Radioactive material has leaked at the site of the former Dounreay nuclear power station in Caithness, it has been confirmed.
Radioactive liquid effluent is understood to have leaked inside a treatment facility.
The Scottish Environment Protection Agency (Sepa) said the leak was minor and did not get outside the plant.
Sepa has launched an investigation. Dounreay is currently undergoing a £2.6bn decommissioning process.
Dounreay Site Restoration Limited (DSRL) said that nobody was put at risk by the incident.
The local SNP MSP praised the staff dealing with the leak, but repeated his opposition to nuclear power.
Rob Gibson, who represents Caithness, Sutherland and Ross, said: "Thankfully, no staff from Dounreay have been harmed in this incident and I trust that the investigation will assure the public that the surrounding area will not be affected.
"These events underline both the complexity of the decommissioning processes and the possibility that errors may still be made.
"Nuclear energy has no place in a safe Scotland."
Dounreay was constructed in the 1950s as an experimental nuclear power complex, but has not generated electricity since 1994.
Radioactive liquid effluent occurs when a reactor and its equipment are cleaned.
The treatment plant at Dounreay removes some radioactivity from the waste liquid as part of the process.
A section of the treatment plant has been shut down for investigations.
DSRL is in the process of dealing with 100 tonnes of breeder reactor material.
The fuel is being stored securely while the dismantling of the site continues.