Assessment of crushed salt consolidation and fracture healing in a nuclear waste repository in salt

An assessment is made of the degree to which consolidation of crushed salt backfill and fracture healing might enhance the waste isolation capabilities of a high-level waste repository in salt. The paper reviews the properties of crushed salt and the nature of fracture healing as known from laboratory testing and presents analytical methods for predicting the rates at which the processes will occur in a repository. The rate at which crushed salt consolidation will occur is found to be highly uncertain because of contradictory evidence from laboratory testing regarding the creep consolidation characteristics of crushed salt. Moreover, the consolidation rate is likely to vary considerably from one site to another due to differences in room closure rates resulting from differences in the creep characteristics of intact salts. Present indications are that consolidations to low porosities should occur within hundreds of years at most candidate repository sites for locations within the repository close to the waste. Fracture healing should occur relatively rapidly, within tens to hundreds of years, as the confining stress across the fractures approaches lithostatic levels. Healing of fractures caused by excavation may be enhanced by placing relatively rigid concrete plugs in the tunnel openings