However, safety and operational concerns set a limit to the load variability: in France the load variability is limited to ±4 up to ±8 assemblies, depending on the reactor type and the fuel management option. This variability allows for an increase of the natural cycle length if more new assemblies are charged in the reactor or for its reduction, if fewer assemblies than normal are substituted. The fuel enrichment varies generally from 3% and 5%, while average burn-up ranges from 33 to 50 GW^/ tHN (GW days per tonne of heavy metal). The flexibility is already a fundamental requirement for the nuclear industry, and will become increasingly important in the future.