Many different rock types have been considered for HLW disposal, for example salt, anhydrite, granite and clay. The most important mechanism for repository degradation involves barrier corrosion and radionuclide transport in groundwater, and hence site selection specifically aims to ensure that groundwater fluxes through the repository are very low and that the chemical environment is favourable. The geological processes and changes in most formations are very slow in relation to the time for which the waste needs to be isolated. The properties of potential host rocks are not only investigated from the surface but in many cases laboratories have also been constructed underground - Major past or present underground research laboratories . Such underground laboratories allow development and application of a wide range of hydrogeologic, geophysical and geochemical techniques for rock characterisation and also opportunities to test models of the processes involved in repository evolution.
Mathematical models are key components of the safety assessment of waste disposal options.
A chain of such models is used to describe the long term degradation of the engineered barriers and the eventual release of radionuclides into groundwater and their transport to the biosphere. Very large national research programmes support the performance assessment models and their associated databases. Finally models can be tested by comparison with natural analogues - similar processes occurring in natural systems. On one scale, such analogues include those which demonstrate the basic feasibility of geological disposal. An example is the movement of radioactive elements from the nuclear reactors that occurred spontaneously in very rich uranium ores at Oklo in Gabon, starting about two thousand million years ago.