Tailoring Inorganic Sorbents for SRS Strontium and Actinide Separations: Modified Monosodium Titanate Phase 3 Final Report

This document provides a final report of Phase III testing activities for the development of modified monosodium titanate (mMST), which exhibits improved strontium and actinide removal characteristics compared to the baseline MST material. The activities included characterization of the crystalline phases present at varying temperatures, solids settling characteristics, quantification of the peroxide content; evaluation of the post-synthesis gas release under different conditions; the extent of desorption of {sup 85}Sr, Np, and Pu under washing conditions; and the effects of age and radiation on the performance of the mMST. Key findings and conclusions include the following. The peroxide content of several mMST samples was determined using iodometric titration. The peroxide content was found to decrease with age or upon extended exposure to elevated temperature. A loss of peroxide was also measured after exposure of the material to an alkaline salt solution similar in composition to the simulated waste solution. To determine if the loss of peroxide with age affects the performance of the material, Sr and actinide removal tests were conducted with samples of varying age. The oldest sample (4 years and 8 months) did show lower Sr and Pu removal performance. When compared to the youngest sample tested (1 month), the oldest sample retained only 15% of the DF for Pu. Previous testing with this sample indicated no decrease in Pu removal performance up to an age of 30 months. No loss in Np removal performance was observed for any of the aged samples, and no uptake of uranium occurred at the typical sorbent loading of 0.2 g/L. Additional testing with a uranium only simulant and higher mMST loading (3.0 g/L) indicated a 10% increase of uranium uptake for a sample aged 3 years and 8 months when compared to the results of the same sample measured at an age of 1 year and 5 months. Performance testing with both baseline-MST and mMST that had been irradiated in a gamma source to a total dose of 3.95 x 10{sup 6} R, indicated little to no affect on the performance of the material to remove Sr and actinides. Previous testing established that mMST releases oxygen gas during the synthesis, and continues to off-gas during storage post synthesis. The post-synthesis gas release rate was measured under several conditions, including varying the pH of the wash water and at elevated temperature (49 C, typical of bounding summertime storage without air conditioning). Results indicated that a high pH (basic) wash reduced the initial gas release rate, but after 2 days the release rates from all different pH washed samples were not statistically different. The gas release rate at 49 C, a temperature at which the material may be exposed to during shipping and storage, was consistently about 2.5 times higher than the rate at room temperature. All gas release results indicated that vented containers would be necessary for shipping and storage of large quantities of material. Suspension of sorbate-loaded solids into diluted solutions representing intermediate and final stages of washing for 24-hours revealed no evidence of desorption of Sr, Pu or Np from the mMST solids. Based on the results of the Phase III testing as well as that from earlier studies (Phases I and II), SRNL researchers recommend adopting the use of the mMST material for the removal of strontium and actinides from the SRS HLW supernatant liquids in the Actinide Removal Process and Salt Waste Processing Facility. Given the decrease in Sr and Pu removal performance for the mMST having an age of 4 years and 8 months, we recommend that mMST be used within 30 months of production. Furthermore we recommend that DOE provide funding to conduct pilot-scale testing of the mixing and settling characteristics of the mMST and impact, if any, on the generation of hydrogen during processing in the Defense Waste Processing Facility (DWPF)