6.45. In view of the complexity and variability of these factors, it is not possible to establish a universal timescale over which meaningful quantitative results from modelling can be obtained. For above surface disposal facilities (e.g. for waste from mining), the uncertainties in modelling results will already be substantial when considering periods of several hundred years, and quantitative estimates may become meaningless already beyond a period of a thousand years. For engineered near surface disposal facilities, which are subject to processes that may affect their integrity (e.g. erosion, human intrusion) to a lesser degree or with a smaller probability, modelling periods of a few thousand years may still be reasonable. For deeper facilities, such as geological disposal facilities for high level waste, modelling for periods of tens of thousands of years and beyond may still result in meaningful estimates of upper bounds of possible radiation doses.