Engineering and Process Development in Support of the Removal of Residual Sodium Coolant from the Prototype Fast Reactor at Dounreay, Scotland

摘要

As part ofthe United Kingdom nuclear power development programme, a sodium cooled fast breeder reactor was constructed at the United Kingdom Atomic Energy Authority (UKAEA) Dounreay site. The Prototype Fast Reactor (PFR) commenced operation in 1974, and supplied electricity to the national grid until the end of its design life in 1994. The facility is now being decommissioned, and the site environment restored. Decommissioning and dismantling operations require the chemotoxic hazard presented by the sodium to be considered, in addition to the radiological hazard inherent in the decommissioning of any nuclear facility.rnRemoval of sodium from the reactor vessel will be in two stages. Firstly, the bulk of the sodium will be pumped out to a dedicated treatment plant. This will leave sodium residues as films on all internal surfaces and as pools in areas of the vessel that will not drain. These residues will then be chemically reacted in-situ, prior to commencing dismantling operations.rnThe Water Vapour Nitrogen (WVN) process methodology has been selected by UKAEA as the preferred technology for the removal of sodium residues. To this end, a dedicated off-site test and development facility has been constructed to prove the WVN process for application to facilities on the Dounreay site. As part of the development, the highly vigorous sodium-water reaction has been captured on video, and pressure and temperature transients analysed to provide a better understanding of the reaction progress.rnThe WVN process has now been successfully used to clean sodium residues from ancillary plant including storage tanks, small plant items with complex geometries, and the secondary circuits. Residual sodium quantities of up to one tonne have been successfully treated. Lessons learned through these successes will be important in the design stage of the system to clean the reactor vessel, in order to ensure its safe, successful cleaning.