Coolant removed from Dounreay Fast Reactor

05 August 2016

A ten-year process to remove 68 tonnes of highly-radioactive liquid metal coolant from the primary circuit of the UK's Dounreay Fast Reactor (DFR) has now been completed, marking a major milestone in its decommissioning.

DFR - NaK disposal plant glove box - 460 (DSRL)
A worker using a glove box on the NaK processing system (Image: DSRL)

Dounreay's experimental fast breeder reactor, housed inside a steel sphere, led British nuclear R&D during the 1950s and 60s. It became the world's first fast reactor to provide electricity to a national grid in 1962. Its 14 MWe output was enough to power a small town like nearby Thurso, with a population of about 9000.

Dounreay Site Restoration Limited (DSRL) announced today that some 68 tonnes of the liquid metal coolant - a blend of sodium and potassium called NaK - have been removed from the primary circuit of the DFR and destroyed over a ten-year period.

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.

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 NaK in the primary circuit was significantly contaminated with fission products during reactor operations. The reactor design allowed the metal to flow through the fuel assemblies. A number of open fuel pins added to the coolant contamination.

The secondary circuit was cleaned out between 1979 and 1981. This contained about 110 tonnes 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 neutralized 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 director of reactors Ken Heider said, "The Dounreay team, in partnership with our supply chain, worked extremely hard over a long time to remove and convert the highly radioactive NaK into safe products. Most importantly, our highly skilled team delivered the work safely and in compliance with our environmental authorizations."

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".

NDA chief operating officer Pete Lutwyche said, "The difficulty of this task can't be understated, and I welcome the news that this work is complete. Everyone involved should be proud of their achievement."

The focus of decommissioning work at the DFR will now be the removal of some 1000 breeder elements that remain in the reactor vessel, DSRL said. This must be completed before cleaning and removal of the reactor and its nine kilometres of cooling pipework.

Researched and written
by World Nuclear News