Modelling Approach to LILW-SL Repository Safety Evaluation for Different Waste Packing Options

The key objective of the work described here was to support the identification of a preferred disposal concept and packaging option for low and short-lived intermediate level waste (LILW-SL). The emphasis of the assessment was the consideration of several waste treatment and packaging options in an attempt to identify optimised containment characteristics that would result in safe disposal, taking into account the cost-benefit of alternative safety measures. The long-term safety of radioactive waste repositories is usually demonstrated with the support of a safety assessment. This normally includes modelling of radionuclide release from a multi-barrier near-surface or deep repository to the geosphere and biosphere. For the current work, performance assessment models were developed for each combination of siting option, repository design and waste packaging option. Modelling of releases from the engineered containment system (the ‘near-field’) was undertaken using compartment AMBER code while detailed unsaturated water flow modelling was undertaken using the HYDRUS code, where the degree of engineered barrier degradation with time is accounted for in each packaging option. The approach proved to be highly flexible, transparent and effective in terms of calculation time.