Geological disposal remains the only long-term solution available. After four decades of study, geological disposal remains the only scientifically and technically credible long-term solution available to meet the need for safety without reliance on active management. It also offers security benefits because it would place fissile materials out of reach of all but the most sophisticated weapons builders. As in all scientific work, progress in achieving geological disposal has been marked by surprises, new insights, and the recognition that for even the best-characterized sites, there always will be uncertainties about the long-term performance of the repository system. Providing convincing evidence that any repository assures long-term safety is a continuing technical challenge. Nevertheless, a well-designed repository represents, after closure, a passive system containing a succession of robust safety barriers. Our present civilization designs, builds, and lives with technological facilities of much greater complexity and higher hazard potential.

Today the biggest challenges to waste disposition are societal. Difficulties in achieving public support have been seriously underestimated in the past, and opportunities to increase public involvement and to gain public trust have been missed. Most countries have made major changes in their approach to waste disposition to address the recognized societal challenges. Such changes include initiating decision processes that maintain choice and that are open, transparent, and collaborative with independent scientists, critics, and members of the public.

Whether, when, and how to move toward geological disposal are societal decisions for each country. This decision process will be lengthy, and the time can be used to improve both the technical and the societal bases for these decisions.

A stepwise process is appropriate for decision making under technical and social uncertainty. Some—but not all—of today's uncertainties in predicting the future behavior of a repository system can be reduced or eliminated by further research and development. Both fundamental knowledge and ways to deal with uncertainty are advancing, and they will continue to do so during the course of a repository development program. A stepwise decision process can utilize this evolving knowledge to make sound decisions on repository siting (including the geological setting), design, and operation.