Completing one of the world’s deepest nuclear clean-up projects will involve considerable challenges and some innovative approaches to accelerate the programme while minimising cost. By Bo Wier
A vertical shaft, which was excavated in the 1950s for the removal of rock spoil during construction of an undersea tunnel for the Dounreay site’s effluent discharge pipes, and later authorised as the UK’s first Intermediate Level Waste (ILW) disposal facility, now represents one of the biggest challenges in the UK’s nuclear decommissioning portfolio.
A major project to decommission the shaft (and the wet silo also used for ILW storage), including recovery and packaging of over 1500 tonnes of radioactive waste, is one that is now the focus of renewed emphasis for the Dounreay team. Babcock Dounreay Partnership, which is responsible for the decommissioning, demolition and clean-up of the Dounreay nuclear site, has committed to accelerate the Dounreay decommissioning programme (by up to 16 years over previous estimates of two years ago) and reduce project costs by well in excess of a billion pounds. The shaft and silo project is one of the key projects being accelerated within this programme.
In 1958 the shaft — which is largely unlined and cylindrical in shape, measuring up to 4.6m across and 65.4m deep — was plugged at the bottom, allowed to fill with groundwater, and authorised by the government for disposal of ILW. More than 11,000 waste disposals (predominantly consigned in 100 litre cans) took place until 1977.
By the mid-1960s, the shaft was becoming full and a decision was taken to construct a new, purpose-built facility for intermediate level solid waste. This facility, the silo, was brought into use in 1971, with the first recorded transfer occurring on 9 August.
The wet silo is a concrete-lined and -roofed box built just beneath the surface. Comprising two interconnected compartments, the silo is more than 9m deep, 8.5m wide and 10m long, with an approximate storage capacity of 720m3. Each compartment has a concrete roof more than 1m thick. Four removable shield plugs (two for each compartment) were removed to allow waste disposals into the compartments.
Waste disposal in the shaft ceased following a chemical explosion in 1977, thought to have resulted from an accumulation of hydrogen. By this time most waste, except for large items, was being consigned to the wet silo, which then succeeded the shaft and was used until 1998 as a storage facility for the site’s ILW.
A decision was taken in the 1990s, as a result of advances in technology, to empty the shaft and silo, and a major programme is now underway to decommission them both.
The first step was to isolate the shaft from the surrounding groundwater by grouting the rock fissures around the shaft, using technology widely used in the construction of road tunnels. This grout curtain was completed in 2008.
The next stage is the challenging removal, treatment and storage of the waste, estimated to total approximately 1220m3 of mixed solid and liquid ILW (including sludge from pond clean-up and solid waste decomposition) that covers a broad chemical and radiological spectrum. Waste is to be recovered from both the shaft and silo and processed for long-term interim storage within shielded containers on site.
With concept design finalised, and detailed design completed in 2013, construction work is scheduled to begin in the second half of 2013 and take up to three years to complete. Waste retrieval is expected to take 18 months at the silo, and 30 months at the shaft, before both facilities e
|History of the shaft/silo decommissioning project|
|1991||Early retrieval studies|
|1995-96||Shaft option studies|
|1998||Decision to empty shaft and silo|
|2000||Formation of projects|
|2001-2002||Shaft and Silo retrieval option studies|
|2003||Shaft and Silo option recommendations|
|2003-05||Waste treatment option studies|
|2005-06||Retrieval & waste treatment design development|
|2006||Active drain diversions and land remediation completed at shaft|
|2006||Buildings demolished in new plant footprint|
|2007||Crane structure above silo removed|
|2007||Site boundary extended and shaft working platform constructed|
|2008||Shaft isolation work completed|
|2009||Active drain diversions|
|2007-9||Trials and development of equipment/methods|
|2010-11||Site competition; project put on hold|
|2012||Babcock Dounreay Partnership wins Dounreay closure contract|
Concept designs have been developed for waste retrieval, treatment and storage facilities, evaluating the techniques that can be utilised and the equipment needed. Two independent retrieval facilities for the shaft and silo, rather than one, have been developed. Each has its own waste-processing capability to minimise the potential for processing pinch points.
Novel features include the use of limited-life construction in preference to heavily-engineered long-term structures, which are less costly to build and to decommission, and the use of modularised plant and equipment systems for ease of commissioning. Modular systems allow critical items of plant to be more easily and quickly substituted in the event of a failure, enabling work to continue. Additionally, the use of self-shielded waste containers for on-site storage helps to optimise cost efficiency by avoiding the need for high capital investment in the construction of shielded waste stores with remote handling.
Importantly, a key feature of the team’s approach is the use of proven, commercially available off-the-shelf (COTS) equipment wherever possible, rather than designing and developing bespoke systems (at, inevitably, higher cost and greater risk). The team will also aim to adopt methods and techniques used in the nuclear and other sectors such as water treatment and mining industries. This approach is being applied to multiple aspects of the project, from shredders, remote vehicles, remotely-operated equipment, and cranes, to waste treatment, waste containers, and assay and monitoring systems.
Work on the shaft will commence with the construction of the waste recovery headworks, with a combination of industrial grabs and robotic mechanisms that can be lowered into the shaft to recover the waste. The challenges faced in undertaking this project are considerable. For example, high radiation levels (up to ~1Sv/hr) prevent personnel access for routine and remediation operations, and if retrieving objects 65m down a vertical shaft were not difficult enough, there is also a need to deploy equipment almost 20m into the side tunnel.
The technical issues needing to be addressed are significant. The plan calls for the water to be kept at the same level as the waste being removed. To do that, water will need to be pumped out of the shaft as waste is removed. As the retrieval depth increases and the water level within the shaft is progressively lowered, the differential pressure between the water table and shaft water level will increase, which will cause an increase in inflow into the shaft, in turn placing a greater burden on the downstream liquid effluent treatment plant.
Furthermore, managing the retrieval of the waste matrix using a grab so it does not snag on the waste is difficult to achieve remotely. In addition, there are challenges associated with the supply of hydraulic and electrical power which grow as the retrieval point gets deeper, and the hose and cable management deployment mechanisms have to increase in size and strength. The plan is to use electro-hydraulic grabs in which the hydraulic power pack built into the grab so only an electrical connection is required. The cables will be controlled with a cable reeler.
Equally, the deployment of lighting, camera systems and monitoring equipment also becomes more complex as depth increases. Cameras and lights will be positioned above the grab as well as on an auxiliary hoist for full coverage. All equipment used also has to be radiation-tolerant.
Deployment of a remotely-operated vehicle (ROV) from a platform into and along the full length the side tunnel, along with all the ancillary power and control systems required, and the retrieval of waste back to the shaft, will inevitably be complex and time-consuming. Additionally, the recovery of sludge and free liquid from a depth of up to 65m is a further challenge, requiring specialist suction or jetting systems to overcome the height over which the active waste will have to be transported.
The physical location of the facilities adjacent to the coast brings its own additional challenges. A reinforced concrete working platform has been built to provide protection from the encroachment of the sea, as well as a secure base on which to mount the retrieval structure, plant and processing equipment. The selection and design of the overbuilding will take into consideration the location and harsh weather conditions that prevail at the Dounreay site.
In line with programme policy, a limited-life structure will be erected, incorporating shielded areas for waste retrieval, waste processing and packaging, waste characterisation, and sludge conditioning. This will include an industrial crane, a modularised ventilation extraction system with HEPA (High Efficiency Particulate Air), and modularised processing plant and equipment (see figure, p48).
Wherever possible, COTS equipment previously successfully deployed within a nuclear decommissioning process or harsh industrial environment will be specified. The high ambient dose conditions will require these systems to be remotely controlled and specified to withstand prolonged exposure within high radiation fields. Plant and equipment will be modularised, enabling the systems to be manufactured and tested off-site, thereby minimising on-site installation and commissioning timescales and, later, decommissioning time.
Both petal and clam shell grabs will be deployed to retrieve waste from the full depth of the shaft, with the crane (20t capacity) having X-Y traverse capability so that complete coverage of the 4.6m-diameter cross-section of the shaft is ensured, although it is anticipated that the waste will slump down into the shaft as waste is recovered.
A deployment platform will be used to transport an ROV down the shaft. The platform will have full manipulator access to all parts of the shaft, with the ability to remove any obstructions on the side walls. The platform will also have the capability of ‘driving’ into the stub tunnel to recover waste back into the shaft, although the platform shown below would be re-configured for this task. The exact detail of the platform/manipulator capability is still being evaluated.
Waste retrieved from the shaft and brought to the surface will be segregated, characterised, and processed for interim storage. The characterisation and processing of the waste is in itself a further key challenge facing the project, with a number of issues to be addressed before consignment for storage.
The waste will be sorted using ROVs with powered manipulators and an assortment of end effectors for sizing and handling waste items. It will be segregated into solid and effluent waste streams. A shredder and size-reducing tooling will be used to shred and screen the debris and cut large items. Waste characterisation at this stage will include gamma and fissile monitoring systems to ensure that all waste is processed in accordance with the UK Letter of Compliance (LoC) regulatory process.
All wastes retrieved will be conditioned and encapsulated or immobilised within self-shielded containers using a cementitious matrix. The self-shielded containers used to package the waste will depend on the waste type. They will be either lead-lined stainless steel drums (TRU-Shield containers) or a pre-cast concrete box (WAGR box). The preference is to use the WAGR box, although if the waste activity is higher than the box permits then the other container will be used. The containers will be transported for intermediate storage on-site in compliance with regulatory requirements.
Recovery and processing of waste from the silo will follow a similar procedure, with variations to meet the silo-specific requirements. A grab will be used to retrieve the bulk solid waste, with a pump for the sludge wastes. Among the differences in the waste recovery procedure from the silo, for example, are the fact that there will be no requirement to deploy a platform-mounted ROV into the silo. On the other hand, the roof slab will need to be removed to improve access to retrieve the waste. Waste from the silo, which is similar to that in the shaft and predominately in 100 litre drums, will also be characterised and encapsulated.
Bo Wier is Babcock Dounreay Partnership’s shaft & silo project director.