The operator of a nuclear complex in Caithness has admitted illegally dumping waste and allowing radioactive particles to be flushed into the sea.
Dounreay is in the process of being decommissioned
The UK Atomic Energy Authority (UKAEA) pleaded guilty to four charges under the Radioactive Substances Act at Wick Sheriff Court.
They relate to activities at Dounreay between 1963 and 1984.
Sheriff Andrew Berry has deferred sentence after hearing from the fiscal and UKAEA's solicitor.
Fiscal Alistair MacDonald told the court that the cost of dealing with the particles would be borne by taxpayers.
For UKAEA, solicitor David Stewart said it was accepted that the events should not have occurred.
He also said that the industry had moved on, learning from its mistakes of the past.
UKAEA's court appearance followed a report to the procurator fiscal by the Scottish Environment Protection Agency (Sepa).
The company admitted illegally dumping solid nuclear waste in a landfill site at Dounreay and three charges of allowing fragments of irradiated nuclear fuel to enter the plant's liquid effluent discharge pipe into the Pentland Firth.
Outside the court, UKAEA director of safety Dr John Crofts conceded "some mistakes were made" during the pioneering days of fast reactor nuclear technology.
Dounreay was Britain's centre for fast reactor research.
Dr Crofts said: "UKAEA deeply regrets that some particles were released from the site.
"Our priority today is to rectify those errors and minimise their impact on the environment.
"The practices which gave rise to these particles ceased long ago and we are now focussed on our mission to remediate the site and deal with the particles issue."