New Methods Developed to Help Isolate Radioactive Waste
|By Allan Chen, A_Chen@lbl.gov
November 18, 1996
BERKELEY, CA -- Using X-ray absorption spectroscopy, scientists at Ernest Orlando Lawrence Berkeley National Laboratory have developed methods to examine the in-situ chemical properties of technetium, a component of radioactive waste. This method holds promise for detecting a variety of other chemical species in radioactive wastes, and for evaluating the safest way to isolate these radioactive elements from the environment.
Technetium (Tc) is element number 43 in the periodic table. The radioactive isotope Tc-99 is abundant in nuclear waste and has a long half-life, about 200,000 years. Released into the environment, Tc-99 is extremely damaging, traveling up the food chain, and causing cancer in humans.
Radioactive wastes are a growing problem throughout the world. The Savannah River Site in South Carolina has some 160 million gallons (600 million liters) of low-level wastes stored on-site awaiting a permanent disposal solution.
"The question is: What is going to be the ultimate method of isolating waste containing technetium? One possibility is to put it into a cement waste form," says Deputy Division Director Norman Edelstein of Berkeley Lab's Chemical Sciences Division.
The problem is that technetium in cement takes on a form that is water-soluble. "Technetium's most common oxidation state is the pertechnetate ion, (TcO4-), which is a very soluble material. So if the method of mixing cement with radioactive waste containing pertechnetate is to be successful, it's necessary to reduce the technetium to a less soluble material, such as TcO2," says Edelstein.
"This research started as a project with colleagues working at the Savannah River Technology Center," says Jerome Bucher, another member of the Chemical Sciences Division team. Chris Langton of the Savannah River Technology Center, and Sue Clark of Washington State University had developed a cement formulation for immobilizing waste that includes blast furnace slag and fly ash mixtures.
"The slag was supposedly a reducing agent, but they were not sure how effective it was. They didn't have a good technique to examine Tc in the cement-blast furnace slag mix. X-ray absorption spectroscopy is ideal for this problem. We were able to look at many of their formulations and determine that the technetium was not completely reduced from the pertechnetate form by the blast furnace slag materials" says Bucher. "Then we became interested in trying to figure out what components of the slag were actually doing the chemical work. So we looked at various additives in the slag."
X-ray absorption fine structure (XAFS) spectroscopy was the method of choice for studying technetium in cement. "It is the only technique that we know of that can determine the oxidation state of metal ions in a cement, and it also provides information on the identity of other atoms near the Tc." says Edelstein.
The oxidation state is determined by the number of electrons added to or subtracted from the elemental atom. An element with positive or negative charge is called an ion. The oxidation state, combined with information about nearby atoms, indicates whether the technetium is in a mobile, water-soluble form, or a in more insoluble form.
To obtain data using XAFS, researchers irradiate a sample contained in a cell just millimeters thick, and about two centimeters long, with X-rays. This ejects an inner shell electron of the ion, and other electrons fall back to the inner shell emitting radiation that is characteristic of the particular element that the ion is based upon. The characteristic radiation emitted from the ion is measured as a function of X-ray energy to determine the oxidation state of and chemical species containing the ion. Because inner shell electrons are bound so tightly to their nuclei, extremely high-energy radiation is necessary to push them into a higher shell -- thus the need for X-rays.
"One of the real benefits of XAFS is that you can get the oxidation state of the technetium in the cement without chemically separating it from the cement, along with structural information about the nearby atoms," says team member David Shuh. XAFS works on materials like cement that don't have a crystalline structure, and are known as amorphous materials.
The team prepared a series of cement mixtures, each containing simulated technetium waste and a different component of blast furnace slag, and then measured the oxidation state of the Tc in each sample. They found that iron sulfide (FeS) and sodium sulfide (Na2S) were equally effective chemical additives for reducing the pertechnetate ion into more stable forms. Their research also demonstrates that XAFS spectroscopy is a useful tool for studying the chemistry of technetium in radioactive waste forms.
The success of this work suggests that XAFS will be applicable to the study of a wider variety of radioactive contaminants isolated in cement, as well as help to evaluate the effectiveness of other methods for isolating radioactive wastes.
"This technique can be used for studying other radionuclides in cement in addition to technetium." says team member Pat Allen. "It can also help improve the processing techniques or the choice of waste form by telling us which waste forms are effective at keeping a species isolated." The team is planning to study other methods of long-term radioactive waste storage, including mixing waste with glass (vitrification), and with ceramic materials.
A paper describing this research will be published in Radiochimica Acta later this year.
Berkeley Lab is a U.S. Department of Energy national laboratory located in Berkeley, California. It conducts unclassified research and is managed by the University of California.