The Challenge
The Dounreay PFR contained 1500t of sodium coolant, and as such is a significant hazard. Although the PFR was carefully designed to be free draining, during drainage several hold up points resulting in between 20 and 40 tonnes of sodium being left in the reactor.

The Solution
The AMRR Alliance (UKAEA, Sinclair Knight Merz (SKM), Babtie, JGC Engineering and Alstec ) was formed to solve the problem of removing the remaining sodium within the reactor. The process designed to solve the problem was the Water Vapour Nitrogen (WVN) system.

This system allows a controlled reaction to occur between the water vapour, held in a heated nitrogen carrier, and the sodium. The resulting reaction creates liquid and gaseous by-products in the form of sodium hydroxide and hydrogen. The ultimate aim being to use the process on the sodium pools within the reactor.

The WVN process is an unknown quantity in the nuclear industry and as such requires extensive proving and development to maintain safety. The design and modification of the process is continuous and as such new challenges occur regularly.

The projects for the WVN process are the treatment of the PFR sodium tank farm, the PFR secondary sodium circuits, various reactor components termed ‘the sodium inventory’, and the reactor and ancillary equipment.

Before the process is used on the above projects, a series of extensive testing projects have been carried out at the Janetstown off-site facility using non-contaminated sodium. This testing phase has provided vast amounts of data and operational experience to enable future projects to progress. A second WVN rig has been designed and manufactured to complete the treatment of the PFR sodium tank farm and sodium inventory.

The testing phase is nearing completion, with all bulk sodium treatment tests complete. The size of test ranged from 5kg to 1000kg of clean sodium.

Due to the complex geometry of the reactor vessel interior, testing involving the cleaning of varying diameter pipework and valves is currently being undertaken.

Conclusions
The experience and expertise of the Alliance members has proved an effective team at resolving the challenges faced; the perfect platform to design, develop, manufacture, operate and analyse the WVN process for use within the nuclear industry. T3|UK facility has proved to be key in the gaining of operational experience in controlling the WVN process for future projects, allowing for the creation and implementing safe systems of work, such as a permit to work system to enable safe operations.