Engineers believed they could reduce the levels of radioactivity in effluent from a new liquid metal destruction plant by up to a thousand times at most.
But the £15 million plant built at Dounreay in northern Scotland has reported decontamination levels reached four million during its first full run.
This is 4000 times more than the designers thought possible and means emissions from the plant have been a lot cleaner.
Andy Swan, the engineer in charge of the project, says its success is down to a unique resin developed for Dounreay by scientists in Finland.
“The clean-up factor is well in excess of anything the plant designers envisaged and means the impact on the environment from the destruction of the liquid metal is barely even detectable,' he said.
Randall Bargelt, site director at the Nuclear Decommissioning Authority, said: 'This sort of performance is why Dounreay is renowned for its decommissioning progress - excellent results, with minimal impact on the environment.
'And now that Dounreay has been granted stabilised funding of up to £150m, we can be confident that we will continue to see this sort of progress.'
The liquid metal is a remnant of Britain’s quest in the 1950s to generate electricity too cheap to meter from fast breeder nuclear reactors.
Almost 1700 tonnes of sodium and potassium used as reactor coolant has been destroyed already during the decommissioning of Dounreay, Britain’s centre of fast reactor research until 1994.
The 57 tonnes remaining inside the Dounreay Fast Reactor are heavily soiled with radioactive caesium from the fuel used in the core.
Its destruction is one of the national priorities of the UK Government’s Department of Energy and Climate Change.
The £15m destruction plant lifts the highly-reactive, alkali metal from the reactor system in 200-litre batches.
Each batch is reacted with water in a nitrogen atmosphere to create a hydroxide solution that is neutralised with nitric acid.
The process turns it into 2000 litres of effluent that is about twice the strength of standard household drain cleaner.
But it cannot be discharged to the sea yet, because it still contains high levels of radioactive caesium, so the effluent is cleansed through a special resin fitted inside an “ion exchange”.
“We knew from tests carried out on the resin by Prague University that it had the potential to exceed the expectations of the plant’s designers,” explained Andy Swan.
“Once the chemical hazard has been dealt with, we’ve been very careful with the operation of the ion exchange to get the maximum performance from the resin and so reduce the environmental impact of the effluent to an absolute minimum. It has been so successful that the level of caesium in the effluent discharged to sea is barely even detectable.”
The treatment plant has now completed its first campaign, destroying more than 10 tonnes in the process.
It’s now shut down for planned maintenance, including the installation of a new resin column in the ion exchange. The old column, soaked with caesium, was removed safely, packed in a drum and sent to a store at Dounreay for intermediate-level waste.
The destruction plant is scheduled to restart early in February. It is on target to destroy all 57 tonnes by 2013.