Utility requirements are tending to become ever more demanding as they attempt to extend cycle lengths further and further and at the same time demand increased fuel discharge burnup. If this trend continues, it is likely that burnable poisons will be used even more extensively and with ever-increasing sophistication (which usually means more heterogeneous distribution of the poison material radially and axially within the fuel assemblies). The use of enriched 10B, already available commercially, is likely to increase and there may also arise a demand for isotopically enriched gadolinia and erbia. These developments are likely to increase the pressure on fuel manufacturers to develop automated fabrication lines that can more easily deal with more complicated burnable poison loadings, and on the developers of nuclear design codes to more easily accommodate very heterogeneous burnable poison loadings through a higher level of automation. Since there are only around 90 naturally occurring elements in the Periodic Table, the number of candidate burnable poison materials is virtually exhausted and it is unlikely that any new ones will emerge (see Chapter 2.09, Properties of Austenitic Steels for Nuclear Reactor Applications, Chapter 2.15, Uranium Oxide and MOX Production and Chapter 2.19, Fuel Performance of Light Water Reactors (Uranium Oxide and MOX)).