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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.
Te 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.
Innovative approach 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. Tis approach is being applied to multiple aspects of the project, from shredders, remote vehicles, remotely
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operated equipment, and cranes, to waste treatment, waste containers, and assay and monitoring systems.
Major challenges 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. Te 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. Te 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. Tis 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. Te 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
Fig. 2. An overview of the silo at Dounreay.
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