[1] Expressed as a percentage: if 5% of the initial heavy metal atoms have undergone fission, the burnup is 5%FIMA.
This can be computed by multiplying the thermal power of the plant by the time of operation and dividing by the mass of the initial fuel loading.
For example, if a 3000 MW thermal (equivalent to 1000 MW electric at 33.333% efficiency, which is typical of US LWRs) plant uses 24 tonnes of enriched uranium (tU) and operates at full power for 1 year, the average burnup of the fuel is (3000 MW·365 d)/24 metric tonnes = 45.63 GWd/t, or 45,625 MWd/tHM (where HM stands for heavy metal, meaning actinides like thorium, uranium, plutonium, etc.).
Converting between percent and energy/mass requires knowledge of κ, the thermal energy released per fission event.
Higher burnup allows more of the fissile 235U and of the plutonium bred from the 238U to be utilised, reducing the uranium requirements of the fuel cycle.
In once-through nuclear fuel cycles, higher burnup reduces the number of elements that need to be buried.
Unprocessed used fuel from current light-water reactors consists of 5% fission products and 95% actinides (most of it uranium), and is dangerously radiotoxic, requiring special custody, for 300,000 years.
93Zr, having a very long half life, constitutes 5% of fission products, but can be alloyed with uranium and transuranics during fuel recycling, or used in zircalloy cladding, where its radioactivity is irrelevant.
The remaining 20% of fission products, or 1% of unprocessed fuel, for which the longest-lived isotopes are 137Cs and 90Sr, require special custody for only 300 years.
To the extent that fuel is reprocessed on-site, as proposed for the Integral Fast Reactor, opportunities for diversion are further limited.
In addition, expenses will be required for the development of fuels capable of sustaining such high levels of irradiation.
"[10] A study sponsored by the Nuclear Energy University Programs investigated the economic and technical feasibility, in the longer term, of higher burnup.