A prerequisite for the start of decommissioning work is that the nuclear fuel, coolant and the radioactive process waste are removed from the plant. Thus, the original activity inventory is reduced to a very great extent compared to the activity contained in the activated and contaminated components. This residual activity is then mostly present in solid form and amounts after one year to less than one percent of the activity inventory of a plant in operation. Depending on the individual circumstances, three main decommissioning variants exist: safe enclosure, partial removal with safe enclosure and complete removal.