For more than half a century, it represented one of the major radiological hazards in the UK nuclear industry. Now, highly radioactive waste from Britain’s earliest experiment with fast breeder reactors has been made safe for future generations in a landmark for hazard reduction at Dounreay.

Underground tanks used to store the waste liquor from reprocessing of fuel irradiated in the Dounreay Fast Reactor have been emptied and their contents immobilised in cement, more than half a century after the first fuel was reprocessed.

It has been described as a landmark in hazard reduction by the Nuclear Decommissioning Authority, which inherited the waste in 2005.

“This is a great milestone in a history of Dounreay milestones,” said Nigel Lowe, NDA head of programme for Dounreay and Magnox.

The 60MW Dounreay Fast Reactor operated between 1959 and 1977 and was the world’s first fast breeder reactor to supply a national grid.

Fuel from the reactor was reprocessed at the site between 1960 and 1979. A total of 7300 core elements were dissolved in nitric acid to separate the re-usable uranium and plutonium from the fission products.

The waste stream containing the fission products, or raffinate, was consigned to a series of underground storage tanks where it lay undisturbed for half a century.

In 2005, the newly-formed Nuclear Decommissioning Authority inherited a proposal to build a new waste treatment plant at Dounreay at an estimated cost of £150 million.

The NDA suspended design work in 2010 and invited consortia to look for an alternative solution as part of its competition to find a parent body organisation for site licence company DSRL.

The successful tender came from the Cavendish Dounreay Partnership, which proposed modifying an existing plant used since 1996 to treat raffinate from the reprocessing of materials test reactor fuel.

The Dounreay Cementation Plant was originally designed to receive MTR raffinate in 2500-litre batches, neutralise it and immobilise the conditioned liquor with cement in 500-litre stainless steel drums.

When this programme finished in November 2013, work started to modify the plant to receive raffinate from the Dounreay Fast Reactor reprocessing campaigns.

“We needed to make a number of changes, the most significant of which was to install an additional transfer pot to increase the existing conditioned raffinate batch volume per drum from 266 litres to 328 litres, a consequence of the different raffinate chemistry and newly-developed cement formulation for the DFR raffinate,” said Stuart Andrew, plant project manager at DSRL.

Transferred in 3200-litre batches, the DFR raffinate was immobilised on a production line surrounded by thick shielding. The neutralised liquor was mixed with cement inside a 500-litre drum, stirred using a sacrificial paddle and allowed to set before being capped with a layer of grout and the drum lid replaced.

In total, some 232m3 of DFR raffinate was treated this way, creating 875 drums each weighing 1.1 tonnes. These will now be stored at the site in accordance with Scotland’s policy on the long-term management of radioactive waste.

“It’s a milestone for hazard reduction,” said Bruce Covert, who was appointed by Cavendish Dounreay Partnership as director of waste at DSRL. “In its liquid form, there’s a risk the waste can leak and cause serious damage. Now it’s been immobilised in cement, the waste is stable and safe to store for future generations.

“The DFR raffinate represented one of the major radiological hazards left over from the R & D programme at Dounreay. The plant team in particular, and the site as a whole, can take great pride in what they’ve achieved to make it safe.”

Fuel from Britain’s second and last fast breeder, the 250MW Prototype Fast Reactor that operated from 1974 until 1994, was also reprocessed at Dounreay until 1996 This created another 220m3 of raffinate still stored in tanks.

More extensive modifications to the cementation plant will now begin to enable the last of the site’s raffinate to be immobilised, finally eliminating a major hazard of the UK’s 20th century nuclear research.