WASTE FORM DEVELOPMENT AND TESTING

The committee had expressed an interest in having ANL provide a more detailed description of the processing steps (see Appendix D) involved in the preparation of waste forms, beginning with the initial dehydration of the zeolite-bonded forms or powders and ending with hot isostatic pressing (HIP) to produce the final glass-bonded zeolite (GBZ) waste form. The presentations by the ANL staff described in detail the equipment and processing conditions used in each of the steps in the sequence of processing:

  1. Dehydration of the zeolite,

  2. Preloading of the dehydrated zeolite with LiCl-KCl salts,

  3. Molten salt column ion exchange to load the zeolite with fission products,

  4. Removal of excess salt from the zeolite particles in the column and removal of the salt-loaded zeolite particles from the ion exchange column,

  5. Grinding of the salt-loaded zeolite to fine particles,

  6. Hot blending of the fine particles with additional salt-free dehydrated zeolite powder and glass frit, and

  7. HIP processing to prepare the GBZ waste form.

In addition, several committee members and staff were taken on a laboratory tour to observe the equipment used for each of these steps; the procedures employed were discussed in detail.

It appeared to the committee that the equipment and conditions employed in step 1 were well designed and likely to provide satisfactory results. The equipment and procedures for steps 2 through 6 also appear to be suitable.

The pretreatment consisting of dehydration of the as-received zeolite appears to be appropriate and adequate. The use of extremely dry nitrogen and relatively high temperature should ensure the complete removal of water from the zeolite, and the measurement of moisture in the off-gas stream will allow determination of the completeness of dehydration. Conditioning of the dehydrated zeolite by filling the structural voids with the LiCl-KCl salt mixture will help minimize rehydration as well as prepare the zeolite for mixing with molten salt in downstream operations. Storage and handling in a glove box containing a 1-ppm moisture atmosphere in argon further decrease opportunities for readsorption of water.

Samples of material produced in steps 1 through 4 were shown to the committee; they consisted of small beads of approximately 1/16-in. diameter and were free flowing, with only a few of the beads stuck together in small clumps. Thus, unloading of the columns and transfer of material from the columns for treatment in step 5 should work well.

ANL's results from both batch operation and flowing column studies support an ion exchange mechanism for the transfer of fission product elements from the bulk molten salt into the zeolite matrix. As expected, higher loadings were achieved in the dynamic column tests. However, because of the need for zeolite pellete in the column configuration. Additional studies are planned to determine any effects of binder material on the physical and chemical characteristics of the zeolite. The steps involved for an alternative “batch processing” approach were also described. In the batch process, zeolite powder is fed to step 1, and the resulting dehydrated powder proceeds directly to step 6 for hot blending with salt containing fission product elements and glass frit.



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ELECTROMETALLURGICAL TECHNIQUES FOR DOE SPENT FUEL TREATMENT: A STATUS REPORT ON ARGONNE NATIONAL LABORATORY'S R&D ACTIVITY WASTE FORM DEVELOPMENT AND TESTING The committee had expressed an interest in having ANL provide a more detailed description of the processing steps (see Appendix D) involved in the preparation of waste forms, beginning with the initial dehydration of the zeolite-bonded forms or powders and ending with hot isostatic pressing (HIP) to produce the final glass-bonded zeolite (GBZ) waste form. The presentations by the ANL staff described in detail the equipment and processing conditions used in each of the steps in the sequence of processing: Dehydration of the zeolite, Preloading of the dehydrated zeolite with LiCl-KCl salts, Molten salt column ion exchange to load the zeolite with fission products, Removal of excess salt from the zeolite particles in the column and removal of the salt-loaded zeolite particles from the ion exchange column, Grinding of the salt-loaded zeolite to fine particles, Hot blending of the fine particles with additional salt-free dehydrated zeolite powder and glass frit, and HIP processing to prepare the GBZ waste form. In addition, several committee members and staff were taken on a laboratory tour to observe the equipment used for each of these steps; the procedures employed were discussed in detail. It appeared to the committee that the equipment and conditions employed in step 1 were well designed and likely to provide satisfactory results. The equipment and procedures for steps 2 through 6 also appear to be suitable. The pretreatment consisting of dehydration of the as-received zeolite appears to be appropriate and adequate. The use of extremely dry nitrogen and relatively high temperature should ensure the complete removal of water from the zeolite, and the measurement of moisture in the off-gas stream will allow determination of the completeness of dehydration. Conditioning of the dehydrated zeolite by filling the structural voids with the LiCl-KCl salt mixture will help minimize rehydration as well as prepare the zeolite for mixing with molten salt in downstream operations. Storage and handling in a glove box containing a 1-ppm moisture atmosphere in argon further decrease opportunities for readsorption of water. Samples of material produced in steps 1 through 4 were shown to the committee; they consisted of small beads of approximately 1/16-in. diameter and were free flowing, with only a few of the beads stuck together in small clumps. Thus, unloading of the columns and transfer of material from the columns for treatment in step 5 should work well. ANL's results from both batch operation and flowing column studies support an ion exchange mechanism for the transfer of fission product elements from the bulk molten salt into the zeolite matrix. As expected, higher loadings were achieved in the dynamic column tests. However, because of the need for zeolite pellete in the column configuration. Additional studies are planned to determine any effects of binder material on the physical and chemical characteristics of the zeolite. The steps involved for an alternative “batch processing” approach were also described. In the batch process, zeolite powder is fed to step 1, and the resulting dehydrated powder proceeds directly to step 6 for hot blending with salt containing fission product elements and glass frit.

OCR for page 6
ELECTROMETALLURGICAL TECHNIQUES FOR DOE SPENT FUEL TREATMENT: A STATUS REPORT ON ARGONNE NATIONAL LABORATORY'S R&D ACTIVITY The hot isostatic pressing operation is being performed on a variety of glass formulations mixed with zeolite. Results presented by ANL indicate that there is significant variability in the leaching behavior of GBZ derived from different glasses. It appears that an operational procedure for HIP has been established that allows reproducible preparation of the waste form, and it is anticipated that the glass formulation and the glass/zeolite ratio will be finalized in subsequent experiments. In response to questions asked by the committee in December 1995, 10 additional work was carried out by ANL to evaluate zeolite A phase retention during the hot isostatic pressing process. The results showed the conversion of zeolite A to sodalite, and in some cases nepheline, depending on temperature, pressure, and the length of time the material was subject to HIP. These phase transitions show a collapse of zeolite A; the transition to sodalite is accompanied by a decrease in intracrystalline void volume (the site of occluded salt and nuclides) by a factor of three.11 In principle, the transformation should cause a partitioning of the occluded salt and nuclides, resulting in their partial expulsion into the glass matrix. ANL is studying a range of methodologies for establishing waste form performance,12including both short-term product quality control tests and long-term accelerated reaction tests.13 At this preliminary stage, test and analysis protocols previously developed for other waste forms are being used. The appropriateness of these protocols for testing these specific mineral and metal waste forms for geologic disposal remains to be evaluated. Accordingly, any conclusions regarding relative and absolute performance of waste form samples should be viewed as preliminary. 10   See Appendix C in An Evaluation of the Electrometallurgical Approach for Treatment of Excess Weapons Plutonium, National Research Council, Washington, D.C., February, 1996. 11   The ANL estimate of a factor-of-three reduction in intracrystalline void volume came from the observation that the salt-loaded Linde Type A (LTA) zeolite contains 12 chloride ions per 12 framework aluminum atoms, whereas the literature reports that sodalite (the NaCl-containing felspathoid mineral) contains only 4 chloride ions per 12 framework aluminum atoms. (“Zeolite Molecular Sieves: Structure, Chemistry, and Use,” D.W. Beck, Wiley-Interscience, New York, 1974.) 12   Waste Form Acceptance Requirements for Molten Salt Electrorefining of Spent Nuclear Fuel, Chemical Technology Division, Argonne National Laboratory, NT Technical Memorandum No. 8 (ANL-NT-8), September, 1995. 13   Standard Test Method for Static Leaching of Monolithic Waste Forms for Disposal of Radioactive Waste, ASTM Standard C 1220-92.