Nuclear fission reactors: Techniques
Light water, heavy water, fast breeders and factory-built minis
What this tab covers The methods used to capture this energy: how each works, and how they compare on efficiency, cost, life-cycle emissions, maturity and scale.443 operable reactors (405.7 GW, October 2026) generated 2,701.6 TWh in 2025, about 9% of world electricity. Light-water reactors dominate.
Comparison
| Method | Efficiency | Cost | Life-cycle emissions | Maturity | Scale |
|---|---|---|---|---|---|
| Light-water reactors (pressurised and boiling)21122211214 | 33-37% thermal; burn-up 40-55 GWd/t | USD 175-255/MWh unsubsidised LCOE, new US build (Lazard, 2026); overnight capital USD 2,157-6,920 per kW (IEA/NEA, 2020) | 5.1-6.4 g CO2e/kWh (UNECE); 5.49 g for Hinkley Point C | mature | Most of the 405.7 GW fleet |
| Heavy-water reactors (CANDU, Indian PHWRs)12 | Similar to light-water (33-37%) | Not reported separately | No separate figure; no enrichment | mature | For example Darlington, Ontario: four 878 MW units |
| Fast breeder reactors and thorium cycles3 | Net 39-41%; about 60 times better uranium use | Not reported | No separate figure | early commercial | Russia's BN-600 (560 MWe, 1980) and BN-800 (789 MWe, 2014) |
| SMRs, microreactors, molten salt and high-temperature gas reactors101 | Generation IV designs could exceed 45% | Not yet established | No separate figure | early commercial | Two operating (Akademik Lomonosov, 2020; HTR-PM, 2023); 70 active designs |
| Naval reactors9 | Not reported | Not reported | Not reported | mature | Over 160 ships, more than 200 reactors |
| Uranium mining, in situ leaching and enrichment87 | Centrifuges about 50 kWh per separative work unit (SWU); old diffusion plants 2,500 | Enrichment is almost half of fuel cost, about 5% of electricity cost | Included in reactor figures | mature | 60,213 tonnes U mined (2024), 55% by in situ leaching |
Light-water reactors are costly to build in the West, so their electricity costs more than wind, solar or gas, but they offer very low emissions and steady output (83% average capacity factor, 2024). Fast reactors promise far better fuel use but run commercially only in Russia. SMRs aim to cut cost through factory building; only two operate.
How each method works
Light-water reactors (pressurised and boiling)Uranium oxide pellets enriched to 3.5-5% U-235 sit in metal tubes in a steel pressure vessel. Water slows neutrons and carries heat to a steam turbine. Fuel stays three to six years in the core.21122211214
Heavy-water reactors (CANDU, Indian PHWRs)Heavy water slows neutrons well enough to use natural uranium; fuel bundles are changed while the reactor runs.12
Fast breeder reactors and thorium cyclesSodium or lead cools a core with no moderator, so fast neutrons convert U-238 into plutonium, making as much fuel as is burned or more. India plans to breed U-233 from thorium.3
SMRs, microreactors, molten salt and high-temperature gas reactorsReactors of up to a few hundred MW, largely factory-built; some use helium or molten salt, which run hotter than water.101
Naval reactorsCompact pressurised-water reactors, in US submarines using uranium enriched to about 93%, run for decades without refuelling.9
Uranium mining, in situ leaching and enrichmentOre is mined, or dissolved in place by solutions pumped through it (in situ leaching), then converted to a gas and enriched in centrifuges.87
Sources
- Nuclear power plant Energy Education, University of Calgary, 2024
- Nuclear Fuel Cycle Overview World Nuclear Association, 2025
- Fast Neutron Reactors World Nuclear Association, 2025
- World Nuclear Power Reactors and Uranium Requirements (updated 16 October 2026) World Nuclear Association, 2026
- Uranium Enrichment World Nuclear Association, 2025
- World Uranium Mining Production World Nuclear Association, 2025
- Nuclear-Powered Ships World Nuclear Association, 2025
- IAEA flags leading SMR projects Nuclear Engineering International, 2025
- Carbon neutrality in the UNECE region: integrated life-cycle assessment of electricity sources (ECE/ENERGY/141) United Nations Economic Commission for Europe, 2022
- Lazard's Levelized Cost of Energy+ 2026 (unsubsidised LCOE ranges), as reported by pv magazine Lazard / pv magazine, 2026
- Hinkley Point C cleaner than renewables, study shows World Nuclear News, 2021
- Nuclear is most affordable dispatchable source of low-carbon electricity (reporting IEA/NEA, Projected Costs of Generating Electricity 2020) NucNet, 2020