Nuclear Fission Energy Calculator
Estimates the thermal energy released by nuclear fuel, either from its burnup or from complete fission of its U-235, Pu-239 or U-233 content.
Last updated: October 2026
Thermal Energy Released
1,080 MWh
Formula below · 3 sources (physics.nist.gov, laradioactivite-prod-en.apps.wok3.in2p3.fr, Wikipedia) · Updated Oct 2026
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About this calculator
There are two ways to estimate the heat released by nuclear fuel, and the calculator offers both.
From burnup (the normal engineering figure)
Burnup is defined as thermal energy per tonne of heavy metal, so E (MWh) = fuel mass (t) × burnup (MWd/t) × 24 h per day. One kilogram at 45,000 MWd/t gives 45 MWd, or 1,080 MWh. Fuel type and enrichment do not enter, because the burnup already states how much energy was extracted.
From complete fission (the physical ceiling)
The number of fissile atoms is mass × fissile fraction ÷ molar mass × Avogadro's number (6.022×10²³ per mole). Each fission releases about 200 MeV, and 1 MeV = 1.602×10⁻¹³ J, so E (MWh) = atoms × 200 × 1.602×10⁻¹³ ÷ 3.6×10⁹. Complete fission of 1 kg of U-235 gives about 22,800 MWh (8.2×10¹³ J). Pu-239 releases a few percent more per fission than U-235; the calculator uses 200 MeV for all three isotopes, so only the molar mass differs between them.
Results are heat. A light-water reactor turns roughly a third of it into electricity.
How to use
Choose the basis first. Burnup basis (default): 1 kg of fuel discharged at 45,000 MWd/t gives 0.001 t × 45,000 × 24 = 1,080 MWh of heat. Complete-fission basis: 1 kg of uranium enriched to 4% U-235 holds 40 g of U-235, or 40 ÷ 235.04 × 6.022×10²³ = 1.025×10²³ atoms. At 200 MeV each that is 3.28×10¹² J, or about 912 MWh. Pure U-235 (enrichment 100%) gives about 22,806 MWh per kg.
Frequently asked questions
What does fuel burnup mean?
Burnup is the thermal energy extracted per unit mass of heavy metal, usually in megawatt-days per tonne (MWd/t). 45,000 MWd/t means 45,000 megawatt-days of heat per tonne of uranium loaded. Light-water reactor fuel is typically discharged at 40,000-60,000 MWd/t.
Why is the burnup result much lower than complete fission of pure U-235?
Power-reactor fuel is only a few percent U-235, and not all of it fissions before the fuel is removed. Part of the energy also comes from plutonium bred from U-238 during operation. The complete-fission option is an upper bound for the fissile content, not what a reactor achieves.
How much energy is released per fission?
About 200 MeV for U-235, most of it as kinetic energy of the fission fragments, with the rest carried by neutrons, gamma rays, beta particles and neutrinos.
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