A scientific breakthrough by ORNL researchers is allowing millions of gallons of dangerous waste in South Carolina to be removed from the environment and processed for safe disposal.
The waste is a toxic, highly radioactive soup left over from the Savannah River Site’s days creating and processing plutonium. Thirty-seven million gallons was created in a process (invented at ORNL) known as PUREX, for “plutonium and uranium redox extraction.”
The straightforward part of the cleanup has long since been completed. By raising the pH level of the waste with sodium hydroxide, the site’s overseers induced all but one of the most radioactive isotopes to drop to the bottom of the tanks as a sludge. The sludge has been sluiced out and is being turned into glass through a process called vitrification.
But that remaining isotope, cesium-137, was not so cooperative, remaining soluble in the salt portion of the waste. Its presence meant that the bulk of the waste was still highly radioactive, and regulators have demanded that it be separated and vitrified as well.
Science to the rescue
To remove the cesium, ORNL scientists developed a solvent-extraction process that uses both complex chemistry and engineered molecules to isolate, transfer, and ultimately concentrate the isotope. At that point, it is ready to be combined with other radionuclides and vitrified.
The scientists’ chemical achievement, as scaled up, demonstrated, and implemented at SRS, earned the 76-member Salt Waste Disposal Technologies Team a 2013 Secretary’s Achievement Award from Energy Secretary Ernest Moniz.
Team leader Bruce Moyer of ORNL’s Chemical Sciences Division said the accomplishment is noteworthy at least in part because ORNL’s impact had grown out of its science spinning off into environmental management.
“ORNL has really a lot to crow about,” Moyer said. “This isn’t our waste, and we don’t really have a very big EM program at the lab.
“What we are strong at is science. We’d been studying this kind of chemistry for 10 years before we even started on the waste cleanup problem. And so when the real problem came along, we were ready to jump in and make this chemistry work, which no one else really had done.”
The key to the process is a molecule known as a calixarene–crown ether, which takes a ring-like calixarene molecule and adds a separate crown ether molecule to cap one end, thereby creating a cage. After much work, the team came up with a cage molecule so effective that it captures nearly all of the cesium while leaving other elements almost entirely alone.
As good as the cages are, however, they need help. Over the course of a decade and a half, the team had to clear some very complex hurdles.
Challenges to overcome
For instance, the cage molecules are contained in a solvent called Isopar L, but cesium ions won’t go into the solvent unless they are paired with negatively charged ions. So the team added a modifier to the solvent that brings in nitrate anions found within the waste salt solution. Moyer credits Peter Bonnesen, a chemist now with ORNL’s Center for Nanophase Materials Sciences, with engineering the clever structure of this modifier, for which ORNL holds a patent.
In addition, the cesium had to be coaxed out of the cage molecules and removed from the solvent. That took two approaches: first, a molecule that makes the cesium ions pop out of the cage molecules, and second, a boric acid stripping solution that pulls the released cesium out of the solvent.
The process has gone through two iterations, with the first removing 99 percent of the cesium. This was better than the original requirement of about 92 percent, but Moyer and others wanted to do much better. The revised solvent, known as NGS, or “Next Generation Solvent,” includes a more soluble calixarene molecule for capturing the cesium as well as guanidine for stripping cesium out of the solvent.
This improved efficiency better than a hundredfold, to 99.9975 percent, so that the process leaves behind only one part in 40,000 of cesium in the original waste.
Using the tools of science
To get to this achievement, the team used a variety of scientific tools, including organic synthesis, X-ray crystallography, nuclear magnetic resonance spectroscopy, radiometric tracer methods, and computational modeling.
Engineers collaborating on the project—including Joe Birdwell at ORNL, Mark Geeting at SRS, Terry Todd at Idaho National Laboratory, and Monica Regalbuto and the late Ralph Leonard at Argonne National Laboratory—scaled up the process using centrifugal contactors, and Sam Fink’s group at Savannah River National Laboratory was instrumental in testing and demonstration.
“You name it, and our team did it,” Moyer said. “We studied the structure of the complexes of cesium nitrate and other metals with a couple dozen calixarene–crown ethers. We had a couple of really good organic chemists, and we got to be experts at the synthesis. Then we studied the structure using X-ray crystallography and NMR spectroscopy.
“Then we did the thermodynamics—basically figuring out how strong is the extraction and what is the chemical way you can describe and model the extraction.”
The project is using a pilot facility at SRS while the $2 billion-plus Salt Waste Processing Facility is being constructed. Moyer said the project is an enormous achievement in technology transfer for ORNL, even though it is a government-use application rather than a commercial one and brings in no licensing fees.
“There are no royalties involved, even though its impact is billions and billions of dollars. It, therefore, doesn’t show up in the lab’s commercialization metrics, but it’s one of ORNL’s biggest commercial successes.
Separating and treating cesium: the process
STEP 1: Getting to the cesium
1. Start with a kerosene solvent called Isopar L.
The kerosene acts as the oil in an oil-and-water approach.
2. Add calixarene–crown ether cage molecules.
Calixarenes are ring-like molecules. When they are capped at one end with a crown ether molecule, the resulting cage captures cesium so well that it ignores other elements in the waste such as potassium and sodium.
3. Add a solvent modifier called Cs-7SB
The cesium ions must be paired with negatively charged ions—especially the abundant nitrate anions in the waste—before they can be coaxed into the solvent. The solvent modifier brings in the nitrates.
4. Effectiveness: 99.9975%
In other words, the combination of solvent, solvent modifier and cage molecules pulls out nearly all of the cesium, leaving only one part in 40,000.
STEP 2: Separating the cesium from the cage molecules and solvent solution
1. Add a substance called guanidine to the solvent and then strip it with boric acid that has been mixed with water
The solvent solution costs about $10,000 a gallon. That means you have to reuse it, and that in turn means you have to remove the cesium. Guanidine causes the cesium ions to pop out of the cage molecules, and boric acid strips them out of the solvent.
STEP 3: Dispose and reuse
1. Send the boric acid and cesium off for vitrification
The cesium is now in the boric acid and ready to be turned into a glass through a process called vitrification.
2. Send the treated solvent back for reuse
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