Cesium Exchange onto Crystalline Silicotitanate from Blended Hanford Tank Wastes

Abstract

The Tank Side Cesium Removal (TSCR) system is under construction to filter and remove cesium (Cs and 137Cs) from Hanford tank waste supernate in preparation for vitrification. The Cs removal will be conducted with crystalline silicotitanate (CST) ion exchange media. Under the planned waste-processing strategy, the tank waste supernate will be queued for TSCR processing in tank 241-AP-107 (AP-107). Once AP-107 tank waste volume is sufficiently depleted, the waste supernate from tank 241-AP-105 (AP 105, the holding tank before transfer to AP-107) will be transferred to tank AP-107. Supernate from another tank will be transferred to the holding tank, AP-105, for eventual transfer to tank AP-107. These supernate streams will undergo blending in tanks AP-107 and AP-105; the volume blend ratios will be driven by how much the tank waste supernate volumes are depleted before the next tank waste is added. The consequence of tank waste blending on Cs uptake by CST was of interest and was tested via batch contacts; results are reported herein. Testing was conducted on AP-107, AP-105, and 241-AW-102 (AW-102) tank wastes that were first processed through CST ion exchange beds to strip 137Cs (and Cs), reducing the radiation dose and allowing the matrices to be contact-handled by laboratory personnel during this testing. Tank waste volume blend ratios represented the upper and lower bounding volume blend ratios, 21.8 and 3.8, respectively, along with an intermediate blend ratio, 12.8. (Note that a larger value of the volume blend ratio represents a larger difference between the two volumes blended, so the blend ratio of 21.8 results when the maximum amount of supernate has been removed from the feed tank, leaving minimum heel, before supernate from the holding tank is added.) Cs isotherms were then developed for each tank waste blend at two different temperatures (15.7 and 24.3 °C). The blended tank waste results were compared to the unblended tank wastes. Small (10% to 15%) increases in Cs loading Q values (mmoles Cs per g CST) were observed at 15.7 °C for the blended tank wastes, but no substantive changes in Q values were observed at 24.3 °C. Q values agreed within 6% relative standard deviation for a given set of three waste blends at a given temperature. Table S.1 summarizes the Q values for each tank waste blend and temperature. None of the tank waste blends exceeded the TSCR documented safety analysis (DSA) limit for 137Cs loading (141,600 Ci, corresponding to 238 Ci 137Cs/kg CST). Unblended AW-102 tank waste was processed to determine Q values at three process temperatures (13.4, 15.7, and 24.3 °C). The Q value vs. temperature curve fit was similar to those previously reported, where Q = -0.00226 ? T + 0.1008 (corresponding to a DSA threshold at 1.4 °C process temperature, T). There is no conceivable process temperature where AW-102 tank waste could be expected to exceed the DSA 137Cs loading limit without causing other process issues such as extensive salt precipitation.