Separating the wolves from the sheep in spent nuclear fuel disposal
A collaboration of U.S. and Russian scientists and engineers have made treating nuclear waste safer and cheaper with a new, one-step process that separates out much of the radioactive material.
"The idea is to segregate out this very small amount of radioactive material and concentrate this element of the waste into the smallest volume possible," said Scott Herbst, a chemical engineer at the Department of Energy’s Idaho National Engineering and Environmental Laboratory.
This month, he and a team of scientists from the INEEL and Khlopin Radium Institute in Russia received an $800,000 three-year grant from DOE’s Environmental Management Science Program to study and improve their solution. The Universal Extraction, or UNEX, process is the first demonstrated technology of its kind capable of removing multiple radioactive elements from high-level nuclear waste in one step.
Just a sprinkling of radioactive elements turns volumes of waste into "high-level radioactive waste," subject to rigorous and expensive storage standards.
Such waste is a byproduct of nuclear energy and weapons development and usually contains a mixture of intensely radioactive fission products (e.g., strontium-90 and cesium-137) and long-lived radioactive elements (e.g., plutonium and americium, the actinide elements) plus hazardous and toxic materials. Separating most of the radioactive elements from the other materials can shrink the volume of high-level waste, reduce the total disposal cost and minimize potential harm to the people and environment surrounding it.
In the past, however, it has been difficult to remove more than one radioactive element at a time. The default process requires three separate steps. One solution removes cesium-137, the next takes out a group of similar elements called the actinide elements, and the last removes strontium-90. But sending waste through three different steps is time-consuming and expensive. At the moment, most countries don’t go to the trouble and expense of separating out radioactive elements. They simply take the entire volume of high-level waste, solidify it into a glass, and bury it whole in large stainless steel canisters for long-term storage.
In 1994, a few INEEL scientists traveled to Russia to exchange the technologies each country had independently developed for nuclear waste cleanup. "The idea in the Cold War was that we didn’t talk to each other. So the Russians developed separate separations techniques than we did," Herbst said. After examining what each had, they jointly came up with an extractant that works better than any of the original extractants alone. UNEX removes radioactive strontium, cesium and the actinides at once.
"We’re combining three separate operations into one," said Herbst. "I’m mesmerized that we’ve even been able to get this thing to work. It flies in the face of what everyone has attempted to do before."
Using the UNEX process, the scientists trim the volume of high-level waste at least twentyfold; each gallon shrinks to less than a cup. And the twentyfold volume reduction leads to a corresponding reduction in disposal costs. The majority of the waste left over after the UNEX separation is far less expensive to treat and store than the highly radioactive portion.
So why does UNEX work so well? At the moment, the scientists have only a general idea. "I would call the development efforts to date a brute-force method," said Herbst. He knows what happens in a simple extraction. The scientists mix an extracting solution -- something that won’t mix with the waste, like oil and water -- with the waste, and the extracting solution preferentially snatches radioactive molecules out of the mixture. But the UNEX extraction mechanism is far more complicated.
"When you start trying to pull out a number of things it becomes extremely hard to control the process," Herbst said. "The bottom line is you’ve got three different active extractants." The process can be likened to three soccer games occurring simultaneously on the same field. The players are bound to run into each other or in some way affect the other games, just as the extractants -- or the molecules seizing out the hazardous radionuclides -- bump into each other or otherwise affect each other’s work. In this case, Herbst, said, the extractants "tend to complement one another. There is a synergy between the extractants. Each extracts better in the presence of the others than they typically would alone."
To understand the nature of the extractions, and the intriguing synergy between them, the scientists will use three sets of tools.
First, they will apply what Herbst calls "wet chemistry." They’ll perform mini-extractions in the laboratory, and, through a numerical analysis, deduce the molecular structures in the extractant.
The second technique they will apply, spectroscopy, will help them figure out the arrangements of the molecules that form. Chemists use a variety of instruments to take advantage of a molecule’s response to light or a magnetic field and create a partial picture of a molecule and its environment. It is as if the chemists receive a photograph that has been cut up into small pieces.
They then try to puzzle out the correct place for each piece. Herbst doesn’t expect the pictures will be simple. Because the ability of one extractant is enhanced by the presence of the other extractants, large, elaborate complexes are forming. "It will really be unique," he said. "I’m not sure researchers often have the opportunity to examine complicated systems such as this."
The last tool they will use, also a spectroscopy technique, is a powerful technique for determining structures. X-rays generated at the Stanford Linear Accelerator at Stanford University can be used to give structural detail about the molecular compounds that form during the UNEX extraction. The instrument is like a powerful microscope that can show the arrangement of atoms inside a molecule.
In the end, the goal is to understand the one-step UNEX extraction well enough to improve it and further shrink the volume and cost of nuclear waste disposal.
Herbst’s team on the UNEX project includes Vasily Babain and colleagues at the Khlopin Radium Institute in St. Petersburg, Russia, and Sue Clark at Washington State University. His colleagues at INEEL include Tom Luther, Fred Stewart, Terry Todd, Jack Law, Dean Peterman and George Redden. John Bargar from the Stanford Linear Accelerator Center will assist with the X-ray spectroscopy. Their work supports DOE’s environmental management mission.
The INEEL is a science-based, applied engineering national laboratory dedicated to supporting the U.S. Department of Energy's missions in environment, energy, science and national defense. The INEEL is operated for the DOE by Bechtel BWXT Idaho, LLC.
Media contacts: Deborah Hill, 208-526-4723, dahill@inel.gov; Teri Ehresman, 208-526-7785, ehr@inel.gov, Louisa Dalton, 208-526-3176, daltlw@inel.gov
Technical contact: R. Scott Herbst, 208-526-6836, herbrs@inel.gov
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