A fire broke out at the Dounreay prototype fast reactor sodium tank farm on 7 October.
Dounreay Site Restoration Ltd called the fire 'small', that noone was hurt, and that the Dounreay Fire Brigade put it out quickly and made the area safe in 30 minutes.
Work in the tank farm has ceased; an investigation will take place.
The PFR ceased operation in 1994 and is in decommissioning.
The sodium used as coolant in the reactor reacts on contact with water.