(18b) Flammable Gas Generation During Immobilization of Tetraphenylborate in a Cementitious Waste Form

The disposition of 137Cs-containing tetraphenylborate (TPB) waste in Savannah River Site Tank 48 by aggregation with other tank wastes, followed by immobilization in saltstone (a cementitious waste form) has been proposed as a straightforward, cost-effective method of disposal. One of the major technical issues to be resolved in this approach is the generation of flammable gases during processing and in the saltstone vaults. The generation of benzene from decomposition of the TPB was the major flammable gas concern, so tests examining the effects of TPB concentration and temperature were conducted to determine a safe operating envelope for this process. Temperatures as high as 95 °C can be achieved due to the heat of hydration of the cementitious reactions.

Several TPB concentrations, ranging from 30 mg/L to 3000 mg/L in the mixed salt fraction (Tank 48 + other tank wastes) were studied initially at 25, 55, 75, 84, and 95 °C. The saltstone shape used was a cylindrical monolith with only the top surface available for benzene evolution to the vapor. The benzene emitted was collected on coconut shell carbon tubes and analyzed by gas chromatography after extraction by carbon disulfide. The results of these tests showed that operation at 25 °C produces very little benzene and therefore should be acceptable at any of the TPB concentrations tested. The tests at 75 and 95 °C produced unacceptable amounts of benzene except for 30 mg/L TPB at 75 °C. Several tests at 75 °C with higher surface areas were also performed and gave significantly higher benzene emission rates. These tests showed that the evolution rate of benzene definitely depended on the surface area to volume ratio.

The data for the monoliths at 55 °C suggested that benzene evolution should not be significant, but the surface area effects seen at 75°C brought into doubt this conclusion. Further tests at 55 and 65 °C with a range of particle sizes (surface area to volume) have begun to quantify better the actual benzene generation rate and the effects of surface area to volume. Results of these ongoing tests will be included in the conference presentation.