Numerical simulation by a finite element computer code has bee* used to identify key stages in the initiation of confined bulk explosive by cast primers. The simulations have been verified by comparison with experimental field measurements of shock arrival time versus distance above the primer for a relatively sensitive emulsion doped with ammonium nitrate prills. Further simulations were performed examining the effect of primer size in a relatively insensitive blend of ANFO and emulsion. Immediately above the primers, changes in shock velocity in the explosive were caused by the interactions between shocks originating from different surfaces of the cylindrical primers. At a distance up the explosive column corresponding to about one to two borehole diameters, the arrival of shock reflections off the borehole walls caused a sudden acceleration up to the confined steady state detonation velocity. The simulations reproduced experimental observations that primer size has only a weak effect on changes of shock velocity with distance for confined explosives. However, it was predicted that primer size has a strong influence on the intensity and duration of the pressure pulses that bulk explosive send into the rock surrounding the primer region.