One of DOE’s primary remediation challenges is the safe and cost-effective disposal of radioactive wastes currently stored in underground tanks at the Hanford, Oak Ridge, and Savannah River sites. Waste forms must be chemically durable under environmental storage conditions and thermally stable under repository conditions over a geologic time scale. The remediation plan for Hanford’s 177 tanks calls for disposal of highly radioactive waste in the form of borosilicate glass “logs.”

To make processing practical and to reduce the ultimate waste volume, radionuclides in the liquid above the tank sludges—principally cesium and strontium—must be precipitated or separated out. One of the most cesium-selective, stable ion exchangers is crystalline silicotitanate (CST), but borosilicate glass may not be the best waste form for disposing of the loaded ion exchangers, which are nonregenerable.

Titanium dioxide in the CST complicates the vitrification process and affects the quality of the waste form, but limiting its concentration to acceptable levels creates other problems. Because processing takes longer, interim storage is required, raising not only costs but safety issues as well. Dangerous levels of hydrogen may be generated in the wet, used ion exchangers while they await further processing. And the higher volume of the dilute final waste form drives up the expense of processing, transportation, and ultimate storage.

Beyond the issues of time and concentration, dissolving CST in borosilicate glass requires removal and transfer of CST from ion exchange columns, mixing it with glass frit, and melting—all steps that carry risk of contamination to workers and the environment. Moreover, cesium’s volatility is a concern at the temperatures required for vitrification. All of these factors add up to the need for a better method to dispose of CST, one that requires less interim storage, handling, dilution, and heat. This need is all the more pressing because problems with precipitation processes are making separation increasingly the method of choice for treating liquid tank waste.

Finding a simpler solution

Researchers at Pacific Northwest National Laboratory are teamed with others from Sandia National Laboratories and the University of California at Davis in an EMSP project to explore new disposal strategies and waste forms specific to CST—in particular, in situ heat treatment. Direct thermal conversion of cesium-loaded CST ion exchangers is possible because they already contain basic ingredients that can form a ceramic or glass at high temperature. The project, entitled “New Silicotitanate Waste Forms: Development and Characterization,” is generating information on the durability and stability of thermally consolidated CSTs to evaluate the viability of this option for storage and disposal.

This approach would immobilize the radionuclides in the consolidated ion exchanger in a much simpler process with minimal handling and also remove the water and hydroxyl groups that permit radiolytic hydrogen generation during storage. The potential benefits are numerous and substantial:

• Eliminating handling steps would cut the risk of accidental contamination.
• Reducing or eliminating storage prior to treatment would save money and minimize the opportunity for hydrogen generation while waiting for final disposition.
• Waste requiring vitrification into borosilicate glass could be more concentrated and thus less voluminous, saving many millions of dollars in processing and storage costs.
• Lower processing temperatures would reduce issues related to cesium’s volatility.

The project’s research strategy is based on an understanding of ceramic and glass structures and phase stabilities. Experiments at PNNL during the first year of the three-year project showed that thermally converted CSTs have aqueous durability several orders of magnitude higher than borosilicate glass: seven-day leached concentrations of less than 0.01 g/L compared to more than 13 g/L. Heat treatments of 500 and 900°C yielded the most durable ceramics, but the last residual water and hydroxyl groups are not removed until 800°C. Therefore, treatment at 900°C produced the optimal combination of leach resistance and prevention of hydrogen formation from radiolytic decay. Less than 1 percent of the cesium is lost through volatility during such processing. While this rate will still require capture and further treatment, it is a small fraction of that experienced with vitrification.

Phase stability and crystal chemistry studies are vital to predicting short- and long-term performance of waste forms. In its second year, the project team focused on defining the cesium-containing phases and thermodynamic stabilities of compounds related to the ion-exchanged CST and the thermally converted oxides. Transmission electron microscopy revealed that the majority of the cesium is contained in a cesium/X/silicon oxide, where X is a proprietary component of the ion exchanger. With the aid of X-ray diffraction, the new compound was found to have a hexagonal crystalline structure consisting of silica tetrahedra and X octahedra, forming three- and six-membered rings (see graphic below). That the largest free aperture of the rings is smaller than a cesium atom accounts in part for the waste form’s demonstrated high resistance to cesium leaching.

Principal investigator Lou Balmer of PNNL reports that phase identification is nearly complete. SNL researchers are currently studying metastable phases to complete the team’s understanding of the phase development during conversion to a stable ceramic waste form. Researchers at UC Davis are using solution-drop calorimetry to evaluate the thermodynamic stability of compounds fabricated at PNNL and SNL. The researchers aim to have, by the project’s end, sufficient predictive capability to provide the basis for developing and controlling a process to dispose of CST ion exchangers that is not only technically sound, but faster, safer, and substantially more cost-effective than existing alternatives.

If the process is found feasible and applied at sites throughout the complex treating radioactive tank waste, reductions in risk, schedule, and cost could be very important. For further information on this research, contact Lou Balmer, PNNL, (509) 376-2006, lou.balmer@pnl.gov.