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Decommissioning of the former experimental reactor establishment at Dounreay has taken another important step forward with the green light to empty a series of tanks containing a legacy of radioactive liquid waste from the site's reprocessing era.

The liquid will be transferred from underground storage tanks to a modern treatment plant where it will be mixed with cement and set inside 500-litre drums.

Dounreay director Peter Welsh said: "An essential step in restoring the environment of our site requires the conversion of liquid waste like this to a solid condition that is suitable for long-term storage or disposal. I am pleased that we have received the consent of the Nuclear Installations Inspectorate to get on with the job of turning this particular type of waste into a form that is passively safe for the future. This task is one of the key activities in the site restoration plan, so today's announcement represents an important milestone in the delivery of the plan."

The liquid waste is a legacy of reprocessing of fuel from research reactors and the Dounreay Fast Reactor. Dounreay also possesses a quantity of liquid waste from the reprocessing of fuel from the Prototype Fast Reactor and UKAEA is investigating the feasibility of cementing this waste as well.

The cementation plant first came into service during the 1990s. It had processed enough liquid to make 165 drums of cemented waste when its operation was stopped in 1998 by the Direction served on the
site's waste management complex by the Nuclear Installations Inspectorate (NII) of the Health and Safety Executive. Consent has now been received from the NII to resume cementation of the liquid.

It is expected to take 10-15 years to empty and solidify the contents of all the tanks. The contents - an acidic liquor containing fission products - were created by the chemical separation of irradiated reactor fuel undertaken at Dounreay until 1996.

The liquid waste will be mixed with caustic soda and set in cement inside stainless steel drums. In total, it is expected to create approximately 5500 drums of solid intermediate-level waste that will be stacked in a shielded store at the site.

Each 500-litre drum of cemented waste is made up of 178 litres of liquid waste and 610kg of cement to create a "monolithic" waste package weighing 1.25 tonnes. Once fully recommissioned, the anticipated throughput of the cementation plant is 14 drums per week.

UKAEA is currently spending £140-150 million per annum on the decommissioning and site restoration of Dounreay. Decommissioning of Dounreay is currently worth approximately £75 million per annum to the economy of the Highlands.