# Nuclear fission reactors: Techniques Light water, heavy water, fast breeders and factory-built minis Source page: https://www.materialinsights.org/energy/e_fission/techniques/ 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)**^[2](https://world-nuclear.org/information-library/nuclear-fuel-cycle/introduction/nuclear-fuel-cycle-overview)^[1](https://energyeducation.ca/encyclopedia/Nuclear_power_plant)^[12](https://www.pv-magazine.com/2026/07/13/u-s-solar-lcoe-on-the-rise-lazard-says/)^[22](https://www.nucnet.org/news/nuclear-is-most-affordable-dispatchable-source-of-low-carbon-electricity-12-3-2020)^[11](https://digitallibrary.un.org/record/4020227)^[21](https://www.world-nuclear-news.org/Articles/Hinkley-Point-C-cleaner-than-renewables%2C-study-sho)^[4](https://world-nuclear.org/information-library/facts-and-figures/world-nuclear-power-reactors-and-uranium-requireme) | 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)**^[1](https://energyeducation.ca/encyclopedia/Nuclear_power_plant)^[2](https://world-nuclear.org/information-library/nuclear-fuel-cycle/introduction/nuclear-fuel-cycle-overview) | 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 cycles**^[3](https://world-nuclear.org/information-library/current-and-future-generation/fast-neutron-reactors) | 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 reactors**^[10](https://www.neimagazine.com/analysis/smrs-advanced-reactors/iaea-flags-leading-smr-projects/)^[1](https://energyeducation.ca/encyclopedia/Nuclear_power_plant) | 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 reactors**^[9](https://www.world-nuclear.org/information-library/non-power-nuclear-applications/transport/nuclear-powered-ships) | Not reported | Not reported | Not reported | mature | Over 160 ships, more than 200 reactors | | **Uranium mining, in situ leaching and enrichment**^[8](https://www.world-nuclear.org/information-library/nuclear-fuel-cycle/mining-of-uranium/world-uranium-mining-production)^[7](https://world-nuclear.org/information-library/nuclear-fuel-cycle/conversion-enrichment-and-fabrication/uranium-enrichment) | 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.^[2](https://world-nuclear.org/information-library/nuclear-fuel-cycle/introduction/nuclear-fuel-cycle-overview)^[1](https://energyeducation.ca/encyclopedia/Nuclear_power_plant)^[12](https://www.pv-magazine.com/2026/07/13/u-s-solar-lcoe-on-the-rise-lazard-says/)^[22](https://www.nucnet.org/news/nuclear-is-most-affordable-dispatchable-source-of-low-carbon-electricity-12-3-2020)^[11](https://digitallibrary.un.org/record/4020227)^[21](https://www.world-nuclear-news.org/Articles/Hinkley-Point-C-cleaner-than-renewables%2C-study-sho)^[4](https://world-nuclear.org/information-library/facts-and-figures/world-nuclear-power-reactors-and-uranium-requireme) **Heavy-water reactors (CANDU, Indian PHWRs)** Heavy water slows neutrons well enough to use natural uranium; fuel bundles are changed while the reactor runs.^[1](https://energyeducation.ca/encyclopedia/Nuclear_power_plant)^[2](https://world-nuclear.org/information-library/nuclear-fuel-cycle/introduction/nuclear-fuel-cycle-overview) **Fast breeder reactors and thorium cycles** Sodium 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](https://world-nuclear.org/information-library/current-and-future-generation/fast-neutron-reactors) **SMRs, microreactors, molten salt and high-temperature gas reactors** Reactors of up to a few hundred MW, largely factory-built; some use helium or molten salt, which run hotter than water.^[10](https://www.neimagazine.com/analysis/smrs-advanced-reactors/iaea-flags-leading-smr-projects/)^[1](https://energyeducation.ca/encyclopedia/Nuclear_power_plant) **Naval reactors** Compact pressurised-water reactors, in US submarines using uranium enriched to about 93%, run for decades without refuelling.^[9](https://www.world-nuclear.org/information-library/non-power-nuclear-applications/transport/nuclear-powered-ships) **Uranium mining, in situ leaching and enrichment** Ore is mined, or dissolved in place by solutions pumped through it (in situ leaching), then converted to a gas and enriched in centrifuges.^[8](https://www.world-nuclear.org/information-library/nuclear-fuel-cycle/mining-of-uranium/world-uranium-mining-production)^[7](https://world-nuclear.org/information-library/nuclear-fuel-cycle/conversion-enrichment-and-fabrication/uranium-enrichment) #### Sources 1. [Nuclear power plant](https://energyeducation.ca/encyclopedia/Nuclear_power_plant) Energy Education, University of Calgary, 2024 2. [Nuclear Fuel Cycle Overview](https://world-nuclear.org/information-library/nuclear-fuel-cycle/introduction/nuclear-fuel-cycle-overview) World Nuclear Association, 2025 3. [Fast Neutron Reactors](https://world-nuclear.org/information-library/current-and-future-generation/fast-neutron-reactors) World Nuclear Association, 2025 4. [World Nuclear Power Reactors and Uranium Requirements (updated 16 October 2026)](https://world-nuclear.org/information-library/facts-and-figures/world-nuclear-power-reactors-and-uranium-requireme) World Nuclear Association, 2026 5. [Uranium Enrichment](https://world-nuclear.org/information-library/nuclear-fuel-cycle/conversion-enrichment-and-fabrication/uranium-enrichment) World Nuclear Association, 2025 6. [World Uranium Mining Production](https://www.world-nuclear.org/information-library/nuclear-fuel-cycle/mining-of-uranium/world-uranium-mining-production) World Nuclear Association, 2025 7. [Nuclear-Powered Ships](https://www.world-nuclear.org/information-library/non-power-nuclear-applications/transport/nuclear-powered-ships) World Nuclear Association, 2025 8. [IAEA flags leading SMR projects](https://www.neimagazine.com/analysis/smrs-advanced-reactors/iaea-flags-leading-smr-projects/) Nuclear Engineering International, 2025 9. [Carbon neutrality in the UNECE region: integrated life-cycle assessment of electricity sources (ECE/ENERGY/141)](https://digitallibrary.un.org/record/4020227) United Nations Economic Commission for Europe, 2022 10. [Lazard's Levelized Cost of Energy+ 2026 (unsubsidised LCOE ranges), as reported by pv magazine](https://www.pv-magazine.com/2026/07/13/u-s-solar-lcoe-on-the-rise-lazard-says/) Lazard / pv magazine, 2026 11. [Hinkley Point C cleaner than renewables, study shows](https://www.world-nuclear-news.org/Articles/Hinkley-Point-C-cleaner-than-renewables%2C-study-sho) World Nuclear News, 2021 12. [Nuclear is most affordable dispatchable source of low-carbon electricity (reporting IEA/NEA, Projected Costs of Generating Electricity 2020)](https://www.nucnet.org/news/nuclear-is-most-affordable-dispatchable-source-of-low-carbon-electricity-12-3-2020) NucNet, 2020 ## Other sections - [Summary](https://www.materialinsights.org/energy/e_fission/index.md) - [Overview](https://www.materialinsights.org/energy/e_fission/overview/index.md) - [Correlation](https://www.materialinsights.org/energy/e_fission/correlation/index.md) - [Efficiency](https://www.materialinsights.org/energy/e_fission/efficiency/index.md) - [Resources and labour](https://www.materialinsights.org/energy/e_fission/resources/index.md) - [Impact on the biosphere](https://www.materialinsights.org/energy/e_fission/biosphere/index.md) - [Relationships](https://www.materialinsights.org/energy/e_fission/links/index.md)