Nuclear Fuel Burnup Calculator
Calculates nuclear fuel burnup in MWd/kg (the same number as GWd/t) from reactor thermal power, effective full-power days and the uranium mass in the core.
Last updated: October 2026
Burnup
15 MWd/kg
Formula below · 3 sources (nrc.gov, smrintel.com, Wikipedia) · Updated Oct 2026
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About this calculator
Burnup is the thermal energy a reactor has extracted from each unit mass of heavy metal (uranium, plus plutonium in mixed-oxide fuel) that was loaded. It is power multiplied by time, divided by mass:
Burnup (MWd/kg) = thermal power (MW) × effective full-power days ÷ initial heavy-metal mass (kg)
Units
1 MWd/kg is the same as 1 GWd/t and 1,000 MWd/t (also written MWd/MTU). Use the thermal power (MWt), not the electrical output, and the mass of uranium metal, not of uranium dioxide: UO₂ pellets are about 88% uranium by mass. Effective full-power days are calendar days scaled by the average power level, so 100 days at half power count as 50.
What the number means
This calculator gives the core-average burnup gained over the period you enter. Individual assemblies stay in the core for several cycles, and light-water reactor fuel is typically discharged at 40 to 60 GWd/t. In the United States the licensed limit for the peak fuel rod has been 62 GWd/t, and the industry is working on approval for higher values.
How to use
Enter the reactor's thermal power, the effective full-power days and the initial uranium mass in the core. With the defaults (3,000 MWt for 500 days on 100,000 kg of uranium): 3,000 × 500 = 1,500,000 MWd, divided by 100,000 kg gives 15 MWd/kg, or 15,000 MWd/t. Second example: a 3,400 MWt core holding 80,000 kg of uranium that runs 480 effective full-power days gains 3,400 × 480 ÷ 80,000 = 20.4 MWd/kg in that cycle. Fuel that stays for three such cycles ends near 61 MWd/kg.
Frequently asked questions
Is MWd/kg the same as GWd/t?
Yes. A gigawatt-day is 1,000 megawatt-days and a tonne is 1,000 kg, so the two factors of 1,000 cancel. 15 MWd/kg = 15 GWd/t = 15,000 MWd/t.
Should I enter the fuel mass or the uranium mass?
The uranium (heavy-metal) mass. Burnup is defined per unit of initial heavy metal. If you only know the mass of UO₂, multiply it by about 0.88 to get the uranium mass.
What burnup does commercial fuel reach?
Light-water reactor fuel is typically discharged at about 40 to 60 GWd/t. In the US, NRC approvals have capped the peak rod at about 62 GWd/t, and fuel vendors are seeking approval to go higher.
Why is the result lower than typical discharge burnups?
The result is the burnup added over the days you enter, averaged over the whole core. An assembly reaches its discharge burnup over several operating cycles, so one cycle usually adds only a third or a quarter of it.
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Sources & references
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