Simulation studies of Non-Ionising Energy Loss (NIEL) in silicon exposed to various types of hadron irradiation are presented. A simulation model of migration and clustering of the produced primary defects is developed. Although there are many uncertainties in the input parameters it is shown that the model is consistent with experimental observations on standard and oxygen-enriched silicon. However, the model makes the rather dramatic prediction that NIEL scaling of leakage current and effective doping concentration can be violated significantly even in standard silicon. Although there are possible shortcomings in the model which might account for this, it is shown that at the microscopic level there is, indeed, no obvious reason for an exact NIEL scaling. Furthermore, it is argued that, contrary to common belief, even a significant violation of NIEL scaling can still be consistent with experimental data.