Sellafield Legacy Waste processing plant

A new plant being constructed at Sellafield, known as the Silos Direct Encapsulation Plant (SDP), has seen significant progress towards its goal to treat legacy radioactive waste.

SDP is being built to process waste retrieved from the Magnox Swarf Storage Silos (MSSS) which is one of the high hazard decommissioning projects on the Sellafield Site dating back to the 1960s. Intermediate Level Waste mainly comprising Magnox fuel cladding was stored under water in the twenty two individual compartments within the facility.

Recently, the SDP project has manufactured, installed and set to work an important piece of development equipment, known as the Universal Mixing Vessel Rig Mark 2. Constructed in a new dedicated bay within NSG’s facility at Chorley, the work has been directed and managed by Sellafield Ltd’s main SDP engineering contractor, Nuvia.

This huge rig will have to handle many different waste forms as each skip of waste retrieved from the MSSS will be unique. The project has therefore developed a range of about 80 different simulants to cover the likely radioactive wastes to be grouted. These essentially include sludges across a range of shear strengths, varying sizes of Magnox swarf and numerous Miscellaneous Beta Gamma Waste streams and finally uranium.

In order to achieve this, a programme of development trials have been carried out using different Mixing Vessels. Additionally, extensive Tipping Trials have demonstrated that a range of simulants can be tipped from the MSSS skip into the mixing vessel and that their behaviour is fully understood.

David Welsby, SDP Project Manager, said: “We need to demonstrate that we can produce an acceptable product form across a range of very different simulants. This involves varying several parameters (mixing angle, vessel diameter, mixing speed, grout formulation, water content, mixing time etc) to firstly make an acceptable product and then optimise this process.”

The original SDP plant was constructed between 1995 and 2001. However, work on SDP was halted when new information on the waste streams to be processed meant that a decision had to be taken to put the project on hold.

In 2004 work was carried out on the new process options for the plant and in 2005 significant elements of the redundant plant and equipment were stripped out, in preparation for the new plant.

SDP construction is programmed to be completed in 2018, however accelerated programmes are looking to bring this forward. Operation of the SDP will allow historic radioactive wastes to be processed into a form suitable to be safely stored in the long term and placed in a Geological Disposal Facility underground if the decision to build such a facility is taken.

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