There are three different units in common use for describing the state or depletion of the fuel. These units are:
Fuel burnup (also known as fuel utilization) measures how much energy is extracted from nuclear fuel and measures fuel depletion in nuclear engineering. The most commonly defined as the fission energy release per unit mass of fuel in megawatt-days per metric ton of heavy metal of uranium (MWd/tHM) or similar units. Fuel burnup defines energy release as well as defines the isotopic composition of irradiated fuel. Since during refueling, every 12 to 18 months, some of the fuel – usually one third or one-quarter of the core – is replaced by fresh fuel assemblies, and power distribution is not uniform in the core, reactor engineers distinguish between:
- Core Burnup. Averaged burnup over entire core (i.e., over all fuel assemblies). For example – BUcore = 25 000 MWd/tHM
- Fuel Assembly Burnup. Averaged burnup over single assembly (i.e., overall fuel pins of a single fuel assembly). For example – BUFA = 40 000 MWd/tHM
- Pin Burnup. Averaged burnup over a single fuel pin or fuel rod (overall fuel pellets of a single fuel pin). For example – BUpin = 45 000 MWd/tHM
- Local or Fine Mesh Burnup. Burnup significantly varies also within a single fuel pellet. For example, the local burnup at the rim of the UO2 pellet can be 2–3 times higher than the average pellet burnup. This local anomaly causes the formation of a structure known as a High Burnup Structure.