Nuclear Power
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. Te 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
Fig. 3. Waste disposal in the shaft (measuring up to 4.6m across and 65.4m deep) ceased following a chemical explosion in 1977.
challenge, requiring specialist suction or jetting systems to overcome the height over which the active waste will have to be transported.
Te 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. Te selection and design of the overbuilding will take into consideration the location and harsh weather conditions that prevail at the Dounreay site.
Retrieval methodology 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. Tis 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. Te 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
segregated, characterised, and processed for interim storage. Te 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. Te 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
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