Thus, in the Board's view, a site may be suitable even if the repository system placed there has to rely on engineered barriers for waste containment and isolation to a greater degree than was envisioned when the current guidelines were published. Show that the repository design contains significant redundancy so that more than one independent barrier contributes to the capability of the repository system to contain and isolate waste over the period of compliance. Assess the relative roles played by natural and engineered barriers, as well as analyzing their potential interactions. The DOE should add a requirement that performance assessment not only show that the repository system complies with a standard, but that it does so robustly. Sensitivity studies are carried out to show the effects of higher or lower values of variables.
Compliance is shown with a margin of safety.
A robust conclusion about the performance of a repository system should be better able to withstand challenges brought about by new knowledge and changing assumptions. The DOE should specify the level of confidence that must be reached in its performance calculations before it is prepared to make a positive site-suitability determination. Underlying the DOE's proposed revisions to the guidelines appears to be the implicit presumption that clear and obvious conclusions can be drawn from the performance assessment. As noted above, the Board believes that a performance assessment may, in fact, produce values that have substantial uncertainty bands around them, especially if the assessment is carried out in a manner consistent with the recommendations included in this letter. Therefore it is essential that the DOE specify in advance the level of confidence needed to make a positive site-suitability determination.