Burnup
The thermal energy extracted from nuclear fuel per unit mass of initial heavy metal, usually expressed in gigawatt-days per tonne (GWd/tHM).
Burnup measures how thoroughly fuel has been used. It links fuel consumption to energy output and drives fuel cycle costs and spent fuel volumes.
Since complete fission of 1 t of heavy metal would release roughly 940–950 GWd, a burnup of 50 GWd/t corresponds to fissioning about 5% of the heavy metal atoms (FIMA).
Typical discharge burnups:
- Early 1970s light-water reactors: 20–30 GWd/t
- Modern PWRs and BWRs: 45–55 GWd/t, with assembly averages limited in the US to about 62 GWd/t peak rod
- CANDU natural uranium reactors: about 7–8 GWd/t
- High-temperature gas reactors: 80–100+ GWd/t; fast reactors over 100 GWd/t
Higher burnup requires higher enrichment, about 1% of U-235 per 10 GWd/t as a rule of thumb, but reduces the number of assemblies discharged per unit of electricity. A 1 GWe PWR at 33% efficiency and 50 GWd/t burns roughly 22 t of fuel per year. Limits come from cladding corrosion, fission gas release and pellet swelling; the US industry is seeking approval for 75–80 GWd/t with accident-tolerant fuels and enrichment above 5%.
Related terms
All terms →- Spent nuclear fuel
Fuel assemblies removed from a reactor after use, highly radioactive and thermally hot, containing uranium, plutonium, fission products and minor actinides.
- Uranium enrichment
The process of increasing the share of fissile U-235 in uranium above its natural 0.711%, to make reactor fuel or, at very high levels, weapons material.
- Nuclear fission
The splitting of a heavy atomic nucleus such as uranium-235 or plutonium-239 into lighter nuclei after absorbing a neutron, releasing about 200 MeV of energy and further neutrons.
- Pressurised water reactor (PWR)
The most common nuclear reactor type, in which water under high pressure cools the core without boiling and transfers heat to a separate steam circuit.
- Thorium fuel cycle
A nuclear fuel cycle that uses fertile thorium-232, which absorbs a neutron and decays to fissile uranium-233, as an alternative to uranium-plutonium fuel.