Year
2012
Abstract
As a result of historic processing activities, a range of plutonium containing residues are stored on the Sellafield site. Sellafield Ltd, responsible for the management of these residues charged the UK’s National Nuclear Laboratory (NNL) with the task of demonstrating a technology capable of immobilising these materials in such a form that would be suitable for storage and ultimate disposal. In collaboration with the Australian Nuclear Science and Technology Organisation (ANSTO), a hot isostatic pressing route (HIPing) capable of processing and immobilising the residues in a ceramic, glass ceramic and metal based waste forms has been developed and demonstrated. The process uses a hot-isostatic press (HIP) unit to simultaneously generate the high pressures (100 MPa) and high temperatures (circa 1200-1300 o C) necessary to consolidate the monolith. A glass ceramic waste form was identified as being suitable for the majority of the wastes, with the zirconolite phase designed to accommodate plutonium and other actinides with the balance of the largely inactive species being accommodated in the glassy matrix. The glass ceramic waste form was based on a boro-aluminosilicate glass matrix with additions of CaO, ZrO2 and TiO2 to promote formation of crystalline zirconolite (CaZrTi2O7) as an actinide host phase. A metallic waste form was proposed for the immobilisation of a minority of materials that would be difficult to accommodate in a glass ceramic waste form. This waste form encapsulates the waste components into a copper matrix consolidated by warm isostatic pressing of copper powder at 300 °C. Waste form development has been carried out and validated with plutonium and the process has been demonstrated inactively at full scale. A project has now been initiated for the design and construction of a facility to be housed in NNL’s Central Laboratory on the Sellafield site. This facility will demonstrate the process on actual plutonium containing residues to act as a technology demonstrator for the possible disposition of bulk plutonium should that become the UK government policy. The paper will describe the key recent development activities that have been undertaken in producing the flowsheet, how the flowsheet has evolved to overcome the diversity of the feeds and the work to maximise actinide incorporation levels.