A Predictive Model for Waste Package Terminal Velocity in Deep Borehole Disposal

Summary The aim of this work is to provide insight into the sinking rate of waste packages in Deep Borehole Disposal (DBD). An investigation was performed using simplified, scaled down experiments, analytical modelling and molecular modelling. The experiments systematically varied a range of cylinder parameters to understand their influences upon the sinking rate of the cylinder. Results showed that this sinking velocity varied as a function of cylinder diameter, length and density, with diameter being the predominant factor in dictating the sinking rate. An analytical model was subsequently developed using the experiment data as validation. The model was developed by solving the Navier-Stokes equations for the flow within the annular gap, in addition to characterising pressures applied at the front of the cylinder. Results showed good levels of accuracy for low values of clearance, although velocity was increasingly over predicted as clearance increased. Molecular dynamics simulations were used as a method of gaining pseudoexperiment data and further insight into the fluid flow. Sinking disc simulations provided several correlating results with experiments; confirming that sinking velocity decreases linearly with diameter at sinker-container ratios greater than 0.6, and that density appears to increase sinking velocity towards a plateau. Stationary disc simulations illustrated that highly turbulent flow regimes occurred at the wake of objects in confined boundary systems. Several of these flow regimes occurred at significantly lesser streaming velocity for finite boundary systems as opposed to infinite boundary systems. This shows the importance of accounting for turbulence in finite boundary systems, and provides a logical path for the future development of a predictive sinking velocity model.