To demonstrate that the caustic-side solvent extraction (CSSX) process could remove cesium from Savannah River Site (SRS) high-level waste over long periods of time, an improved minicontactor (2-cm centrifugal contactor) was needed that could be operated for several days. In particular, the contactor temperature had to be controlled and contactor hydraulic performance needed to be improved. Because the process was to be continuous, provisions were made for a three-shift operation. With the improvements made and the operators trained, the CSSX process was run in a 33-stage minicontactor over a period of three days to remove cesium from an average SRS simulant for the waste feed. The two key process goals were achieved: (1) the cesium was removed from the waste with decontamination factors greater than 40,000 and (2) the recovered cesium was concentrated by a factor of 15 in dilute nitric acid. These goals were maintained for 71 h as 1.4 L of solvent was recycled 42 times while processing 180 L of SRS simulant. The average decontamination factor for cesium was 157,000 and the average concentration factor was 14.9. The process had to be shut down twice for minor problems, which were fixed and testing resumed. This multi-day demonstration confirmed that the CSSX process could be used to help decontaminate the millions of gallons of SRS waste now stored in underground tanks.
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