Technical Report NTB 93-23

Integrated performance assessments of proposed repositories for radioactive waste involve identification of possible paths of future evolution of the repository system and quantification of the consequences of these possibilities using deterministic or probabilistic modelling approaches. The results of such studies are presented as profiles of consequence (e.g. dose to a selected population) as a function of time extending into the distant future. Although individual components of the model chains can be tested, ensuring the validity of the overall assessment is much more difficult, as is presenting the results in a manner which gives confidence in their applicability.

In this report, the Kristallin-I safety assessment of a HLW repository sited in the crystalline basement of Northern Switzerland is compared with assessments of similar HLW disposal concepts, and also with three recent assessments of direct spent fuel disposal, in order to demonstrate the "reasonableness" of the Kristallin-I results.

Similarity of disposal concepts allows a quantitative comparison between Kristallin-I, its predecessor, Project Gewähr, and the Japanese assessment PNC H-3. From this comparison, it is possible to show that differences in near-field and geosphere performances arise from 3 main factors which themselves result from differences in emphasis in the three assessments and, particularly, from the greater maturity of the Swiss geological investigation programme for Kristallin-I.

A detailed examination of the consequences of different disposal concepts and modelling approaches used in Kristallin-I and the three spent-fuel assessments (SKB 91, TVO 92 and AECL 94) leads to the identification of several key factors which influence the results of the performance assessments. The significance of these key factors does, however, vary between these assessments.

Overall, these comparisons gave no indications of obvious errors or deficiencies in the Kristallin-I performance assessment. The results obtained, in terms of long-term consequences, are similar to those reported by the other assessments.

In order to try to build confidence in the results of Kristallin-I, examples from natural analogues are used to illustrate the reasonableness of predictions for key processes in the repository system. The doses and associated risks arising from the HLW repository are compared with doses and health risks from other forms of radiation (e.g. terrestrial, cosmic, man-made), and with risks associated with toxic materials (e.g. from smoking), ordinary illness and disease, and everyday behaviour which has associated hazards (e.g. flying or driving). These comparisons give a wider context for the presentation of the Kristallin-I results, and put them into a form more easily appreciated by the layman (or, indeed, technical audiences outside the waste disposal field).