# Nuclear fission reactors: Efficiency

Huge energy from tiny fuel, yet 99% of uranium's potential unused

Source page: https://www.materialinsights.org/energy/e_fission/efficiency/

Huge energy from tiny fuel, yet 99% of uranium's potential unused What this tab covers How much of the energy available is actually captured or extracted, in per cent: theoretical limits, laboratory records, typical new equipment and in-service averages, each with exactly what is being measured.

Fission is very efficient at turning a little fuel into a lot of heat, but today's reactors use less than 1% of the energy in mined uranium and convert only about a third of their heat to electricity. Fast reactors could use uranium about 60 times more efficiently.

Chart: Efficiency ranges
 Commercial In service

#### Efficiency by measure

| Measure | Range | Type | What is measured |
|---|---|---|---|
| **Thermal efficiency, light-water and heavy-water reactors** | 33 to 37%^[1](https://energyeducation.ca/encyclopedia/Nuclear_power_plant) | Commercial | Reactor heat to electricity at the plant |
| **Thermal efficiency, sodium-cooled fast reactors** | 39.35 to 41%^[3](https://world-nuclear.org/information-library/current-and-future-generation/fast-neutron-reactors) | Commercial | Net efficiency: BN-800 39.35%, China's CFR-600 41% |
| **Share of uranium's energy used in today's reactors** | 1%^[3](https://world-nuclear.org/information-library/current-and-future-generation/fast-neutron-reactors) | In service | Less than 1% of the energy potentially available from mined uranium |
| **Fuel burn-up, light-water reactors** | 40 to 55 GWd per tonne^[2](https://world-nuclear.org/information-library/nuclear-fuel-cycle/introduction/nuclear-fuel-cycle-overview) | In service | Heat released per tonne of uranium in the fuel: historically limited to about 40, now 55 possible with 5% enrichment |
| **Fuel burn-up, fast reactors** | 66 to 100 GWd per tonne^[3](https://world-nuclear.org/information-library/current-and-future-generation/fast-neutron-reactors) | Record | BN-800 average 66, with potential increase to 100; experimental US metal fuel reached 19% of atoms fissioned |
| **Theoretical limit of burn-up** | 920 GWd per tonne^[2](https://world-nuclear.org/information-library/nuclear-fuel-cycle/introduction/nuclear-fuel-cycle-overview) | Theoretical limit | All heavy-metal atoms fissioned (calculated from 60 GWd/t being about 6.5% of atoms fissioned) |
| **Energy return on energy invested (published estimates)** | 1 to 60 ratio^[20](https://www2.mdpi.com/2071-1050/3/10/1796) | In service | Studies range from below 1 to almost 60 depending on method and quality correction; older reviews gave 5-8 |
| **World average capacity factor (2024)** | 83%^[5](https://world-nuclear.org/our-association/publications/world-nuclear-performance-report/preface-to-world-nuclear-performance-report-2025) | In service | Actual output as share of maximum possible output, 410 reactors |

#### Where the energy is lost

| Step | Loss |
|---|---|
| **Waste heat rejected to cooling water or towers** | about 63-67% of reactor heat (calculated from 33-37% efficiency)^[1](https://energyeducation.ca/encyclopedia/Nuclear_power_plant) |
| **Uranium-238 not fissioned, and U-235 left in enrichment tails and spent fuel** | over 99% of uranium's potential energy^[3](https://world-nuclear.org/information-library/current-and-future-generation/fast-neutron-reactors) |

A reactor splits heavy atoms, mostly uranium-235, releasing heat that boils water to drive a steam turbine. Water-cooled reactors run at modest steam temperatures, which caps their efficiency at about a third. Natural uranium is only about 0.7% U-235; the rest is U-238, which ordinary reactors barely use, though some is converted to plutonium, which supplies about a third of a typical plant's output.

Fast reactors keep neutrons energetic so they can convert U-238 (or thorium) into new fuel, which is why they can extract far more energy per tonne of uranium.

#### 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. [Preface to World Nuclear Performance Report 2025](https://world-nuclear.org/our-association/publications/world-nuclear-performance-report/preface-to-world-nuclear-performance-report-2025) World Nuclear Association, 2025
5. [A Review of the Past and Current State of EROI Data (Gupta and Hall, Sustainability 3(10))](https://www2.mdpi.com/2071-1050/3/10/1796) MDPI, 2011

## 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)
- [Techniques](https://www.materialinsights.org/energy/e_fission/techniques/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)