25 March 2008
The natural thing to think of when demolishing nuclear facilities is radiation and the risk of contamination. While this is the first, most obvious hurdle at Sellafield, UK, the challenge of demolishing old buildings with many structural ‘unknowns' is also problematic.
Sellafield is the place most people in the UK would think of if asked about nuclear power. It was home to the world's first fully operational commercial nuclear power station – Calder Hall, opened in 1956. The site also houses the first prototype Advanced Gas cooled Reactor in the world, the Magnox and Thorp nuclear fuel reprocessing plants, the Sellafield Mixed Oxide Fuel manufacturing plant and a host of ancillary waste management and treatment facilities. All of these facilities are crowded together on a compact footprint, often only yards apart.
Decommissioning work began on the site during the late 1980s, but with a slow start. Today, this work is accelerating and some of the major installations are now coming up for removal.
In total, clearing the site will entail the demolition of 170 major nuclear facilities and 2200 other buildings dating from 1940 to 2006; 1 million m3of concrete above ground and the same amount below ground; 37 km of roads, 15 km of railway, 400 km of surface works, 120 km of sewers and 65 km of water piping; 7 km of pipe bridges, 16 km of ducts and trenches.
While this is going on, parts of the Sellafield complex will remain in operation until 2012 or beyond, and the final completion of all this work is a distant prospect. Today's estimates suggest the work will be underway for the next 70 years at a massive cost. Currently, of the order of UK£ 500 million (€ 750 million) is being spent per year on decommissioning and demolition work, so a simple extrapolation suggests that the total cost will be in the order of UK£ 30 billion (€ 45 billion) – at today's prices, of course!
Currently, the site is owned by a UK government body called The Nuclear Decommissioning Authority, on whose behalf a private sector company, British Nuclear Group Sellafield Ltd, manages and operates on-site activities.
To coordinate demolition activities, a dedicated Demolition Group was formed in 2005, charged with delivering “smarter, cheaper accelerated decommissioning with safety and environmental performance being paramount.”
Work in Progress
The current focus for the group is the demolition of Sellafield's Uranium Purification Plant (UPP) – the first major nuclear facility, and the first legacy building, on site to be brought down. Built in 1953, it was a major component in Britain's nuclear weapons programme and comprises a heavy steel portal frame with asbestos cement cladding. Internally, there is a brick and concrete structure, divided into four cells, that housed the process plant, and which may (the ‘unknown' factor) have formed part of the roof support structure.
Decommissioning the UPP, involving the removal of process vessels and pipe work from inside and outside the cells, began in 1992 and was completed in 2005. This cleared the way for demolition of the structure to get underway, but this is not without its difficulties.
Pipe bridges run around three sides of the building and a major pedestrian route runs along the fourth side. In addition, a rail line used for transferring nuclear fuel to the Magnox reprocessing facility runs along one side of the building. Finally, with other buildings in close proximity with sensitive processes ongoing, the transfer of vibrations from demolition activities must be minimal and possible site accidents involving the high collapses onto these buildings was a major concern.
All in all, quite a problem for the contractor, Euro Dismantling Services (EDS), which was appointed to carry out the job in December 2005. In addition to this, the company was new to working at Sellafield and had to come to terms with stringent safety requirements. The nature of the work and methods had to be compiled into a Safety Case, which had to be approved before demolition could start. This meant it was August 2006 before work was scheduled to get underway, and a further six week delay followed while concerns raised by the Safety Case were addressed.
Being new to nuclear demolition, EDS also had to get used to the site's controlled area barrier safety procedure. This meant all workers have to wear full protective clothing and undergo a contamination examination every time personnel exit the area.
When CE visited the site last month, more than half the building had been demolished safely and without incident, with a Liebherr long reach excavator working bay to bay and floor to floor within each bay. Steel beams are cut and lowered to the ground by the high reach, during which time no other demolition activities take place.
Once the Liebherr has ceased operations the steel beams are cut into 1,8 m (6 ft) maximum lengths on the ground to prepare them for processing. BNG Sellafield Ltd has found that the many coats of paint applied over the years have absorbed a degree of low level radiation contamination but once the paint is removed the steel can be released for re-use.
This is done by feeding the steel through an onsite wheel abrator several times to strip the paint. It is expected to take more than 100 weeks, at 8 tonnes per week, to process all the steel from the site – there is obviously a storage issue for the steel while it awaits this process.
The brick and concrete cells suffer from contamination, and as a result will have to be separated into seven individual areas during demolition. At this stage of the work, BNG Sellafield Ltd is unable to quantify the volume of demolition debris that will finally result – another of those historical unknowns – but is confident that the vast majority will be recyclable.
There is still much to do at Sellafield, and currently the Demolition Group is looking at the next legacy building to be dealt with. This will be the removal of a 61 m reinforced concrete chimney stack, which sits on top of the 61 m, 11 storey tall Primary Separation Plant (PSP).
It is the next in a series of challenges on the site which will doubtless continue to crop up for decades to come.