Nuvia Limited was contracted to design, build and operate a waste treatment plant to stabilise a quantity of about 300m3 of active sludge stored in the External Active Storage Tanks (EAST) at the former United Kingdom Atomic Energy Authority (UKAEA) research site at Winfrith, UK. At the end of this process both the old and new plants are to be decommissioned and demolished with the minimisation of waste material volumes. The sludge was produced in the Steam Generating Heavy Water Reactor (SGHWR), which is now in the early stages of decommissioning. As part of the reorganisation of UKAEA, responsibility for the site now lies with RSRL (Research Sites Restoration Limited) with funding provided by the Nuclear Decommissioning Authority (NDA). The process of stabilization of the SGHWR sludge from the EAST tanks within 500 litre stainless steel drums in the newly constructed Winfrith EAST Treatment Plant (WETP) was completed in March 2010. The plant will now enter a full decommissioning phase since no further work on any other waste materials has been identified by the client. This paper describes the development of a decommissioning plan for the mixture of old and new structures such that this next process can be undertaken without delay. This involved the completion of a decommissioning safety case together with supporting documents to demonstrate best practice and close attention to ALARP issues. The potential techniques for decommissioning are discussed leading to the final processes selected for this project. The challenges to be overcome, particularly with the older plant items where local contamination of their structures is suspected, are also identified and addressed. A process of recovery of concrete core samples for radiochemical analysis and their outcomes will be included. In contrast, the means by which the newer WETP facility was shown to be capable of unrestricted demolition as a result of the care taken during the design and construction phase will also be explained in the context of the practicalities, minimisation of the costs and timescale of such operations. Finally, issues concerning the minimisation of waste volumes, particularly where tritium contamination is present, are addressed within a UK-based disposal environment. The link between these outcomes and the planning and execution of the various structure surveillance tests will also be described and potential waste volumes identified. These considerations may be of interest to other groups undertaking similar projects on mixtures of older and more recent plants in varying states of contamination.

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