Human intrusion and effects on multi-barrier disposal systems

Human intrusion has long been recognised as a potentially important post-closure safety issue for solid radioactive waste repositories. The quantitative assessment of the likelihood and effects of human intrusion is problematic, not least because of the inherent difficulties in predicting future human activities. Nevertheless, a variety of human intrusion scenarios has been postulated to provide illustrations of the possible risks to future generations. However, most of those analyses have been concerned with the risks to those involved in the intrusion. Relatively little attention has been given to the effects on the containment provided by the natural and man-made components of the barrier system and the consequent risks to third parties. This paper briefly reviews international guidance and regulatory background, examines the treatment of the intrusive effects on barrier systems in performance assessments of shallow and deep facilities and discusses the implications for the future. While intrusion within any particular year is unlikely the cumulative chance that some intrusion event has occurred, say within 10,000 years is not so low. Thus the scope for damage to barriers as a result is also not so low. Important time frames to be considered are: the initial period when site controls might still be effective so that intrusion is unlikely, the time frames over which components of the barrier system are needed to be effective, e g according to radionuclide half-lives etc.; and any cut-off for assessment externally provided by the assessment context, for example given by the regulatory body