One of the world’s leading independent nuclear experts described the admission as “astonishing” and would cost “millions” to solve.
The experimental fast breeder reactor at Dounreay led British research and development of nuclear energy during the 1950s and 60s.
Housed inside a steel sphere, it was built between 1955 and 1958 to test the concept, and became the first fast reactor in the world to provide electricity to a national grid in 1962.
Its 14MW output was enough to power a small town like Thurso, with a population of 9000. It closed down in 1977 and Dounreay is currently being decomissioned at a cost of £1.6bn, in what its operators describe as “one of the most significant challenges in the UK today.”
The reactor was one of only two ever built in the UK to run on liquid metal - an alloy of sodium and potassium known as NaK.
Following its closure, the reactor was defuelled, the liquid metal removed from the secondary circuit and some of the breeder material taken out.
Construction started in 2003 on the plant needed to remove the rest of the breeder material material and destruction of the bulk liquid metal in the primary cooling circuit was completed in 2012.
Once the residues of liquid metal have been cleansed from the circuits and all the nuclear material removed, work will begin to dismantle the reactor structure, including the containment sphere. The current timescale is by 2021 - though the latest discovery could alter that date significantly.
Site operators DSRL has confirmed that last year a specialist camera was sent into the reactor to survey the state of the breeder fuel that remained - and captured the first footage for over 50 years of the core.
Nitrogen cooled digital cameras were inserted into the reactor and recorded the images of the internal structures and the breeder fuel matrix.
The images will enable Dounreay to prepare accurate plans for the “safe removal” of the remaining fuel and ultimately dismantling the reactor, housed inside the site’s famous - and increasingly rusting - dome.
But the camera made a startling discovery.
“The camera showed that a larger proportion of the remaining fuel was stuck than had been believed,” said a spokeswoman.
“The time and cost will form part of the reprofiling of the decomissioning programme.”
She was unable to give any estimate of the cost of the operation. It is hoped that a revised plan - which will also take into account other changed issues at Dounreay such as security and nuclear fuel transportation - will be revealed in November.
But world renowned independent nuclear expert Dr John Large, who oversaw the salvage of the Russian submarine Kursk, described the discovery as “concerning.”
“It will cost millions but what I find astonishing is that they didn’t know they had left so much of highly radioactive material in the core,” said Dr Large, who advises governments and groups like Greenpeace and who is primarily known for his work in assessing and reporting upon nuclear safety.
“If this material - which will be 93 percent uranium - has fused, in other words ‘melted’ - inside the core it will be a major problem to get it out. It is quite alarming that this material was left when it was defuelled decades ago. What is going on?
“Instead of dealing with irradiated material you now have fission products which are a different kettle of fish. It is not just a case of knowing it is radioactive you have to know the chemistry of what’s there too.
“The reactor did run on plutonium for a while and given the current situation this is quite worrying. They appear - at this stage - to not know how to get it out, if it’s fused in - let alone the cost.”
“They have a big problem - it changes the whole dimension of things. What they should do is reassess the length of the job, resources and timings as well as the issue of radioactive waste management.”
Dounreay’s purpose was to research and develop more efficient ways to generate electricity from uranium and plutonium
A variety of fuel types was tested in reactors and examined over a 40-year period.
By 1994, when the UK Government decided Dounreay had served its purpose, the site overlooking the Pentland Firth had accumulated approximately 100 tonnes of uranium and plutonium fuels, much of it in a variety of specialist forms.
Dounreay is currently at the centre of a row over plans to ship cargoes of radioactive nuclear waste around the north coastline from Caithness to Cumbria.
The proposed trial is part of efforts to find an alternative to the controversial practice of sending spent fuels from Dounreay by rail for reprocessing at Sellafield - but critics are warning against the risks of navigating rough seas around Cape Wrath and the Minch.