Dounreay fast reactor (DFR)

This is the iconic steel sphere on the north coast of Scotland. It was built between 1955 and 1958 and was the first nuclear reactor in the world to supply electricity to a national grid in 1961. It was cooled with a eutectic mixture of sodium and potassium (NaK) and produced 14 MW. It was shut down in 1977.

Early decommissioning included removing the conventional electrical generation installations, removal of the fuel and destruction of the secondary circuit NaK. There was then a lengthy period of care and maintenance during which numerous theoretical studies and large off-site and on-site practical experiments were carried out to investigate a way in which to decommission the reactor. The major challenges were how to remove, treat and destroy the 57 tonnes of highly contaminated (400TBq) primary circuit NaK and then remove the uranium and plutonium breeder fuel elements that had provided the blanket around the core. Both challenges are among the most hazardous and difficult in NDA’s UK decommissioning programme.

During the period 1999-2008, the original plant used to destroy the primary NaK was refurbished and revalidated along with major refurbish­ment of the DFR infrastructure. A successful campaign of destroying the NaK started in 2009 and is nearing completion. The process involves react­ing the NaK with an aqueous solution of NaOH in a comprehensively monitored and controlled reaction vessel and then neutralisation of the aqueous waste stream produced with HCl. The radioactive caesium is removed from this waste stream by ion exchange plant using hexacyanofer — rate inorganic resin. The decontaminated salty water is discharged to sea through the site’s low level liquid effluent treatment plant (LLLETP). The contaminated ion exchange medium is then stored as solid ILW.

The internal surfaces of the reactor and the 9 km of heat exchanger circuit pipework require the residual NaK to be removed. A tightly controlled wash out process utilising water vapour and nitrogen, or a dry process associated with dismantling could be possible methods.

Special equipment has been designed and manufactured to cut out the remaining uranium and plutonium breeder elements. When removed, they will be treated and packaged in the specifically designed shielded cells that have been constructed as a new facility abutting the sphere. The packaged breeder fuel will then be transferred to Sellafield for reprocessing.

There is then a programme of decommissioning the pipework, NaK processing vessels and the reactor vessel itself in the period up to the 2020s. The current strategy is for the sphere itself to be demolished unless a future viable initiative emerges for it to be retained as a historical industrial herit­age monument.

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