Nuclear fission reactors: Efficiency
Huge energy from tiny fuel, yet 99% of uranium's potential unused
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 serviceEfficiency by measure
| Measure | Range | Type | What is measured |
|---|---|---|---|
| Thermal efficiency, light-water and heavy-water reactors | 33 to 37%1 | Commercial | Reactor heat to electricity at the plant |
| Thermal efficiency, sodium-cooled fast reactors | 39.35 to 41%3 | Commercial | Net efficiency: BN-800 39.35%, China's CFR-600 41% |
| Share of uranium's energy used in today's reactors | 1%3 | In service | Less than 1% of the energy potentially available from mined uranium |
| Fuel burn-up, light-water reactors | 40 to 55 GWd per tonne2 | 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 tonne3 | 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 tonne2 | 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 ratio20 | 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 | 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 |
| Uranium-238 not fissioned, and U-235 left in enrichment tails and spent fuel | over 99% of uranium's potential energy3 |
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
- Nuclear power plant Energy Education, University of Calgary, 2024
- Nuclear Fuel Cycle Overview World Nuclear Association, 2025
- Fast Neutron Reactors World Nuclear Association, 2025
- Preface to World Nuclear Performance Report 2025 World Nuclear Association, 2025
- A Review of the Past and Current State of EROI Data (Gupta and Hall, Sustainability 3(10)) MDPI, 2011