Burnup of fuel is the amount of energy extracted per mass of initial fuel loaded, and the units are generally megawatt-days per metric ton of heavy metal loaded (MW d/ MTHM). For example, if a 500 megawatt (MW) thermal SMR uses four metric tons (MT) of uranium contained within the fuel and operates at full power for one year, the average burnup of the fuel will be:

Average burnup = (500*365)/4 [4.1]

which is 45 625 MW d/MTHM, or 45.6 GW d/MTHM.

On this basis, it can be seen that the burnup requirements are derived from the utility needs, both in terms of the energy output of the reactor, and the refueling frequency, e. g., yearly or every 18 months.

In order for the fuel to achieve its design burnup, both for cycle of operation when it is first loaded, and for its design lifetime, sufficient excess reactivity has to be present in the fuel, and the fuel has to be replenished on an appropriate frequency. For a typical iPWR, this could range anywhere between one and five years, although is more typically 12 to 24 months for modern LWRs.

Although the excess reactivity is not a design limit or requirement directly, it is key to ensuring that the PWR core maintains criticality at full power operating conditions throughout the cycles of operation, including compensation for fuel depletion, buildup of fission product poisons (such as xenon and samarium), and loss of reactivity due to changes in temperature of fuel, moderator, etc. There is no specific limit on the amount of excess reactivity allowed either, but there are other design parameters such as negative reactivity coefficients or shutdown margin (see Sections 4.2.2 and 4.2.4) that are affected by the level of excess reactivity. Since the multiplication of neutrons varies in distribution in the core from one cycle to the next, and during the cycle of operations itself, it is important that the excess in reactivity is controlled both globally within the core, and also locally within the fuel assemblies, and the designer has a number of ways in which the core global and local reactivity and hence power can be controlled; these are described in Section 4.3.2.

There are however, limits on burnups, partially due to vendor warrantees associated with the fuel, but also licensing limits for the maximum fuel rod average burnup, but this is fuel design and vendor specific in both cases. Fuel rod average burnups of 60 to 62 GW d/MTHM are typical for large PWRs and since many iPWR designs rely on the fuel experience gained in large PWRs, a similar limit can be envisaged for all iPWRs, at least within the first few cycles of operation, i. e., before greater experience of iPWR operation and fuel irradiation is gained.