STABILISATION OF SGHWR SLUDGE AND THORIUM METAL IN A COMMERCIAL PLANT IN THE UNITED KINGDOM

Nuvia Limited is contracted to design, build, commission and operate a waste treatment plant to stabilise the active sludge stored in the External Active Storage Tanks (EAST) at the former United Kingdom Atomic Energy Authority (UKAEA) research site at Winfrith, UK. The sludge was generated during the operational lifetime of the Steam Generating Heavy Water Reactor (SGHWR), which is now in the early stages of decommissioning. This is 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. Recently UKAEA has been reorganized and responsibility for the site now lies with Research Sites Restoration Limited (RSRL) with funding provided by the Nuclear Decommissioning Authority (NDA). The process of stabilisation of the SGHWR sludge from the EAST tanks within 500 litre stainless steel drums in the Winfrith EAST Treatment Plant (WETP) is now almost complete with 1033 drums stabilised to date and only a few more anticipated. At this point it was originally planned to decommission and demolish the WETP facilities but RSRL have contracted Nuvia to undertake in the plant another stabilisation project involving 11Te of thorium metal waste. As a result the plant is in the process of being revised to provide facilities for encapsulation of the thorium metal within modified 500 litre drums together with a number of changes to the plant control system. The concrete-shielded WETP facility consists of five cells separated by shield doors and is designed to maintain strict contamination control. There is 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. The process of recovery and homogenization of the residual sludge in the bottom of each tank to the required specification has been described earlier to include use of a tall steel filtration/separation vessel within one concrete tank to hold the final batches such that they can be homogenised as required. The final stages of this process will be given particular attention in the paper. In the revised cell line configuration to deal with the thorium waste, the wet cell will be isolated and not used. The 500 litre drums are to be introduced as before but taken to the original rework cell located to one side halfway along its length. The modification to this cell to make it suitable for handling the thorium waste within a drum loading enclosure will be described together with changes to the drum to centralize the bars under revised grouting arrangements using Ordinary Portland Cement and Pulverised Fly Ash.