In situ Removal of Actinides and Strontium from High Level Waste Tanks, Tea Bag versus Adsorption Column

Abstract

SummaryInitiatives are underway at the Savannah River Site (SRS) to accelerate the disposition of the supernate and salt portions of the waste in the SRS High Level Waste (HLW) tank farm system. Significant savings in processing time and overall cost could be achieved by in situ treatment of waste supernate or dissolved salt inside a tank farm waste tank. For treatment of actinides and strontium in waste, the baseline method is sorption onto monosodium titanate (MST), an engineered powder with mean particle size of ~10 microns. In a separate study at the Savannah River National Laboratory (SRNL), engineered forms of MST were developed and compared on a small (250-mL) scale in batch tests. In the current study, a promising form of engineered MST was tested under two conditions: a traditional ion exchange (or adsorption) column and a porous, flowthrough device called a "tea bag", immersed in solution. Both tests used the same amount of engineered MST to treat 10 L of simulated waste solution containing plutonium (108 µg/L) and strontium (710 µg/L).In the column test, engineered MST succeeded in treating 2900 bed volumes (BV) of simulated salt waste solution. There was no significant strontium (Sr) breakthrough and only 7% plutonium (Pu) breakthrough at the end of the run which operated at 5.3 BV/hour and lasted 23 days. Stated another way, the column of engineered MST achieved an average decontamination factor (DF) of 70 for Sr and a Pu DF ranging from 140 initially to 15. In the tea bag test, activity levels for Sr and Pu were reduced by 82% and 80%, respectively, in four weeks, though most of the Sr removal occurred during the first week. The tea bag DFs were 5.5 for Sr and 4.9 for Pu.Estimates are available for the DFs required to meet Z-Area limits. For the average waste concentration, a Sr DF of 4.5 and a Pu DF of 12 are needed. Based on the performance of the engineered MST in the column tests, this treatment approach would be sufficient to treat much of the SRS waste. The tea bag method, however, would be limited to wastes that have Pu activities below the average composition or used in tandem with other treatment methods for wastes with the average Pu waste composition or higher.Though tea bag and column equipment could be deployed in a large tank, the column approach is more attractive for several reasons: ability to meet decontamination requirements at higher alpha activity, no observed fines generation, and relative maturity of column-related technology. Another important advantage to the column approach is that it enhances loading because the adsorbent is in equilibrium with feed adsorbate levels which are higher than product levels. Enhanced loading yields more efficient use of MST, reducing the amount of MST solids that transfer downstream for vitrification.We scaled the results of bench-scale tests for deployment in a million-gallon waste tank. Using a 70-gal column operating in once-through mode, we would expect a DF of 70 for Sr and about 30 for Pu for the treatment of the initial 200,000 gal in a...