Experience with Stabilization of SGHWR Sludge in a Commercial Plant in the United Kingdom - 9044

In July 2000, following a competitive tender, Nuvia Limited was contracted to design, build and commission a waste treatment plant to stabilise the active sludge stored in the External Active Sludge Tanks (EAST) at Winfrith, UK. The sludge was generated during the operational lifetime of the Steam Generating Heavy Water Reactor (SGHWR), which was in the early stages of decommissioning. This was in support of UKAEA's mission, which is to carry out environmental restoration of its nuclear sites and to put them to alternative uses wherever possible. Latterly, a new body, the Nuclear Decommissioning Authority (NDA), has become responsible for managing the UK decommissioning legacy and since 2004 UKAEA has been contracted to the NDA to deliver decommissioning work at Winfrith and other UK sites. The purpose of this commercial plant is to stabilise the radioactive sludge by encapsulation into a cement matrix within a purpose-designed 500 litre steel drum. The drum design incorporates a lost paddle mixer used to maintain homogeneity of the sludge as well as mixing it with the stabilising powders. The sludge in the EAST tanks is prepared for recovery by a process of homogenisation using in-tank stirrers. The means of reaching a narrow ratio of suspended solids within an aqueous medium will be described together with some of the problems encountered and the practical solutions devised. The material is transferred to the purpose-built Winfrith EAST Treatment Plant (WETP), where it is held in stainless steel tanks in a process area prior to being metered into a 500 litre stainless steel drum in the cell line for stabilization with powders. The cell line consists of five cells separated by shield doors designed to maintain strict contamination control. The line has a wet cell where the drums are filled with the sludge and powder, a cell with stations for curing and grouting the drums, a cell for lidding, bolting and QA inspection, a maintenance and gamma monitoring cell and a buffer store to hold the completed drums. After completion, drums are moved in a shielded overpack to the Treated Radwaste Store located on a different part of the Winfrith site. More than 300m 3 of active sludge, held in four adjacent concrete tanks, has now been stabilised into 978 drums that have been placed into the dedicated store. The process of recovery and homogenization of the residual sludge in the bottom of each tank to the required specification will be described together with the means of recovery and disposal options for a quantity of unexpected materials found at the bottom of each unit. The means of dealing with the final quantities of sludge and water from the last two concrete tanks by recovering it into a tall steel filtration vessel located within one concrete tank will also be described. The final aspects of the paper will briefly describe the approach to be adopted for the final decontamination and demolition of both the EAST facility and WETP plant. The WETP plant is now in the latter stages of commercial operation leading to a second programme of stabilisation of a quantity of thorium metal for UKAEA ahead of its final decommissioning and demolition.