In the circumstances it is hardly surprising that the amount of nuclear waste can only be estimated. Low level solid waste, whether military or civil, is stored in drums at Drigg, down the coast from Sellafield,in a series of trenches. Civil intermediate level solid waste is largely stored at Sellafield awaiting a decision on a national storage facility. Both civil and military high level solid waste is generally moved to Sellafield for temporary storage. Because of this very long time scale, it can never be sealed up and forgotten.
Containers corrode with time.
There are earth movements. Water seeps through rocks. The waste will have to stored in such a form that it cannot be stolen and misused and in such a way that it can be inspected and if necessary retrieved and moved. Plans to dig a trial deep storage facility under the Sellafield site were thrown out in 1997. Geological evidence suggested that the local rock was too fissured and liable to be affected by water seepage.