Cementation plant

Dounreay reprocessed spent fuel from reactors for almost 40 years. The original design of the site included storage tanks for the waste liquor, or raffinate, that would be generated. Initially there was no long-term strategy for managing this waste other than keeping it in tanks.

Years of research and development lead to a decision in the 1980s that the most appropriate way to deal with this waste was to immobilise it with cement set inside drums. The acidity would be neutralised and the liquor solidified, making it safe for long-term storage or disposal as solid intermediate-level waste.

In the 1980s and early 90s, the UK Atomic Energy Authority invested in a cementation plant at Dounreay. This was designed to condition a particular kind of raffinate – the waste from the reprocessing of research reactor fuel.

The cementation plant is a remotely-operated process line surrounded by thick shielding to protect workers from the radiation hazard. The first stage in the process is the receipt of batches of liquor in a reception vessel where the radioactivity and chemistry is analysed. Sodium hydroxide is used to neutralise the acidity of the liquor.

Each 500-litre drum receives 266 litres of conditioned raffinate and 610kg of cement powder. This is mixed using a sacrificial paddle. It is allowed to set for 24 hours before a cap of grout is added and the drum lid screwed on.

All of this work takes place on a conveyor belt system that runs through a series of cells. Workers view the process through thick windows that are specially designed to block the radiation being emitted from the waste.

Each drum to emerge from the process line weighs 1.25 tonnes.

The drums are transferred to a heavily-shielded store adjacent to the cementation plant where they are stacked.

The plant produces an average of 14 drums of solid intermediate-level waste a week.

The cementation plant can also receive other types of intermediate-level waste. These are solid items, such as a fuel wrappers, that arrive loose inside drums. The drums are overpacked and moved to the adjacent store pending the eventual immobilisation of their contents with cement in a proposed new treatment plant.

The cementation plant opened in 1996 but work was interrupted in 1998 by a regulatory direction that effectively stopped all work in the Fuel Cycle Area at Dounreay. The plant restarted in August 2003.

In 2005, a number of errors during the operation of the plant resulted in a spillage of liquor and cement in the process line. This blocked the process line and contaminated the cell but did not result in any environmental discharge or exposure of workers. The plant restarted in 2008 following repair and refurbishment.

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Image: Moving empty drums into position for entry to the shielded cell area area for filling

Moving empty drums into position for entry to the shielded cell area area for filling

Image:

Empty drums are checked before they move inside the shielded cell for filling

Image: Inside the shielded cell where drums are filled with a mixture of raffinate and cement

Inside the shielded cell where drums are filled with a mixture of raffinate and cement

Image: Workers can watch the filling operation inside the cell through thick windows

Workers can watch the filling operation inside the cell through thick windows

Image: A grout cap is added to each drum before the lid is screwed down

A grout cap is added to each drum before the lid is screwed down

Image: Finished drums are transported by conveyor belt for storage

Finished drums are transported by conveyor belt for storage