25 March 2008
When you first think of the demolition of nuclear installations, automatically the topic of radiation contamination comes to mind as the main challenge that has to be overcome. At Sellafield, however, this is the first obvious hurdle, yes, but it is not necessarily the greatest.
The Sellafield complex of nuclear installations traces the history of the UK's involvement in this field of science, boasting as it does structures used to develop the country's independent nuclear deterrent at the end of World War II and the first fully operational commercial nuclear power station in the world (Calder Hall, opened in 1956). In addition, the site 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 crowd 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.
To give an idea of the scale of work required on what is considered to be the single most challenging nuclear site management programme in the world, some basics would be useful.
The scale of work involves 170 major nuclear facilities and 2,200 other buildings dating from 1940 to 2006; 1 million m3 of 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.
To top all this, there will be ongoing commercial operations on the site until at least 2012. In addition, no decision has been reached as to what degree the site should be reinstated to once all decommissioning/demolition work is completed and what use it will eventually be put to.
I say ‘eventually’ because the final completion date is ‘out there somewhere'–today's estimates suggest that work will be underway for the next 70 years. Currently, of the order of £500 million (US$965) is being spent annually on decommissioning/demolition work, so a simple extrapolation suggests that the total cost will be in the order of £30 billion (US$58 billion)–at today's prices, of course!
Currently, the site is owned by The Nuclear Decommissioning Authority, on whose behalf British Nuclear Group Sellafield Ltd manages and operates on–site activities, which centre on remediation, decommissioning, accelerated hazard reduction and clean–up of the historic ‘legacy'. The term legacy is used for those buildings and structures on the site that date back to the early days of Britain's nuclear activities, such as the Uranium Processing Plant (see below).
Planning for demolition
To coordinate demolition activities, a dedicated Demolition Group was formed in 2005, as part of the Nuclear Decommissioning and Major Projects Group. The main Group has been tasked with delivering “smarter, cheaper accelerated decommissioning with safety and environmental performance being paramount”. Its work will involve diverse and highly constrained decommissioning environments, which require meticulous planning and efficient management with the numerous interested parties on site.
Where many ‘legacy’ structures are concerned, the single biggest challenge to be faced is simply that of the unknown–existing structures not matching available plans, plans not available, etc. It is fair to say that the buildings concerned were built with no thought being given to their removal, creating some interesting challenges for the Demolition Group.
Within that, the Demolition Group has its own unique set of challenges to overcome. The first is to safely demolish structures using conventional demolition techniques on a nonconventional site. Minimal disruption to other activities on site is to be achieved, since many of the buildings are located in heavily congested areas and are surrounded by other facilities, sensitive sites and services. The Group also has to ensure that the best practicable option is used to maximise opportunities for reducing and recycling waste materials. It came as a bit of a surprise when Dyan Foss, Head of Programme Delivery End State, told D&Ri that: “We expect in most cases to achieve approaching 80% recycling rates”. While certainly not unusual for a ‘normal’ demolition job, this was not what I had imagined would be the case for such a ‘sensitive’ location.
The final challenge facing the Demolition Group is that of convincing the regulators that the methods proposed meet their stringent requirements to allow work to go ahead–something that can frequently take much more time that actually carrying out the work. For example, it apparently took over six months to prepare the plans and gain the necessary approvals to carry out a three day demolition job!
To date, the Demolition Group has demolished 35 structures on site, freeing up more than 7,000 m3 of space for use–often new construction is already underway on these spaces for buildings that will be used for further decommissioning activities and that will then be demolished some time in the future when their work is completed. A good example of this is the new sludge packing plant, required to speed up decommissioning, that is going up on the site of the demolished Reprocessing Miscellaneous Compound Area/Drum Store.
These structures were a mix of permanent concrete/steel buildings, temporary portacabins and storage tanks.
So far, this demolition work has resulted in the production of 3,000 tonnes of concrete that has been certified as clean and transferred out of the restricted access Separation Area for reuse, along with 300 tonnes of steel.
Work in progress
Currently the Group's major activity in the Separation Area is the demolition of the Uranium Purification Plant (UPP)–the first major nuclear facility, and the first legacy building, on site to and was a major component in Britain's nuclear weapons programme. The building consists of 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 (that ‘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. It is at this point that the shear scale of the predemolition preparation work required for such installations is driven home, with the process of selecting the most appropriate method actually beginning almost 10 years ago, in 1997, while decommissioning was still underway.
A number of options were considered, ranging from fully scaffolding the building and carrying out a piece by piece strip down, through temporary steel supports and lifting down major sections by crane, to the use of a high reach demolition excavator and aerial work platforms. The latter method was the one finally identified as being the best for meeting the main project driver–reducing the potential hazards to the workforce by minimising the number of hands–on operatives.
However, the job still entails the overcoming a number of major obstacles. Pipe bridges run around three sides of the building, in close proximity, and a major pedestrian route runs along the fourth side that is used by at least 200 people three times a day. 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 had to be minimal and possible site accidents involving the high reach 82 d&ri and aerial work platforms collapsing onto these buildings was a major concern.
All in all, quite a problem for the contractor, Euro Dismantling Services, who was appointed to carry out the job in December 2005–but EDS's problems did not stop with dealing with the above. EDS was new to working with Sellafield and immediately found it was going to have challenges all of its own, mainly to do with the way work had to be conducted in such a sensitive facility.
BNG Sellafield Ltd had been developing a Safety Case, detailing the nature and methods of the work, which had to be approved before work could start. Once the contractor was appointed, this could be completed, since the precise method of demolition could now be determined. However, this meant that work actually was not scheduled to start until August 2006.
By the due date, EDS had brought in the equipment specified for the job–a Liebherr 954 Litronic with a 25 m (82 ft) boom, a Komatsu PC300, a JCB 812 NLC and a couple of aerial platforms. Getting this equipment to the site was no mean feat in itself, since the Sellafield site had originally been selected because of its physical isolation away from main population centres–and hence away from main transport routes.
On the day work was due to start and the EDS personnel turned up on site, a plant adjacent to the demolition site raised an issue regarding the Safety Case, which halted work for six weeks while their valid concerns were addressed. Thus the equipment had to be transported off site and then brought back when work could actually start.
But EDS's problems did not stop there. Being new to nuclear demolition, all the EDS personnel were unused to the controlled area barrier safety procedure, which necessitates the wearing of full protective clothing and undergoing a contamination examination every time personnel exit the area.
But it was also the working methods and safety environment that EDS was new to. In conjunction with the Safety Case, an extremely detailed and inflexible Method Statement detailing how the demolition was to be conducted had been prepared that had to be adhered to at every step of the work. Operatives were also new to the Safety Case terminology and the way that authorisation of work activities had to be obtained each day through the Works Control Authorisation System. Before work could start, a Command and Control Summary Document had to be completed because of the need to confirm a number of conditions and prerequisites from the surrounding plants–it could, on occasion, take up to three hours to gain all the required confirmations.
Another complication was the six week delay. This meant the demolition work was now to be carried out at the same time as a pipe replacement programme on the pipe bridges on three sides of the building that took high level scaffolding into the collapse radius of the high reach and the access platforms in the event of an accident. This required extremely good liaison between the two projects.
When D&Ri visited the site, more than half the building had been demolished safely and without incident, with the Liebherr 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 and stored on the site on palletised loads. 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 on–site wheel abrator multiple times, hence the need for palletised loads for easier handling. It is expected to take more than 100 weeks, at 8 tons per week, to process all the steel from the site–there is obviously a storage issue for the palletised 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.
Despite work now progressing smoothly and efficiently, with EDS having adapted to the unusual working conditions very rapidly, BNG Sellafield Ltd has learned a number of lessons that it will apply to future contracts.
When inviting bids for contracts, the ‘peculiarities’ involved with working in the Sellafield Separation Area will be written into the contract, allowing bids to be submitted from contractors who will be fully aware of the working conditions–ensuring a level playing field.
It now hopes to get contractors involved in the planning phase much earlier, since they have the expertise and may be able to suggest others ways of dealing with specific problems. This will also prepare them for the peculiarities of working on the site–safety and security requirements, reductions in daily working hours, etc.
More importantly from a contractor's perspective, Method Statements will be made more flexible and user friendly to allow them to work more like they do on normal contracts–of course, safety will remain paramount, both where structures and the workforce are concerned.
In addition, internal liaison between the various parties that will be affected by the demolition activities will be improved to ensure that everyone is fully aware of what is going to be done, when and how, to guarantee that there are no delays similar to the one experienced with the Uranium Processing site.
There is still much to do at Sellafield, and currently the Demolition Group is looking at the next legacy building to be dealt with. And boy, do they have a major challenge.
On the face of it, the job sounds simple enough–the removal of a 61 m (200 ft) reinforced concrete chimney stack. But (and it is a big but) the stack sits on top of the 61 m, 11 storey tall Primary Separation Plant (PSP) for the first uranium metal fuel reprocessing facility. Built in the late 1940s, it was commissioned in 1952 and operated until 1964, at which time it was superseded by the Magnox fuel reprocessing facility.
Actually the tallest structure on the Sellafield site, removal of the stack will pose a number of problems. The roof of the PSP is light structural steel, with the stack sitting on a central core that contains a number of cells constructed of thick reinforced concrete, surrounded by the structural steel operating annulus and cladding. As a result, roof loadings during the demolition work may require additional reinforcement to be put in place.
Not only that, but the building itself is an operational building that is also undergoing decommissioning work in a number of areas that will be ongoing while the chimney is removed. This of course means that great care will have to be taken to ensure that no demolition debris can fall onto the roof.
Given the height of the structure, bad weather could have a major impact on work. Indeed, when D&Ri visited the site, a storm was blowing in from the Irish Sea. As a result, access was really only possible to one side of the roof and even there it was a case of “hang on to your hard hat”.
Access to the chimney from the ground is also another major issue, as the separation plant is in close proximity to numerous buildings, including other nuclear facilities and sensitive plant and services in heavily congested areas. But the problems do not stop there. Inside the concrete stack is a steel flue that was installed in sections that were then bolted together in situ, and this will need to be removed before demolition of the reinforced concrete can even start.
BNG Sellafield is currently examining a number of potential options for this challenging demolition job and would welcome input from contractors who believe they may have a feasible solution.