The long-term behavior of a waste disposal facility is a function of the entire disposal system, including the waste form, engineered barriers, and
surrounding environment. In order to assess the ability of a given disposal concept to meet regulatory requirements, it is necessary to consider the influence of each of these system components on short — and long-term performance. This is accomplished through the performance assessment (PA) process. For HLW, many countries are proposing long storage life for the canistered glass waste forms during geological repository siting and preparation. During that time, a great many of the radionuclides will decay leaving the long-lived radionuclides as the primary sources that need be considered in a PA.

Figure 6.5 is a schematic of a generic high level waste repository. It shows the relative role of the waste form, the role of the multiple barriers (canis­ters, containers, overpacks, and casks) in the waste disposal system. It is the multi-barrier concept — a barrier within a barrier within a barrier as dis­cussed in Chapter 1. Ultimately the role of the repository or disposal envi­ronment is to isolate the waste from the biosphere until all the barriers have failed at which time almost all of the radionuclides will have decayed.

While the waste form is the source term and should be as durable as reasonably possible, multiple barriers must corrode before the waste form will be exposed to groundwater. As a result of the research programs over the past several decades, there is now an extensive database and substantial understanding of the behavior of nuclear waste glasses in a variety of dis­posal environments [159]. The present challenge is to model glass behavior

in the near-field of specific geologic repository environments and to develop a fundamental understanding of the long-term corrosion rate [160].