June 2000

Lab technology vies for Savannah River project role

Another ORNL technology has been added to the list of candidates for a cesium removal project at the Savannah River Site. ORNL Reporter described the collaboration between ORNL and the South Carolina facility in last October’s issue. SRS is studying methods to remove cesium and other contaminants from high-level salt waste stored in underground tanks there.

Based on results obtained in the coming year by collaborators at ORNL, Argonne National Laboratory and the SRS, a technology will be selected and ultimately imple- mented in a billion-dollar facility at the SRS.

Two technologies that have been under consideration are small tank TPB precipitation and an ion-exchange process.

Since the October article appeared, a young technology has been rapidly developing and is now being considered. It’s a solvent extraction process called the CSEX process, for cesium extraction. The Chemical and Analytical Sciences Division’s Bruce Moyer is the lead researcher.

“Our technology is very new, but it was a front runner early on,” Moyer says. “Back in 1998 when the choices were being narrowed down, the other two technologies were much further along. Owing to the aggressive schedule at the SRS, CSEX was actually dropped from the list for a while.”

A National Academy of Sciences report on the project recommended resuming testing on solvent extraction, which removes cesium from a waste liquid by use of an extractant that has a strong and selective affinity for the element.

“It is a very interesting molecule,” Moyer says of the extractant, called a calixarene. “The molecule forms a cavity that is very complementary to cesium but much less so for other metals.”

By itself, the calixarene is a poor extractant, but Peter Bonnesen in CASD’s Chemical Separations group devised an additive, or “modifier,” that boosts the calixarene’s performance. The solvent, composed primarily of the calixarene and the modifier in a process-suitable diluent, is mixed with the waste, resulting in the extraction of cesium into the solvent. The mixing is carried out in seconds in elegant centrifugal contactors being demonstrated in ORNL’s Chemical Technology Division and at Argonne.

After the cesium is extracted, the loaded solvent easily releases its cesium upon mixing with very dilute nitric acid, regenerating the resusable solvent. One of the primary advantages of the CSEX process is that it produces a purified stream of concentrated cesium that is highly compatible with vitrification, Moyer says.

Moyer admits that the calixarene is expensive, but because of its low concentration and the use of high-throughput contactors, the overall cost of the process remains competitive in cost with the other two technologies. One of the pleasant lessons he says he has learned in developing this technology is that elegant molecules have a place in industrial processes such as nuclear-waste cleanup: High performance offsets the high cost of the molecule.

Testing on the solvent-extraction process will be performed at ORNL, Argonne, and the SRS in the summer and fall of 2000. “What makes the CSEX process exciting for ORNL is that it is an ORNL invention,” Moyer says.—B.C.