The operators of the Dounreay nuclear site have been fined £2m for a spillage of radioactive liquid.
Radioactive liquid was poured onto a sealed drum
The United Kingdom Atomic Energy Authority was penalised by the Nuclear Decommissioning Authority over the incident last September.
The spill took place as the liquid was being mixed with cement and poured into steel drums at the Caithness plant.
Dounreay's operators are still trying to clear up that part of the plant, which remains closed.
An investigation into the incident found that the automated system in the plant tried to fill a 500 litre drum with the highly active liquid despite the fact that the lid was already on.
More than 250 litres of radioactive fluid and 300 kilos of cement spilled onto the floor of the heavily shielded area, then set solid.
Officials said nobody had been harmed or exposed to radioactive waste as a result of the incident.
However, the Nuclear Decommissioning Authority said the incident was such a major safety breach that it would be withholding £2m of the fee it pays Dounreay.
The incident was regarded as all the more serious because it happened after the authority had demanded a safety review be conducted at all nuclear plants following a separate incident at the Thorp plant at Sellafield.