Near-field solubility constraints on radionuclide mobilization and their influence on waste package design

Abstract
The paper addresses the physical and chemical processes that can serve to immobilize waste radionuclides within the confines of an underground repository. These processes, which can be made largely independent of the chemical nature of the host rock, depend for their efficacy upon the maintenance of a very low flow rate of groundwater through the repository constituents. The very long-lived waste products, in particular the actinides, are very insoluble in water under conditions of alkalinity and oxygen potential that will exist in a repository when cement is used as a backfill or as a matrix for the waste. The same chemical conditions tend to favour a fairly long life for steel containers. The paper suggests how these factors may be used to immobilize wastes and draws some conclusions on the relative value of the various engineered features of a repository. Some natural geological analogues are explored as a means of assisting the extrapolation of waste behaviour over very long times.