Dounreay Decommissioning

Dounreay is the second largest civil nuclear site in the UK, located 110 miles north of Inverness on Scotland’s Atlantic Ocean. Once it was the focal point of the UK’s fast breeder reactor research and development programme. Nuclear research ceased in the 1990s and Dounreay is now internationally recognised as one of the world’s most complex nuclear site closure projects. Its collection of reactors, fuel plants, laboratories and waste facilities are all at different stages of decommissioning. Removing 1810 litres of radioactive sodium coolant from a shallow ‘heel’ pool at the base of the Prototype Fast Reactor (PFR) reactor vessel, after most of the sodium coolant had been removed, is an example of the decommissioning challenges that the workforce faces. Removing the remnant sodium from the base of the reactor was an important stage in decommissioning PFR. Safety studies indicated that the pool had to be reduced to less than 150kg before the water vapour nitrogen (WVN) treatment could be applied to the reactor vessel. This presented many  engineering challenges. The most obvious was the fact that the sodium was in a solid state across the base of the vessel, with no means of re-melting it, and was located in an area which was very difficult to access with pumping equipment. The first challenge was to design and install a means of melting the sodium so it could be pumped out. The approach was to design, manufacture and install 11 external heating arrays, each with six petals containing ceramic heaters. These were installed in contact with the reactor vessel through holes drilled through the vessel leak jacket and deployed in a manner similar to opening an umbrella. The design allowed sufficient sprung force to provide hard contact with the underside of the vessel for good thermal conductivity and heat transfer. Nuclear Engineering International 18th Jan 2022  https://www.neimagazine.com/features/featuresodium-success-story-at-dounreays-pfr-9408761/