Selection and Characterization of a Reference Crystalline Matrix

ANL plans to select a reference crystalline matrix for incorporation of radionuclides by October 1997. Alternative crystalline host phases being considered include the following:

  1. Linde Type-A (LTA) zeolite alone, or LTA zeolite with chabazite (CHA) zeolite, or

  2. Zeolite converted to sodalite (a felspathoid).

The conversion of the radionuclide-loaded zeolite to a denser form (e.g., sodalite) will require particular attention to the uniformity of the distribution of the salt between zeolite crystals. Because the denser phase contains less salt, it may be necessary to limit the highest salt loading in each individual zeolite crystal to no greater than the amount that can be contained in the individual sodalite crystals. Otherwise, portions of the salt (and radionuclides) can be expected to reside outside of the sodalite and, possibly, to be more easily leached from the final waste form. ANL should remain alert to issues regarding the scale-up of the manufacturing process for any of these waste forms, especially with regard to the conditions (time, temperature, mixing, etc.) required to achieve an adequate degree of uniformity of salt loading inside each of the crystalline host phases.

Preparation of Ceramic Waste Forms

ANL expects to produce radioactive samples of its ceramic waste form by the end of the EBR-II demonstration period in 1999. It plans to demonstrate all of the key steps for ceramic waste form preparation, including creating radionuclide-loaded zeolite (or sodalite) and HIP of these crystalline phases with glass to manufacture the final waste form. The samples planned are as follows:

  1. A Pu-238 loaded zeolite, to be tested to assess potential alpha-recoil damage;

  2. A zeolite loaded from the “spent” processing salt of the EBR-II demonstration at ANL-W (the so-called “Throw-Away Option”); and

  3. A zeolite batch ion-exchange equilibrated with salt in the electrorefiner, also to be done in parallel with the ANL-W demonstration (the so-called “Batch Equilibration Option”).

None of these samples closely matches the composition and radionuclide loadings to be expected in zeolites from actual column ion-exchange operations. Such samples are postulated, however, as providing preliminary and representative information regarding the performance of radionuclide-bearing zeolites that will be prepared in actual column ion-exchange operations after the EBR-II demonstration.

DOE must, therefore, consider the question of whether this approach will provide adequate information to proceed beyond the June 1999 demonstration. Implementation of the “Throw-Away

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