Nuclear reactors differ in coolant, moderator, fuel form, pressure boundary and safety strategy. The choice affects everything from licensing posture to spent-fuel chemistry to economics. This article summarises the main families operating or under development internationally.

Light-water reactors (LWR)

Light-water reactors use ordinary water as both moderator and coolant. They dominate the world's operating fleet (~85% by unit count).

Pressurised Water Reactor (PWR)

Two coolant loops: primary water (kept liquid by ~155 bar pressure, ~290-325°C) flows through reactor and steam generators; secondary water boils in the SGs to drive the turbine. The primary system is normally non-radioactive at the turbine. Most common type worldwide. Designs: Westinghouse 2/3/4-loop, Combustion Engineering, Framatome/AREVA EPR, KEPCO APR1400, AP1000, ATMEA1, VVER-1000 / V-320 / V-491 / V-466, CPR-1000, Hualong One (HPR1000).

Boiling Water Reactor (BWR)

Single coolant loop: water boils inside the core (typically 70 bar, 285°C) and steam drives the turbine directly. Direct radioactive contamination of the turbine (mainly N-16, half-life 7.1 s). Designs: GE BWR/3/4/5/6, ABWR, ESBWR, BWRX-300 (small modular).

VVER-440

Soviet-designed PWR with six primary loops and horizontal steam generators. Operating examples include Dukovany (CZ, V-213 version) and Loviisa (FI, V-213 version with western containment). Older V-230 units (Kola, Armenian, NPP-1) have been the focus of major safety upgrades or shut down.

Heavy-water reactors

Use heavy water (D2O) as moderator and/or coolant. The neutron economy permits natural uranium fuel without enrichment.

CANDU

Canadian-designed PHWR with horizontal pressure tubes, heavy-water moderator and coolant. On-power refuelling. Operating examples: Bruce, Darlington, Pickering (CA), Embalse (AR), Cernavoda (RO), Kanupp (PK), Wolsong (KR), Qinshan III (CN). Specific safety issues include positive void coefficient (mitigated by control elements and design provisions) and tritium production in the heavy-water moderator.

Gas-cooled reactors

Historical UK lineage: Magnox (CO2-cooled, graphite-moderated, natural uranium metal fuel) and Advanced Gas-cooled Reactor (AGR, CO2-cooled, graphite, enriched UO2). Magnox fleet fully shut down by 2015; AGR fleet decommissioning since 2018-2024.

High-Temperature Gas-cooled Reactor (HTGR)

Helium-cooled, graphite-moderated, TRISO fuel particles. Operating: HTR-PM (CN, ~2021). Historical: AVR, THTR-300 (DE), Fort St. Vrain (US), HTR-10 (CN), HTTR (JP). Selling points: high outlet temperature for process heat; passive decay-heat removal at small scale. Regulatory issues: graphite oxidation, dust, TRISO failure under accident.

Fast reactors

Fast-neutron-spectrum reactors do not use a moderator. They can breed fissile material from U-238 and burn long-lived actinides, but require liquid metal coolant (sodium, lead, lead-bismuth) or gas coolant (helium) due to incompatibility with water.

Sodium-cooled Fast Reactor (SFR)

Operating fleet: BN-600 and BN-800 (RU, Beloyarsk), CFR-600 (CN, since 2023). Historical: Phénix and Superphénix (FR), Monju (JP), Joyo (JP, restart in progress), EBR-II (US), PFR (UK), KNK-II (DE), FBTR (IN). Specific safety issues: sodium reactivity with water and air, sodium-fire mitigation, in-vessel inspection.

Lead-cooled / Lead-Bismuth Fast Reactor (LFR / LBE)

Soviet submarine experience (LBE in Project 705 "Alfa" class). Current civilian designs include the Russian BREST-OD-300 (under construction at Seversk) and several European projects (ALFRED, MYRRHA — research reactor).

Small modular reactors (SMRs)

Designs with output below ~300 MWe per unit, intended for factory fabrication and modular site assembly. Active licensing efforts include:

  • NuScale (US, NRC certified 2023) — integral PWR.
  • BWRX-300 (Hitachi-GE, ONR GDA, CNSC pre-licensing review, Tennessee Valley Authority site) — passive simplification of a BWR.
  • Rolls-Royce SMR (UK, in GDA) — three-loop PWR.
  • Hualong One small variant ACP100/Linglong One (CN, under construction at Changjiang).
  • RITM-200 (RU, floating units in operation; land-based under licensing).
  • Holtec SMR-300, Westinghouse AP300, X-energy Xe-100 (HTGR), Kairos KP-FHR (US designs in early NRC licensing).

Reading "passive safety" honestly

"Passive safety" in regulatory documents has a precise meaning (IAEA Specific Safety Guide SSG-31): a safety function that does not rely on external power, operator action or external mechanical input, driven instead by physics (gravity, natural circulation, decay heat sinks, intrinsic feedbacks). Operators' marketing materials frequently use the term more loosely.

Critical reading questions for any passive safety claim:

  • Is the function passive in the IAEA sense, or does it need at least valve actuation or instrumentation power?
  • How long is the unattended grace period (typical advanced PWR claims: 72 hours)?
  • What happens after the passive function is exhausted?
  • Has the function been demonstrated in integral effects tests, or only in code calculations?
  • How is single-failure criterion satisfied for instrumentation supporting the passive function?

Passive safety reduces dependence on active systems and on operator action, but it does not eliminate the need for licensing-quality evidence, instrumentation, or eventual human intervention. The 2011 Fukushima sequence illustrates how passive isolation condensers in BWR-3 (Unit 1) ended up disabled by operator action and station blackout — the "passive" label is necessary but not sufficient.