Lorraine Mann: "Increased monitoring"

The particle, uncovered by a vehicle-mounted Geiger counter, is about the size of a grain of sand.

Dounreay director Peter Welsh said everything possible was being done to trace the source of the particles.

He said efforts had focused on a disused underwater chamber through which the plant's low-level radioactive waste used to be discharged into the sea.

However, Ms Mann said that theory did not add up.

She is urging UKAEA to start considering other possible sources.

Members of the public cannot make an informed choice about whether to use Sandside beach or not while Dounreay's operators play down the risk posed by the particles, she added.