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.
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. Read More