Implementation of a Geological Disposal Facility (GDF) in the UK by the NDA Radioactive Waste Management Directorate (RWMD): The Potential for Interaction Between the Co-Located ILW/LLW and HLW/SF Components of a GDF

Abstract

In June 2008 the UK government published a ‘White Paper’ as part of the “Managing Radioactive Waste Safety” (MRWS) programme to provide a framework for managing higher activity radioactive wastes in the long-term through geological disposal. The White Paper identifies that there are benefits to disposing all of the UK’s higher activity wastes (Low and Intermediate Level Waste (LLW and ILW), High Level Waste (HLW), Spent Fuel (SF), Uranium (U) and Plutonium (Pu)) at the same site, and this is currently the preferred option. It also notes that research will be required to support the detailed design and safety assessment in relation to any potentially detrimental interactions between the different modules. Different disposal system designs and associated Engineered Barrier Systems (EBS) will be required for these different waste types, i.e. ILW/LLW and HLW/SF. If declared as waste U would be disposed as ILW and Pu as HLW/SF. The Geological Disposal Facility (GDF) would therefore comprise two co-located modules (respectively for ILW/LLW and HLW/SF). This paper presents an overview of a study undertaken to assess the implications of co-location by identifying the key Thermo-Hydro-Mechanical-Chemical (THMC) interactions that might occur during both the operational and post-closure phases, and their consequences for GDF design, performance and safety. The MRWS programme is currently seeking expressions of interest from communities to host a GDF. Therefore, the study was required to consider a wide range of potential GDF host rocks and consistent, conceptual disposal system designs. Two example disposal concepts (i.e. combinations of host rock, GDF design including wasteform and layout, etc.) were carried forward for detailed assessment and a third for qualitative analysis. Dimensional and 1D analyses were used to identify the key interactions, and 3D models were used to investigate selected interactions in more detail. The results of this study show that it is possible for ILW/LLW and HLW/SF modules to be co-located without compromising key safety functions of different barrier components, and this reflects international precedents, e.g. the Andra and Nagra repository designs. There are two key technical issues that need to be managed in designing the geometry of the co-located GDF: (i) the heat flux from the HLW/SF module interacting with the ILW/LLW module, and (ii) the potential for development of an alkaline plume from the ILW/LLW module interacting with the HLW/SF module; particularly within fractured host rocks.

Interactions between the co-located intermediate-level waste/low-level waste and high-level waste/spent fuel components of a geological disposal facility

“…The relative importance of heat transfer processes may be assessed by evaluation of the Péclet number, which represents the ratio of the advection and conduction terms. GDF concepts generally envisage host rocks in which natural groundwater flow rates are small, and Watson et al (2009) determined that the Péclet number is likely to be much less than unity in such rocks. Therefore, heat transfer will be dominated by conduction even in higher strength rocks where groundwater flow may occur in fractures.…”

“…Estimates of the effects of heat generated in one disposal module on the temperature in another may be made using analytical solutions to the linear one-dimensional equation for heat transfer in a water-saturated porous medium (Watson et al, 2009). Based on typical parameter values for the higher strength and lower strength sedimentary host rocks and assuming a constant heat source, it would take more than 1000 years for heat to be conducted from one disposal module to the other for a disposal module separation distance of 500 m. It is expected that on this timescale the thermal output from the wastes will have reduced significantly and the thermal interaction between disposal modules will be insignificant.…”

“…Three-dimensional groundwater flow models have been developed to evaluate interactions between disposal modules for the higher strength rock and lower strength sedimentary rock example GDFs using the FEFLOW code (Watson et al, 2009). The modelling was limited to the sub-areas of the GDF in which interactions are most likely to occur (parts of the two disposal modules in closest proximity and the connecting access tunnels).…”

“…Evolving thermo-hydro-mechanical-chemicalgas (THMCG) conditions in one disposal module during the construction, operational and post-closure phases could influence conditions in the other disposal module. This paper is concerned with the analysis of potential THMCG interactions between disposal modules and presents results from a study undertaken on behalf of the RWMD by Watson et al (2009). The approach to identifying THMCG interactions and an overview of results was presented by Towler et al (2009).…”

“…This paper is concerned with the analysis of potential THMCG interactions between disposal modules and presents results from a study undertaken on behalf of the RWMD by Watson et al (2009). The approach to identifying THMCG interactions and an overview of results was presented by Towler et al (2009).…”