, MOX and thorium fuels differ, and why HALEU supply limits advanced reactors.
  • Fusion and fission compared How fusion and fission differ in the physics, the fuels, the neutrons, the waste, the safety case and how far each has come, set side by side with the numbers.
  • Small modular reactors What makes a reactor a small modular reactor, where the IAEA draws the size lines, why size is judged per unit rather than per plant, which ones actually operate, and why their cost claims are not yet proven.
  • Microreactors What microreactors are, how heat-pipe, gas-cooled and sodium-cooled designs work, what they are meant for, why most need HALEU or TRISO fuel, and where the leading projects actually stand.

All guides