Archaeologists are surveying fields next to Dounreay prior to their development as a site for the disposal of low-active waste from its decommissioning.
Dounreay Site Restoration Ltd hired Headland Arachaeology Ltd to carry out the evaluation, a condition of the planning consent for the £100 million development.
A series of trenches dug across the 40 hectare site did not reveal any significant new archaeological features.
A separate excavation of a small mound first identified in 2000 was carried out over a five-week period and uncovered a sizeable prehistoric burial cairn.
The cairn was constructed from local flagstones with a kerb demarcating the extent of the monument.
A central cist made from large rectangular slabs placed in an upright position is now being excavated.
The lack of burial remains or artifacts detected so far indicates the monument may have been robbed sometime in the past.
According to Headland, the find adds to the knowledge of prehistory in the Dounreay area.
An older Neolithic chamber cairn, dating back 5000 years, is recorded at nearby Cnoc-na-h’Uiseig and provides evidence of earlier bural practices.
The cairn now being examined dates from around 4000 years ago – the Bronze Age – when people were buried individually, often with possessions for the afterlife.
Construction work in the area is scheduled to begin in 2011 following the award later this year of a design and build contract.