Dounreay Site Restoration Ltd

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Primary coolant

Liquid metal was used in fast reactors to transfer the heat from the nuclear reaction in the core to the steam-generating plant where electricity was produced. The metal flowed between the core and the steam-generating plant through circuits linked by heat exchangers.

The Dounreay fast reactor used an alloy of two alkali metals, sodium and potassium, or NaK. This produced little waste during operation but the volatile compound needs unique and complex treatment during its decommissioning. The alloy combusts in contact with air or moisture and is normally kept safe in a fire-suppressant atmosphere such as nitrogen.

The liquid metal “coolant” in the primary circuit of DFR became highly contaminated during its operation until closure in 1977. The design allowed the metal to flow through the fuel assemblies and burst experimental fuel pins added to the coolant contamination.

The secondary circuit, containing approximately 110 tonnes of NaK and a thousand times less radioactivity, was cleaned out between 1979 and 1981. The methods selected were similar to those used to decommission an experimental fast reactor in the USA and involved a mixture of spray burning and conversion to hydroxide. After chemical treatment, the hydroxides were discharged to the sea.

Destruction of the NaK in the primary circuit started in 1983 but was stopped after just 100kg because of concern about the high level of radioactive caesium contamination in discharges to sea. This led to a period of research and development to identify a safer and more environmentally acceptable method of destroying the NaK in the primary circuit.

The method chosen transfers the NaK in batches from the nine kilometres of pipework in the primary circuit to a shielded treatment plant built beside the reactor inside the sphere. Here the NaK is reacted with water under a blanket of nitrogen to create a hydroxide solution that is neutralised with nitric acid.

The salty water created by this process is still too radioactive to be discharged to sea, so it is passed through filters called ion exchange columns that absorb the caesium contamination. This greatly reduces the radioactivity in the water to a level that is within limits authorised by the Scottish Environment Protection Agency for sea discharge. The contamination absorbed on the columns is retained and stored as intermediate-level waste.

The NaK disposal plant was designed by AEAT. It was modified between 2001 and 2007 by NUKEM at a cost of approximately £15 million to meet the exacting standards for modern radioactive plant. Active commissioning started in March 2007 and, subject to regulatory agreement to begin full operation, is expected to take two and a half years to complete destruction of the bulk liquid metal.