N. D. Hutson, C. A. Herman and J. R. Zamecnik
Westinghouse Savannah River Company
Aiken, South Carolina 29808

S. K. Sundaram and J. M. Perez, Jr.
Pacific Northwest National Laboratory
Richland, Washington

S. L. Hoeffner
Clemson University
Anderson, South Carolina

D. O. Russo and M. E. Sterba
Comisión Nacional de Energía Atómica
San Carlos de Bariloche, Argentina

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Under the Science and Technology Implementing Arrangement for Cooperation on Radioactive and Mixed Waste Management (JCCRM), the U.S. Department of Energy (DOE) is helping to transfer waste treatment technology to international atomic energy commissions. As part of the JCCRM, DOE has established a collaborative research agreement with the Comisión Nacional de Energía Atómica (CNEA). The CNEA is investigating treatment and disposal options for organic ion exchange resins currently stored at two nuclear power plants in the Republic of Argentina. The Atucha plant creates 2.83 m³ of waste per year and has a current inventory of spent resin is approximately 42 m³. The Embalse plant creates 9.5 m³ of waste per year and currently stores more than 130 m³. A treatment and disposal method is needed due to continued generation of the resins and limited storage capacity at both plants.

Vitrification has been used to convert hazardous and/or radioactive wastes to a form suitable for permanent disposal. This technology is currently being applied at several DOE facilities and many international sites for the stabilization of high-level radioactive wastes. The Environmental Protection Agency (EPA) has declared vitrification to be the Best Demonstrated Available Technology (BDAT) for high-level radioactive liquid waste. The technology is also being applied to low-level and mixed wastes. Releases of radioactive and hazardous components of the vitrified waste (as measured from standard leach testing) are low due to the chemical bonding of metal oxides in the glass structure.

Ion exchange resins are widely used in the nuclear industries (including at the facilities of the DOE) for purification of various aqueous streams. Unfortunately, their use creates a waste stream that can be very high in both organic and radioactive constituents. Therefore, disposal often becomes an economic problem because of the large volumes of resin produced and the relatively few technologies that are capable of economically stabilizing this waste. Vitrification treatment presents a reasonable disposable alternative.

The major hazards of the ion exchange resins are their organic composition and the contaminants that are present on the resins after purification processes. The principal contaminants are usually the radioactive species that are removed.

For these studies, actual non-radioactive resins from Argentina's Embalse and Atucha plants were tested. The properties of the Embalse and Atucha resins are shown in Table 1.

Extensive bench-scale studies were performed at the Savannah River Technology Center (SRTC) in fiscal year 1997 to determine the proper limits for vitrifying Argentine ion exchange resins. The selected composition was an iron-enriched borosilicate glass, which maximized waste loading and minimized melt temperature and radioactive material volatility. The target glass composition is given in Table 2.

In fiscal years 1997-1999, SRTC completed bench-scale studies and melter demonstrations with organic ion-exchange resins. Bench-scale studies were performed using both types of Argentine ion-exchange resin. In the bench-scale studies, ~30 wt% waste loadings were found which represented a ~65% volume decrease (Note: In this case, "waste loading" is defined as the amount of resin which is processed per the amount of glass produced). This was accomplished using the aforementioned iron-enriched borosilicate glass composition and direct vitrification of the resin material. The data from the studies were used to perform a melter demonstration with one of the Argentine ion-exchange resins. In all studies, homogeneous and durable glasses were produced.

Irradiation studies with the Argentine resins were also performed in fiscal year 1997. These studies showed that no significant degradation of the resins occurred because of radiolysis effects. The studies also showed that some potential flammability concerns exist if large amounts of the resins are stored in small storage containers that are not vented.

In FY99 two vitrification demonstrations using resins representative of Argentine Emblase and Atucha ion exchange material were performed at the Clemson Environmental Technologies Laboratory (CETL) under a South Carolina Universities Research and Education Foundation (SCUREF) contract. The demonstrations were performed in the 232-cm² (surface area) Stir-Melter using the iron-enriched borosilicate glass composition developed at the SRTC. The melter tank was constructed of Inconel^® 690 with a surface area of 232 cm² and a depth of 40.6 cm. The impeller was approximately 7.6 cm in diameter and was also fabricated from Inconel^® 690. The impeller provided the stirring action that results in much higher mixing and production rates than can be achieved by convective mixing alone.

The first test used resin representative of that stored at the Embalse Plant. Through adjustment of operating conditions such as melt temperature, spindle height and flow rate, feed rate, and percent solids of the feed, the maintenance of a target 50% cold-cap was demonstrated throughout much of the campaign. The off-gas from the melter was analyzed for permament gases, uncombusted organic compounds, particulates and metals. The amounts of permanent gases in the off-gas were fairly consistent over time. Some of the highest gas concentrations in the off-gas occurred when the solids content of the feed slurry was high.

In the second study, using Argentine Atucha resin, particulates were removed from the off-gas system using a dust collector. To prevent condensation from occurring in the off-gas line and to increase the velocity of the off-gas, approximately 100 scfm of dilution (room) air was introduced via a 7.6 cm duct just after the cleanout port on the off-gas line at the melter exit. Performance of the off-gas system was assessed by testing the off-gas for metals and particulates before and after the dust collector. In addition, the effect of melter operating conditions on the composition of the off-gas was investigated by testing the off-gas while varying the feed rate, melter vacuum and cold cap coverage.

The glass produced during both runs was durable was measured by the Product Consistency Test (PCT). The product had a predictable, mostly amorphous composition throughout the demonstrations; though there was some evidence of the formation of clinopyroxene crystals. The immobilized product represented an approximately 70% volume reduction from the simulated Argentine ion exchange resin (i.e., a reduction from the volume of as-stored wet resin to the volume of the ultimate borosilicate glass product). For all runs, the radioactive surrogate retention was near 100%.

In FY00, additional melter studies were performed in the Pacific Northwest National Laboratory (PNNL) Research Scale Melter (RSM) using Atucha and Embalse resins that had been doped with inactive Cs, Sr and Co. The RSM, which is located in the Applied Process Engineering Laboratory (APEL) building of PNNL, is a small refractory-lined Joule-heated melter that is capable of processing melter feed on a continuous basis. This capability is key for determining the relationships between the properties of the feed and the properties of the final glass produced. Production of glass in a continuous manner is also more representative of a full-scale system. Detailed offgas analyses (including analysis of fixed gases and particulate measurements) were done. Operating parameters for a conventional slurry-fed Joule heated melter were also obtained.

The results obtained thus far show that spent ion exchange resins can be effectively treated using vitrification to immobilize the contained radionuclides. This work is continuing into FY01. The planned research includes the continuation of vitrification tests on resins representative of those used in the Embalse plant and doped with representative levels of radioactive Co-60, Sr-90 and Cs-137 (supplied by the U.S.). A qualitative analysis of the offgas pollutants and a comparison with previous non-radioactive tests performed at PNNL, CETL and SRTC will be done. This vitrification demonstration will be performed in the shielded cells at the Savannah River Technology Center

During the radioactive demonstration, data will be obtained on wasteform durability performance (as determined using U.S. HLW acceptance criteria, i.e., the PCT), radionuclide retention and partitioning, and organic destruction efficiency.