The discovery was made on Thursday on Sandside Beach, just one-and-a-half miles from the complex in Caithness.

But environmental campaigners Friends of the Earth said the operators waited two days before revealing the find.

It prompted the SNP to condemn what it called Dounreay's "appalling record of secrecy and cover-up".

The party intends to raise the alleged delay in announcing the find in the Scottish Parliament and at Westminster.

Operators deny any cover-up

But a Dounreay spokesman said he was waiting for the particle to be analysed before making the incident public.

Last Thursday's discovery of a metallic fragment of radiating atomic fuel was only the fourth ever to have found found on Sandside Beach, at Reay, Caithness.

Over 200 particles have been found washed ashore on the fenced-off foreshore at Dounreay since the early 1980s.

Top officials at Dounreay believe that a disused radioactive effluent pipeline is the likeliest source of the hot-spots.

Dounreay's administrators informed the watchdogs Scottish Environmental Protection Agency and the Nuclear Installations Inspectorate about the latest find.

They also told the Scottish Executive and the Department of Trade and Industry.

Cover-up denied

Dounreay spokesman Derrick Milnes denied there had been any delay, or cover-up, in informing the public about the find.

He said: "We immediately told the necessary official bodies, but we intended to wait until the particle had been analysed in detail before releasing the news to the general public."

He said he was not aware that SEPA officials had asked officials there to issue an immediate press release.

But anti-nuclear campaigner Kevin Dunion, of Friends of the Earth Scotland, accused the complex's operators of "once again showing themselves to be secretive and dangerous".

He added: "After two decades of radioactive leaks, it is still leaking like a sieve and posing a threat to the general public".

Political moves

The SNP intend to table questions in the Scottish Parliament and Westminster on the issue.

Transport and environment spokesman Kenny MacAskill MSP said: "Dounreay has an appalling record of secrecy and cover-up, and this is the latest in a long line of examples.

"Apparently, Dounreay was advised by the Scottish Environment Protection Agency to inform the public about this potentially lethal leak via a news release on Friday, but failed to do so.

"The leaking of a radioactive particle is an extremely serious matter - another sign of real problems at Dounreay, and an important public health issue.

"For the public to be kept in the dark about this happening on a local beach is a shocking state of affairs.