Technischer Bericht NTB 83-03

The report NTB 83-03 forms part of an intermediate status report on waste disposal work in Switzerland. The specific issue treated concerns the preparations for analyses of the long term safety of final repositories. The accompanying report NTB 83-02 presents the overall national disposal strategies and describes the progress of the numerous component programmes.

The principal objective of radioactive waste disposal is to ensure adequate protection of the public at present and at all future times. Various criteria have been formulated with a view to helping ensure that this objective is realised in concrete repository planning projects. The criteria range from broad qualitative requirements on the total disposal system down to detailed numerical specifications on single components. Guidelines quantifying overall system performance are judged most suitable and the relevant proposals of the Swiss nuclear safety authorities are reproduced in an appendix to the present report.

The system of overlapping individual safety barriers which is proposed for providing the specified overall level of safety is briefly presented. lncluded are solidification matrix, container, backfill, host rock, overlying geological formations and biosphere dilution.

The role of the analyses which lie on the core of the safety reports accompanying disposal projects is to demonstrate that the barrier system described can be relied upon to ensure the safety requirements laid down. The methodology of these analysis is shortly discussed. For project "Gewähr" a traditional approach using scenario and consequence analyses is envisaged, with no great use being made of probabilistic methods.

All analyses aimed at demonstrating the safety of disposal must be summarized within the framework of a safety report. This is a procedure commonly followed today for various complex technical facilities or processes. For nuclear facilities with sparse accident statistics, a predictive modelling approach is used. Waste repositories are a recent kind of nuclear facility so that there is no commonly accepted form for presentation of the safety reports. Based upon existing suggestions and upon existing safety studies a possible structure for the project "Gewähr" safety report is suggested.

The suggestion presented is used as a framework for initiating discussion on expected contents, extent and weighting of the report. Chief characteristics of the proposed contents-list for a HLW safety report are the traditional determination character, the emphasis upon quantitative assessment of effects of data uncertainties, and the importance attached to maximising the information which can be gained by comparison of disposal system behaviour with natural analogue systems.

In an appendix to the current report, the mathematical models which have been developed or implemented in Switzerland for use in safety analyses are reviewed. The brief descriptions cover the purpose of the different models, their special features and their status. A further appendix is devoted to a summary of the corresponding data requirements. The individual data items which are needed as direct model input or for calibration of model operation are concisely listed. This is followed for each safety system component by an assessment of the current status of the data and of any important developments which are expected before submission of reports for the safety guarantee project "Gewähr''.