Near-field processes and release models for granitic rocks

The system of enginnered barriers contained in the near-field is the only part of the multibarrier system over which direct control can be exercised, in terms of its effect on containment of the waste. A combination of experiment and modelling of release processes which treats the principal mechanisms involved in the breakdown of the waste form and in subsequent migration, can be used to help define the behaviour of these barriers during the thermal and post-thermal periods. A review of the results of such studies is presented, principally of the hydrothermal dissolution behaviour of borosilicate glass in a granitic environment, the effects of waste-rock-water interaction, and of the sorption mechanisms for specific nuclides in contact with altered granite fracture surfaces. These results are combined in a model of release from the near-field, and migration through the far-field