Bo Wier looks at the challenges involved to complete one of the world's deepest nuclear clean-ups, and at some of the innovative approaches being deployed to accelerate the programme whilst minimising cost.
A vertical shaft, 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 authorised in 1958 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.
This major project, to decommission the shaft (and wet silo also used for ILW storage) including recovery and packaging of over 1500 tonnes of radioactive waste, is now the focus of 'renewed emphasis' for the new Babcock Dounreay Partnership management team at Dounreay. The team, which is responsible for the decommissioning, demolition and clean-up of the Dounreay nuclear site (the first major closure project in the UK), having taken over in April this year, is taking the decommissioning programme at Dounreay to its interim end state, and has committed to accelerate the 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.[Page Break]
Waste disposal in the shaft (measuring up to 4.6m across and 65.4m deep) ceased following a chemical explosion in 1977, thought to have resulted from an accumulation of hydrogen. By this time most of the waste was being consigned to a nearby wet silo (a concrete lined and roofed box built just beneath the surface with an approximate storage capacity of 720m3), which was used until 1998 as a storage facility for the site's ILW.
ILW consigned to the shaft and silo now comprises items of solid waste plus sludge from the pond clean-up and decomposition of some of the solid waste, and covers a broad chemical and radiological spectrum. 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.
The first step (completed in 2008) was to isolate the shaft to reduce the ingress of ground water, minimising potential contamination. Up to 400 boreholes were drilled around the shaft in a boot shape (to a depth of 80m around the vertical shaft and nearly 20m long side tunnel - the former liquid effluent discharge tunnel). A fine grout was injected at high pressure into the boreholes, to form a barrier and create a giant containment around the shaft and side tunnel, considerably reducing the amount of water getting into the shaft.
The next stage is the highly challenging removal, treatment and storage of the waste, estimated to total approximately 1220m3 of mixed solid and liquid ILW. Waste is to be recovered from both the shaft and silo and processed for long-term interim storage within shielded containers on site.[Page Break]
Concept designs are being developed for waste retrieval, treatment and storage facilities, evaluating the techniques that can be utilised and the equipment needed, and a number of innovative approaches have been introduced by the new management team at Dounreay, helping to accelerate the project while minimising cost.
Two independent retrieval facilities are to be developed for the shaft and silo, each having 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, being less costly to build and decommission, and the use of modularised plant and equipment systems, for ease of commissioning and enabling critical items of plant to be more easily substituted in the event of a failure, allowing work to continue. Additionally, the use of self-shielded waste containers for on-site storage also helps to optimise cost-efficiency by avoiding the need for high capital investment to construct shielded waste stores with remote handling.
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), and adopting 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.[Page Break]
Work on the shaft will commence with 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 are considerable. High radiation levels preventing man access for routine and remediation operations, and the difficulties of retrieval from a vertical shaft down to a depth of 65m are two examples, as is the need to deploy equipment almost 20m into the side tunnel.
The technical issues are also significant. For example, 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, causing an increased inflow into the shaft, in turn placing a greater burden on the downstream liquid effluent treatment plant.
Further, managing the retrieval of the waste matrix and ensuring that the grab does not snag on the waste is difficult to achieve remotely. This is in addition to the challenges associated with the supply of hydraulic and electrical power, as well as lighting, camera systems and monitoring equipment, which become greater as the retrieval point gets deeper. The equipment must also be radiation tolerant to withstand the radiation fields encountered.
Deployment of a remotely operated vehicle (ROV) from a platform into and along 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 a complex and time-consuming procedure. 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 brings its own additional challenges. A reinforced concrete working platform has been built that will provide protection from the encroachment of the sea, as well as providing 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.[Page Break]
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. 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.
Petal and clam shell grabs will be deployed to retrieve waste from the full depth of the shaft, with the crane having X-Y traverse capability to ensure complete coverage of the cross section of the shaft. The crane will also be specified to ensure that it can retrieve any weight of waste likely to be encountered. An ROV will be transported down the shaft from a deployment platform, with the ability to remove any obstructions on the side walls and recover waste from the side tunnel into the shaft, for retrieval back to the headworks using the crane.
Waste retrieved from the shaft will then be segregated, characterised, and processed for interim storage. The waste will be sorted using ROVs with power manipulators and various end effectors for sizing and handling waste items, and segregated into solid and effluent waste streams, using equipment including a shredder and size-reducing tooling to shred and screen the debris and cut large items. Product trials of the processed waste will be conducted to ensure that the wasteform meets regulatory requirements. All wastes retrieved will be conditioned and encapsulated or immobilised within self-shielded containers using a cementitious matrix, and the containers transported for intermediate storage on-site in full 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 (for example, there will be no requirement to deploy a platform-mounted ROV into the silo but removal of the roof slab will be required to improve access to retrieve the waste). Waste from the silo will also be characterised and encapsulated.
The retrieval of radioactive waste and sludges from a narrow vertical shaft and from the silo represents one of the biggest projects in the Dounreay decommissioning and clean-up programme, involving numerous unique and significant challenges. Key milestones include achievement of concept design at the end of this year, completion of detail design in autumn 2013 and initial operations in mid-2015, with a view to ultimate completion of retrieval actions for this project, one of the UK's most significant nuclear decommissioning and engineering challenges, by the end of 2020.
Bo Weir is Babcock Dounreay Partnership's Shaft & Silo Project Director, Dounreay, Caithness, Scotland. www.dounreay.com