Spent nuclear fuel, also called the used nuclear fuel, is a nuclear fuel that has been irradiated in a nuclear reactor (usually at a nuclear power plant or an experimental reactor) and that must be replaced by a fresh fuel due to its insufficient reactivity. Spent nuclear fuel is characterized by fuel burnup which is a measure of how much energy is extracted from a nuclear fuel and a measure of fuel depletion. Due to fuel depletion and fission fragments buildup, spent nuclear fuel is no longer useful in sustaining a nuclear reaction in an ordinary thermal reactor and it must be replaced by fresh fuel. Depending on its point along the nuclear fuel cycle, it may have considerably different isotopic constituents.
Spent Nuclear Fuel – Burnup
As was written, spent nuclear fuel is characterized by fuel burnup, which defines energy release as well as it defines isotopic composition of irradiated fuel. Reactor engineers distinguish between:
- Core Burnup. Averaged burnup over entire core (i.e. over all fuel assemblies). For example – BUcore = 25 000 MWd/tHM. This value do not characterizes spent fuel.
- Fuel Assembly Burnup. Averaged burnup over single assembly (i.e. over all fuel pins of a single fuel assembly). For example – BUFA = 50 000 MWd/tHM. This value is typical for spent fuel.
- Pin Burnup. Averaged burnup over single fuel pin or fuel rod (over all fuel pellets of a single fuel pin). For example – BUpin = 55 000 MWd/tHM. Fuel rods never have identical burnup. They slightly differ and limitations are set usually for fuel rod burnup due to fuel rod design considerations (e.g. internal pressure).
- Local or Fine Mesh Burnup. Burnup significantly varies also within 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 formation of a structure known as High Burnup Structure.
LWR with High Burnup Fuel
Current light water reactors are typically designed to achieve burnup about 50 GWd/tU. With newer fuel technology, and particularly the use of advanced burnable absorbers, these same reactors are now capable of achieving up to 60 GWd/tU. Some studies shows that in the near future, even with the present enrichment limit (5 wt %), fuel burnup could be extended near to 70 MWd/kg. Further increase in fuel burnup is impossible without the relaxation of the present enrichment limit (5%).
See also: High Burnup Fuel