Talks between the owner of a beach where radioactive particles have been found and the organisation leading the clean-up of Dounreay have broken down.
Geoffrey Minter posted a response on his website
The United Kingdom Atomic Energy Authority (UKAEA) and landowner Geoffrey Minter had held discussions about Sandside Beach, near the plant.
Mediation concerned the best way to deal with particles found there.
But site operator UKAEA said it had ended the process. Mr Minter said he was "sorry" at its decision.
In a statement, the authority said: "UKAEA regrets to announce that the mediation process, which we began with Geoffrey Minter in May 2005, has ended.
"We wanted to ensure that we could find a fair, reasonable and documentable resolution to this issue.
"Unfortunately, despite every effort, it has not been possible to achieve this."
UKAEA said it would seek a long-term solution to managing particles found in the area with "full public consultation" and continue to monitor Sandside.
Mr Minter has been a fierce critic of UKAEA in the past.
He posted a response to the UKAEA announcement on his estate's website and said he would not say anything else.
His statement reads: "I am sorry that UKAEA has decided to break off talks on the best way of tackling the radioactive contamination of Sandside Beach.
UKAEA is heading the decommissioning of Dounreay
"Nonetheless, I personally remain committed to finding a solution to the severe and on-going environmental damage to this part of the Caithness coastline."
Mr Minter said UKAEA had to find a way to clean up particles and stop more waste washing up on the beach.
On Sunday, it emerged another radioactive particle had been recovered from Sandside. It was the 67th hotspot to have washed ashore.
Mr Minter previously went to court over particles found on the beach.
His case reached the Court of Session in Edinburgh in 2003.
In a written decision, the judge, Lady Paton, said the law did not allow her to order the detailed clean-up which Mr Minter had demanded in place of the current monitoring regime.
But she held that the UKAEA, which operates the Dounreay plant, had failed its duty under the Nuclear Installations Act 1965.
Lady Paton said UKAEA must ensure that nuclear material causes no injury or damage to property.
She added that, despite the stress and anxiety claimed by the Minters and the effect on their property, no personal injury had been suffered.