The UK’s Dounreay Site Restoration Limited (DSRL) announced on 5 August that some 68t of the liquid metal coolant, a blend of sodium and potassium (NaK) have been removed from the primary circuit of the Dounreay Fast Reactor (DFR) and destroyed over 10 years. Most of the NaK had been removed by 2012, since when work has been under way to remove the last of the coolant from the difficult to access pipework and base of the structure.
Dounreay's 14 MWe experimental fast breeder reactor, housed inside a steel sphere, was key to UK R&D during the 1950s and 60s. It was the world's first fast reactor to provide electricity to a national grid in 1962. The NaK in the primary circuit transferred heat from the reactor to the steam generating plant. This steam was used to produce further steam to generate electricity. The reactor design allowed the metal to flow through the fuel assemblies and the NaK in the primary circuit was significantly contaminated with fission products during reactor operations. A number of open fuel pins added to the coolant contamination. The secondary circuit was cleaned out between 1979 and 1981. This contained about 110t of NaK with a thousand times less radioactivity than the primary circuit NaK.
DSRL said it used a specially-built plant and removal system to process the NaK. This process reacted the NaK with water and caustic to create a hydroxide solution. As NaK reacts readily when exposed to air or water, it had to be kept under a nitrogen gas blanket. The hydroxide solution was then neutralised and sent through an ion exchange bed to remove residual radioactivity. The cleaned solution - basically salt water - was discharged to sea. The radioactivity absorbed by the ion exchanger was packaged and stored as intermediate-level waste. Some 1000 trillion becquerels of caesium-137 was extracted from the coolant.
DSRL said the destruction of the DFR's liquid metal coolant has removed "one of the highest hazards remaining in the Nuclear Decommissioning Authority (NDA) estate". The focus of decommissioning work at the DFR will now be the removal of some 1,000 breeder elements that remain in the reactor vessel, DSRL said. This must be completed before cleaning and removal of the reactor and its 9km of cooling pipework.