(480b) Impact of Alkali Source on Vitrification of Srs High Level Waste

The Defense Waste Processing Facility (DWPF) Savannah River Site is currently immobilizing high level nuclear waste sludge by vitrification in borosilicate glass. The alkali content of the final waste glass is a critical parameter that affects key glass properties (such as durability and viscosity) as well as the processing characteristics of the waste sludge. Two sources of alkali contribute to the final alkali content of the glass: sodium salts in the waste supernate and sodium and lithium oxides in the glass frit added during pretreatment processes.

Increasing the alkali content of the glass, either by washing the sludge solids less or by increasing the alkali content of the frit has been shown to improve the production rate of the DWPF. However, the total alkali in the final glass is limited by constraints on glass durability and viscosity. Studies were conducted to determine if the melt rate would be improved by less washing and removal of alkali from the frit or by more washing to allow increased alkali content in the frit.

The tests were performed using nonradioactive simulants of the waste sludge prepared with varying amounts of washing. Sludge Batch 3 was used as the compositional basis for the simulant. Frits were prepared and the pretreatment process performed so that the final glass from all tests would have the same composition. The impact of the varying wash levels in the simulated waste on the pretreatment and vitrification process was determined using laboratory scale testing to simulate the DWPF process.

The results indicated that an optimum point exists in sludge washing for maximum melt rate of Sludge Batch Three at a point with approximately 21% sodium in the sludge (oxide basis) and 16% total alkali content in the frit. Alkali metals act as a flux during melting. It is speculated that melt rate is reduced as the frit or waste becomes more refractory on either side of the optimal point.

The impact of the varying amounts of sodium salts on the pretreatment process was also evaluated. As sodium salts in the waste were increased, more acid was required during sludge acidification. The additional acid led to increased processing time and higher hydrogen evolution during processing. The melter feed is concentrated to 50% solids during pretreatment. The higher percentage of insoluble solids in the more washed sludge led to difficulties with pumping and required dilution to 45% solids prior to melter testing. The optimal point for melt rate (21% sodium in sludge on an oxide basis) did not lead to processing difficulties.