June 20, 2001 COLLEGE STATION - Disposal of nuclear waste has always been a hot topic, but a Texas A&M University chemist's new approach could lead to new waste treatment procedures - and even a boost to nuclear medicine.
A main component of President George W. Bush's energy policy is to increase use of nuclear energy. However, according to Abraham Clearfield, a professor of chemistry at Texas A&M, "to accept this part of Bush policy, the general public must be confident that nuclear waste disposal will be effectively dealt with."
One of the most common ways to dispose of highly radioactive waste is to use devices similar to water softeners called ion exchangers, which are either inorganic - mineral-type - compounds or synthetically produced organic resins.
An ion exchanger usually contains a harmless element such as sodium, present in ordinary salt, which is exchanged for a harmful element such as cesium 137, present in radioactive waste, says Clearfield.
Clearfield has been developing inorganic ion exchangers for more than 30 years. He has been studying their role in nuclear waste for 10 years in collaboration with Pacific Northwest National Laboratories and the Savannah River Site, a weapons research facility based in South Carolina.
Nuclear waste coming from nuclear weapons plants is made of highly radioactive elements, mainly strontium 90, cesium 137 and plutonium 239 and 240, as well as other less radioactive elements.
The highly radioactive waste is either extracted by a solution that does not mix with the waste solution - a process called solvent extraction - or is removed by ion exchangers. The high-level wastes are then to be immobilized in a special glass, placed inside steel drums and buried about 1,000 feet deep in salt mines, in sites to be designated. The remaining low-level waste may then be encased in cement and stored on site at Hanford, Wash., and the Savannah River Site, S.C.
The inorganic ion exchangers remove cesium and strontium 90, while plutonium is handled separately. Clearfield and his collaborators have devised more than a dozen of these exchangers. Among them is a class of crystals called titanium silicates that have tunnel structures containing sodium ions. One of the most important was developed at Sandia National Laboratory, by the late Robert Dosch and Rayford Anthony of Texas A&M's Department of Chemical Engineering.
"In these tunnels, sodium ions are very loosely held," explains Clearfield. "Because cesium ions are bigger than sodium ions, when a cesium ion goes in and replaces a sodium ion, it cannot move around like the sodium ion. Instead it gets trapped."
In other inorganic ion exchangers, the ingoing and outgoing ions can each have different charges or the channels have different sizes. To study the exchangers' properties, Clearfield and his collaborators study their crystal structure by X-ray diffraction before and after the exchange of different types of ions.
"We try to make compounds in which either a sodium or a potassium ion is exchanged, and then we do the crystal structure," says Clearfield. "We try to exchange a given ion species with these crystals and then we do the crystal structure again, and we see what has happened to the ingoing and outgoing species. It can take from a few weeks to many months before we understand what happened."
Inorganic ion exchangers can also be used in nuclear medicine. Radioactive elements with short half-lives currently are used to determine blood flow or to locate a tumor. With the ion exchanger, it might be possible to better target the tumor by sparing surrounding healthy cells.
"If you could target a radioactive species directly into the tumor," says Clearfield, "and the health physicist would calculate, from the size of the tumor, how much radioactivity to inject, you would not damage the healthy tissue around."
Work is in progress and part of a project with Lynntech, Inc., a technology development company based in College Station, where most of the scientists are Texas A&M alumni.
"The first phase of that work has just been completed," Clearfield says. "We are now waiting for a second phase of funding on the project."
Clearfield has shown that inorganic materials exchange ions more efficiently than organic materials, and they can better withstand radiation as well.
"For applications in nuclear waste and nuclear medicine, organic exchangers can only do part of the job," he says," because radioactivity may destroy the carbon-carbon bonds, which are essential in organic compounds."
Clearfield is eager to participate in a major project currently being set up by the European Commission, called the European Consortium. Focusing on the many applications of inorganic ion exchangers, the project will be led by the University of Helsinki in Finland, with groups at the University of Aveiro in Portugal, and the University of Salford in the United Kingdom, and four industrial firms.
Clearfield says that work on inorganic exchangers is far from being over.
"There are thousands of naturally occurring inorganic materials that can be used," he says. "Some of them are clays, others are natural minerals. Having solved their structure, we can use the information to synthesize materials that could select, by removing them, harmful species from the environment or industrial processes."
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