Crystalline Silicotitanate Performance in Modified SRS Tank 9H Supernate for Tank Closure Cesium Removal - 21153

The Tank Closure Cesium Removal (TCCR) process at Savannah River Site (SRS) uses ion exchange columns filled with crystalline silicotitanate (CST) media to process radioactive waste solutions for the removal of Cs-137. TCCR currently focuses on dissolving SRS radioactive tank waste (primarily sodium salt cake solids) within Tank 10H followed by ion exchange column treatment. Three supernate batches from Tank 10H have been processed through the TCCR unit. Plans are to now replace the CST columns and process dissolved salt solution from Tank 9H through Tank 10H and then through the new CST columns installed in the TCCR unit. At the time of this work the new columns were expected to contain either a media similar to an archived CST batch (IE-911) or the R9120-B CST media used in the current TCCR columns (the two materials are fundamentally the same; just different specifications, product names, and preconditioning steps). The media ultimately selected for the new columns is referred to as R9120-B 30 x 60, and has a particle size more similar to the archived IE-911, smaller than the R9120-B used in the original TCCR columns. Samples of Tank 9H dissolved salt cake were received at the Savannah River National Laboratory (SRNL) and characterized. The Tank 9H sample contained a high sodium concentration (∼9.6 M Na⁺) and the tank supernate will require dilution to near 6 M [Na⁺] prior to processing through the TCCR unit. The cesium concentration in Tank 9H is currently significantly higher than was observed with Tank 10H. Three dilutions of the Tank 9H supernate were conducted to mimic possible dilutions that could be conducted in the tank farm prior to TCCR processing using inhibited water, sodium hydroxide, and sodium nitrate solutions. Dilution no.1 was prepared by diluting the Tank 9H supernate by a factor of 1.6 with inhibited water. Dilution no.2 was prepared by diluting the Tank 9H supernate by a factor of 3.7 with a mixed sodium hydroxide/sodium nitrate diluent. Dilution no.3 was also prepared by diluting by a factor of 3.7, but with sodium hydroxide only. All three dilutions have similar Na⁺ concentrations (∼6 M), but Dilutions no.2 and no.3 contain significantly less cesium. Batch contact cesium equilibrium loading tests were conducted with the Tank 9H dilutions and the two different CST media batches being considered for use in the new TCCR columns. ZAM (Zheng, Anthony, and Miller) isotherm modeling was also performed for comparison to the experimental results. The highest cesium distribution coefficient and percent removal were observed with Dilution no.3. IE-911 CST was more effective at removing cesium than the more recently prepared R9120-B CST media, though both media samples removed >88% of the cesium and the differences may not be statistically significant considering the overall uncertainty. Based on the results, maximum cesium loadings were calculated for each CST media type and Tank 9H dilution. In general, maximum cesium loadings from dilutions of this supernate batch are quite high (approaching 0.1 mmol total Cs⁺/g CST for Dilution no.1). The highest calculated maximum Cs-137 loading for the Tank 9H dilutions using the ZAM model with input of the tank compositions and batch contact results was 7.66 E+12 Bq/kg CST (207 Ci/kg) (Dilution no.1 with IE-911 CST). In all cases, higher maximum cesium loading values were predicted for IE-911 CST versus R9120-B, although the differences varied considerably between the dilutions. The maximum loading value for IE-911 CST with Tank 9H Dilution no.1 was only 7% higher than the maximum loading for R9120-B. The maximum loading value for IE-911 CST with Dilution no.3 was 24% higher than the maximum loading for R9120-B. Dilution no.2 was intermediate between these values. Note that these maximum loading values are the theoretical calculated values, and actual operating conditions under dynamic flow conditions will impact the loading. A CST binder dilution (correction) factor near 0.7 (relative to pure powder CST) was required for each batch contact test with IE-911 engineered CST using the three Tank 9H dilutions. Correction factors calculated for R9120-B CST ranged from 0.56 to 0.66 for the three dilutions. Following the batch contact tests, the CST samples were isolated from the Tank 9H solution, washed, dried, and digested in acid. Total cesium loading values determined by CST digestion were similar to the calculated loading values based on solution analysis for all samples. In addition, the CST was observed to load calcium (R9120-B sample only), iron, strontium, lead, uranium, and plutonium after contact with the waste supernate, as has been observed previously. (authors)