Swiss legislation requires the safe final disposal of radioactive wastes. Final disposal is defined by the safety authority guidelines as follows: ‘After a repository has been sealed, no further measures shall be necessary to ensure safety. The repository must be designed in such a way that it can be sealed within a few years.’ The time for repository sealing is not specified.

In Switzerland, the Wellenberg site was selected for the LIILW repository after an extensive site evaluation process and, in June 1994, the application for the general licence was submitted to the Swiss Federal Government. In addition to the federal licence, a mining concession has to be granted by the canton in a public referendum. The decisive cantonal vote in June 95 was negative (52%) and led to a temporary set-back in the project.

Among the reasons for this negative result was the proposed repository emplacement concept. It was planned to successively backfill the voids between the emplaced disposal containers in the caverns layer by layer. Retrieval of the waste would therefore involve high expenditure and effort. The concept therefore implies closing part of the repository at a very early stage. The outcome of the referendum reflected an unwillingness on the part of the population either to accept this situation or to make a decision today on final disposal. In many discussions, a stepwise implementation of the repository was requested, not only from the point of view of technical feasibility but also to allow political and legal aspects to be taken into consideration. A final decision - particularly regarding closure of the repository - could then be taken in the future, after giving the coming generation an opportunity to think over our proposed solution.

To meet this requirement, GNW/Nagra studied modified repository emplacement concepts within the framework of Swiss legislation. In the proposed new project, concrete containers are emplaced in the caverns without any backfill, allowing easy retrieval until the decision is taken to close the facility. This decision may be taken only after completion of container emplacement, allowing for further monitoring of the filled repository caverns. A decision on how to proceed is currently being prepared by a federal working group with representatives from the canton, the safety authorities and GNW/Nagra.

Studies have shown that, with an appropriate design, drainage and ventilation system, the required stability of the modified repository cavern can be achieved also under seismic loading and that the requirements of operational safety can be met. The potential long opening times of the repository caverns show some minor effects on certain repository components, such as e.g. slight carbonatisation of the container backfill. Overall, it can nevertheless be demonstrated that the long-term safety is not adversely affected.