Specialised reactors, primarily used for research or marine propulsion, use highly enriched uranium (HEU), which contains >20% 235U, as fuel. This presents particular proliferation risks since the fuel contains weapons-usable uranium and, over the last 20 years, considerable efforts have been made to remove HEU from research reactors and to reconfigure them to use lower enriched fuel. HEU fuels typically contain lower plutonium contents than natural or LEU fuels, since they have a lower content of the 238U precursor. As well as the high fissile content, HEU fuels often have unusual compositions and structures which make them difficult to reprocess in a conventional plant.