Changes in chemical composition of tetraphenylboron slurry during pretreatment for vitrification

摘要

This paper presents results from a study of changes in the composition of a high-level waste (HLW) stream at two stages of its treatment. The waste stream that was studied was a tetraphenylborate precipitate slurry of a HLW supernate from a storage tank at the Savannah River Site. Changes to the slurry composition were studied prior to treatment (a hydrolysis process) and then again after hydrolysis of the slurry. In addition, the success of the hydrolysis is briefly discussed. This study showed that prior to hydrolysis, the solubility of cesium 137 and potassium increased significantly under the influence of the beta and gamma radiation from cesium-137 decay. This was due to radiolytic decomposition of the tetraphenylborate precipitate. The concentration of soluble boron was also expected to increase. Although the boron concentration did seem to increase, the change could not be considered statistically significant. Composition of the hydrolysis product also changed. Concentrations of soluble mercury, calcium, and magnesium all increased by more than 2X in a six month period following the hydrolysis. Concentrations of soluble copper, boron and zinc decreased by more than 2X. The concentrations of the most soluble species in the sample, sodium and potassium, did not change significantly. Comparisons of results from four hydrolysis demonstrations with radioactive precipitate slurry indicated that the elemental composition of the hydrolysis product did not vary greatly even when the process was scaled down. The one exception was the concentration of soluble copper. This concentration was significantly lower in two of the four studies.