• Step III: subsequent migration of radionuclides, either into secondary solid phases or as mobile aqueous species (or colloids) undergoing diffusive or advective-diffusive transport.

With respect to Step I, one issue to be resolved is whether the encompassing, nonradioactive glass binder will provide physical isolation of the radionuclide-loaded zeolite/sodalite, limiting the available area of the ceramic contacted by water. Demonstrating physical protection for long periods of time may be difficult and should be based on consideration of the full range of thermal, mechanical, hydrological, and chemical conditions of planned geologic repositories.

In contrast to its physical effects, the presence of the glass binder will undoubtedly have an impact on the chemical reactions occurring between water and the radionuclide-bearing zeolite/sodalite. For example, glass dissolution may be a source of solvated cations that could expedite the subsequent ion-exchange release of radionuclides from the ceramic waste form. Furthermore, water-glass reactions typically lead to increases in pH and alkaline conditions, which could increase the dissolution rate of zeolitic materials.

ANL does not yet have enough QT data on its proposed ceramic waste forms to unambiguously distinguish which release mechanism (Step II)—ion-exchange or dissolution—controls the primary release of radionuclides. Ion-exchange is now acknowledged as one likely mechanism, supported by comparison of the release behavior of nonradioactive ceramic waste forms by ANL using dilute brine contacting waters. ANL plans an extensive set of both long-term and short-term tests (see following section) to attempt to evaluate which primary release mechanism dominates at what time period.

Step III relates to the impact of transport conditions of a repository on the long-term rate of migration of radionuclides released from the waste packages. Information on the expected flow and transport conditions from DOE's high-level waste (HLW) repository program is needed so that reasonable test conditions can be simulated in planned long-term QT of the ceramic waste form.

Qualification Testing Methods

ANL has listed a broad range of planned testing techniques that will form the initial phase of QT for its ceramic waste form. ANL noted that many of the same staff and much of the same analytical equipment necessary to support QT must be shared with parallel activities such as waste form preparation and characterization.

In view of ANL's limited human and facility resources, it would be prudent for ANL to involve external technical experts to review the need for a full set of tests to support QT. There may be needless overlap and redundancy among these proposed tests. Furthermore, alternative unsaturated



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