But the particle, measuring about 2MBq, was found in a section of Dalgety Bay still open to the public.
The particle is believed to date from a time when aircraft were scrapped at the bay as landfill was later used to reclaim the foreshore.
Radium luminised dials on some of the waste can be traced back to military aircraft.
Although the discovery was made in April, it was only made public on Monday when the Scottish Environment Protection Agency chaired a meeting of the Dalgety Bay Forum.
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A spokesman said: "It was not in the cordoned off area, but was in a section of the beach that is not often used and is covered over at high tide. The source, which further demonstrates the need for a comprehensive investigation of the contamination, is currently undergoing testing."
SEPA's role is predominantly an overview at present, as the Ministry of Defence (MoD) carries out its investigations into the presence and distribution of radioactive particles on the foreshore. The MoD is also undertaking regular monitoring and retrieval.
Although "significantly higher" than other particles found on the beach, this one had lower radioactivity than the two "very high activity" particles which had previously been discovered. They measured 76MBq and 13MBq.
A full report on the beach is due next May, ahead of a clean-up.
(NE)
