Experts believe they may have found the most radioactive particle to be recovered so far from a beach near the Dounreay nuclear plant in Caithness.
The Scottish Environment Protection Agency (Sepa) has been informed of the discovery at Sandside.
Initial tests suggest it contained between one a two million becquerels (Bq) of radiation - at least twice as "hot" as a particle found in 2007.
Additional monitoring is now being carried out at the beach.
The particles are linked to the reprocessing of nuclear fuel rods at Dounreay during the 1960s and 1970s, when they were flushed into the sea through the plant's liquid discharge pipe.
The particle, which was found on 14 February, was the 208th to be recovered from the beach by contractors.
Dounreay Site Restoration Limited (DSRL) said further analysis may confirm the find as "significant". If so, it would be the first particle from Sandside to be given that classification.
Initial tests estimated that an isotope called Strontium-90 in the particle had a radioactivity of between one and two million Bq.
The most active particle found at Sandside so far was the one discovered in 2007 with a reading of 500,000 Bq.
In March 2010, a particle containing 270,000 Bq was detected.
A particle recovered from the seabed in 2010 contained 100 million Bq of radiation.
DSRL has been leading a clean-up of the tiny fragments from beaches and the seabed near Dounreay.
It said it had informed Sepa of the initial findings from the latest discovery and the need for additional tests to understand the chemistry of the particle and verify its potential hazard.