Taxpayers must foot a £1m bill to clean up a spill of highly radioactive liquid waste at the Dounreay nuclear complex in Caithness.
The clean-up operation is costing £1m
The plant's cementation plant has been closed until next summer as a result of the accident.
Four workers were suspended but no disciplinary action has been taken against them.
The problem at the reprocessing site occurred during the night shift on 26 September.
An operator failed to see that the lid of a drum was sealed shut as he prepared to release a batch of highly-radioactive waste into it.
The liquid poured over the lid. A number of alarms were ignored and a total of 260 litres of the liquor ended up settling into sumps on the floor.
An extensive in-house investigation has found that no single factor was to blame though it noted there had been an over-reliance on automated controls.
Operator, the United Kingdom Atomic Energy Authority (UKAEA) expects to receive two formal enforcement notices in the wake of the spillage from its regulator, the Nuclear Installations Inspectorate.
Less than three weeks later, another part of the site was closed in a second alert.