Decontamination and decommissioning
Getting the gloves on22 February 2008
Complex problematic sites like Dounreay require a range of decommissioning tasks – from the simple size reduction of gloveboxes to removal of stuck breeder elements from an ageing reactor. By Corrina Thomson
The Dounreay nuclear plant in northern Scotland is a former reprocessing and fast reactor site with a complex range of decommissioning tasks required, from the dismantling of contaminated areas, to reacting volatile sodium coolant and removing fuel elements stuck in one of the site’s fast reactors.
A necessary step in removing this fuel and decommissioning the Dounreay Fast Reactor (DFR) containment sphere has now taken place with the sphere being cut open to connect it with a new breeder fuel removal building.
DFR still contains fuel despite being shut down in 1977, as a significant number of fuel pins and breeder elements were buckled, welded and damaged in the reactor at the time operations were ended.
Some 929 outer breeder elements, 46 ‘gagging skirt’ breeder elements, 2 inner breeder elements and one core experimental fuel assembly remain in the reactor. The composition of the experimental fuel assembly has been withheld by UKAEA on security grounds.
Finding a way to remove this reactor waste has posed a problem for the plant’s operators for decades.
A section of DFR's steel sphere was removed during 2007 in preparation for the connection of the reactor and new plant.
The cover on the outer steel section was first removed revealing the inner containment steel box. A section of this was then removed and a transfer tube carefully installed and secured linking the two buildings.
The installation of the tube is required to allow transfer of the breeder elements from the sphere to the adjacent building.
The contract value of £26.7 million covered construction, and all equipment and manufacturing for the building and sphere work.
Construction of the breeder removal building was carried out for UKAEA by Alstec with local contractors: G & A Barnie, D Gow & Son, Nicolson Engineering Services, Petrie, GMR Henderson, KDS, County Joiners, JCL and Mowat Technical & Design Services.
Construction of the building began in October 2005, while ventilation and electrical work is continuing.
Completion of all work is expected in March 2008.
Following completion, there will be a period of inactive commissioning with active commissioning expected to start in 2009/2010, followed by a three-year operational period.
Another of the areas requiring special treatment is the site’s fuel cycle area, where half a century of research took place. Over a quarter of the fuel cycle area’s laboratory complex has now been decommissioned. This includes Lab 75, the largest of the heavily shielded labs to be dismantled so far.
It was used to prepare samples of irradiated steel for examination to determine the effects of radiation on various steel alloys. The results were used to improve the operating life of reactor components and increase the performance of fuel pins.
The main stainless steel alloys investigated were EN58B, M316 and FV548. The variations in their chemical compositions resulted in different reactions to neutron-induced voidage, pressure and compatibility with fuels. Using the information from tests on these alloys, reactor components were manufactured from the most suitable steel depending upon their life cycle in the reactor.
The lab equipment was inactively commissioned at an engineering site at Warrington, near the Risley site in England, around 1985. It was then dismantled, shipped to Dounreay, reassembled and installed in Lab 75, and actively commissioned. It was fully operational by mid 1986.
The laboratory housed eight interconnected gloveboxes, heavily shielded by concrete, lead and stainless steel. Workers had to dismantle the shielding before size reduction of the gloveboxes took place. One aspect of the design that made it easier to dismantle was that the shielding was mechanically fixed together and had it's own lifting points.
The suite of cells took 12 months to decommission, which included all post-operational clearing out work. The shielding removal and size reduction took up half of this time. Some of the steel plates were 55mm thick and took 15 minutes to cut through using an evolution circular saw with a tungsten-carbide-tipped blade.
The result was 62t of exempt waste, 4.5t of low-level waste and 840kg of intermediate-level waste.
The £300,000 contract was carried out by UKAEA supervisors, and operators from contractors NDSL and Doosan Babcock.
Corrina Thomson is deputy editor of Nuclear Engineering InternationalRelated ArticlesDown the drain HEU not ‘lost’ but found £7.4 million ventilation overhaul Dounreay privatised Babcock buys UKAEA Ltd for GBP50 million Shred it up and start again UKAEA plans to sell off decommissioning arm UKAEA Ltd