The concept of burnup (unit: MWd/t) is a fairly general measure of fuel depletion, which is defined as thermal energy output (unit: MWd) per unit mass (unit metric ton, t) of heavy metal content in the initial fuel.

For a reactor which loads a heavy metal content of 100 t and operates at a thermal power of 3,000 MW for 1,000 days, the average burnup of the discharge fuel will be

30,0 MWd/t.

The thermal energy production by fission of 1g heavy metal fuel, assuming that the fuel is 235U and its fission energy is 200 MeV, is as follows.

The fission of 1 g heavy metal corresponds to about 1 MWd. For a discharge fuel of

30,0 MWd/t burnup, this means that about 30 kg per initial heavy metal of 11 has fissioned (that is, about 3 % of the initial heavy metal has fissioned).

The average burnup of typical LWRs is 30,000-50,000 MWd/t. Fast breeder reactors are being designed with the average burnup of 100,000-150,000 MWd/t as a development target.