Purdue News
Purdue News
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May 6, 2002 New research could spearhead permanent nuclear waste storageWEST LAFAYETTE, Ind. – Researchers armed with a laser are closer to knowing how to prepare millions of gallons of highly radioactive nuclear waste for permanent storage. Their study is the first to describe the chemistry of waste formed by aluminum and alkaline, or caustic sodium compounds, mixing with high-level radioactive material, said Cliff Johnston, lead author and Purdue University environmental scientist. It also documents the metamorphosis of the high aluminum content waste from liquid to solid. This knowledge will be applied to the permanent disposal of the 53 million gallons of radioactive material held in 177 giant underground tanks at the Department of Energy's Hanford Nuclear Reservation in Richland, Wash. Congress has ordered removal of the waste to permanent storage. The report is published on the Environmental Science and Technology Web site and scheduled to appear in the journal's June 1 print issue. The radioactive waste is primarily a combination of aluminum-clad nuclear fuel rods and caustic solutions added to the storage tanks to break down the rods and minimize corrosion of the tanks. This created compounds high in aluminum but with unknown chemistry. In studying how the different aluminum compounds in the tanks transform from a soluble liquid to a solid form, the scientists also are learning more about how to handle the highly toxic waste. "We've gained new information about the chemistry of aluminum in these very concentrated waste solutions," said Johnston, a Purdue agronomy professor. "The significance of that is related to two different areas: minimizing high-level nuclear waste volume for permanent storage, and eventually determining what happens to the material when it leaks out of the tank." Scientists want to be able to decrease the waste volume by evaporating as much water as possible from the crusty sludge. This will make transport easier and storage less costly, Johnston said. But in order to do this, they need to understand the chemistry of the waste when it becomes a solid. Johnston and his team of researchers used a Raman spectrometer that employs a laser to measures molecular vibrations so they can study the soluble forms of aluminum in waste material samples. Next, they used a graph, called a phase diagram, to determine at what points the aluminum compounds, or aluminates, cascade from a stable, soluble solution to a solid state. "We have established a clear relationship between the chemical composition of this waste material and the data from the spectrometer," Johnston said. In addition, he said the Raman spectrometer method could be developed into a fiberoptic instrument that would allow testing of material in the tanks from a distance. This would be non-invasive and eliminate the need for someone to physically scoop a sample of the heterogeneous material from the dangerous tank environment for lab testing. The new testing method potentially could save millions of taxpayers' dollars annually. The high-level radioactive waste currently is in temporary tanks at DOE's Hanford site. Most of the storage tanks are at least 50 years old, 30 years older than the original intended usage, according to the Tri-Party Agreement, a consortium of the DOE, the U.S. Environmental Protection Agency and the Washington State Department of Ecology. Of the 177 tanks, 149 of them are have only one outer wall, and 67 of these single-shelled tanks are suspected to have leaked an estimated 1 million gallons of high-level waste. When the waste seeps into soil, it precipitates into a solid phase with unknown chemistry, Johnston said. Ultimately, the knowledge gained in the current study will help researchers determine the chemistry of the solid forms of aluminum compounds inside or outside the tanks, to predict when that change occurs, and to aid in dealing with spilled waste. The combination of the aluminum and the sodium solutions created a material in the tanks with a pH of 14. Soils that will support plant growth have a pH of 4-8. The Raman spectrometer method possibly could act as a sophisticated type of litmus paper, or a color-coded pH sensor, to detect undesirable chemical composition, Johnston said. "We know that with caustic waste under very highly alkaline conditions, the aluminum is stable in solution," he said. "This is important because the tanks are leaking with a high concentration of aluminum and a high pH and forming a new waste form. "The pH is lowered when the material leaks out of the tank and mixes with the soil or into the subsurface material. In order to decrease the volume of the waste and deal with any that has leaked into the soil, we ultimately must learn the conditions necessary for the soluble forms of aluminum to be transformed into a solid." Congressional mandates call for the DOE to begin moving the nuclear waste to permanent storage facilities by 2007. Recently the Tri-Party Agreement pledged to complete cleanup at Hanford between 2025 and 2035, 35 to 45 years sooner than originally expected. The 560-square-mile Hanford site is not the only one the DOE must clean up. The Savannah River Nuclear Reservation in Georgia stores an estimated 35 million gallons of high-level nuclear waste in tanks similar to those at the Washington facility, according to a report done for the U.S. Senate Committee on Energy and Natural Resources. For cleanup and testing at Hanford alone, the government is spending approximately $1.5 billion annually. The DOE is asking for $2 billion for fiscal year 2003. The final repository site must meet the criteria of safely storing the material for 10,000 years. The radioactive stew cooking in the huge storage tanks became a chemical mystery mainly because, as the ingredients broke down, the contents of one tank was poured into another. The mixing of tank contents resulted from lack of storage space, according to researchers. Though the waste is radioactive, the aluminum itself is not. It is an unstable metal and so changes forms relatively easily. When an aluminum compound precipitates and becomes a solid, it can take the radioactive material with it. The other researchers on this study are: Stephen Agnew, Archimedes Technology Group Inc.; Jon Schoonover, Jill Osborn and Rob Corbin, Materials Science and Technology Division, Los Alamos National Laboratory; John Kenney III, Chemical Physics Laboratory, Concordia University, Irvine, Calif.; and Bobbi Page, Chemical Physics Laboratory, Eastern New Mexico University. The Environmental Management Science Program at Los Alamos National Laboratory and the Department of Energy provided support for the study. Writer: Susan A. Steeves, (765) 496-7481, ssteeves@aes.purdue.edu Source: Cliff Johnston, (765) 496-1716, clays@purdue.edu NOTE TO JOURNALISTS: A copy of the study referred to in this release is available from the Agricultural Communication Service. Related Web site: Ag Communications: (765) 494-2722; Beth Forbes, bforbes@aes.purdue.edu; https://www.agriculture.purdue.edu/AgComm/public/agnews/ ABSTRACT Raman Study of Aluminum Speciation Cliff T. Johnston, Stephen F. Agnew, Jon R. Schoonover, The chemistry of concentrated sodium aluminate solutions stored in many of the large, underground storage tanks containing high-level waste (HLW) at the Hanford and Savannah River Nuclear Reservations is an area of recent research interest. Not only is the presence of aluminate in solution important for continued safe storage of these wastes, the nature of both solid and solution aluminum oxy-hydroxides is very important for waste pretreatment. Moreover, for many tanks that have leaked high aluminum waste in the past, little is known about the speciation of Al in the soil. In this study, Raman spectroscopy has been used to investigate the speciation of the aqueous species in the Al2O3 — Na2O — H2O system over a wide range of solution compositions and hydration. A ternary phase diagram has been used to correlate the observed changes in the spectra with the composition of the solution, and with dimerization of aluminate that occurs at elevated aluminate concentrations (> 1.5 M). Dimerization is evidenced by growth of new Al-O stretching bands at 535 and 695 cm-1 at the expense of the aluminate monomer band at 620 cm-1. The spectrum of water was strongly influenced by the high concentrations of Na+ and OH- (> 17 M). Upon increasing the concentration of NaOH in solution, the d(H-O-H) bending band of water (n2 mode) increased in frequency to 1663 cm-1, indicating that the water contained in the concentrated caustic solution was more strongly hydrogen bonded at the higher base content. In addition, the sharp, well-resolved band at 3610 cm-1, assigned to the n(O-H) of free OH-, increased in intensity with increasing NaOH. Analysis of the n(O-H) bands in the 3800 to 2600 cm-1 region supported the overall increase in hydrogen bonding as evidenced by the increase in relative intensity of a strongly hydrated water band at 3118 cm-1. Taking into consideration the activity of water, the molar concentrations of the monomeric and dimeric aluminate species were estimated using the relative intensities of the Al-O stretching bands from the Raman spectra. A constant apparent log Kdimer value was obtained at aluminate concentrations > 1.5 M with a value of 0.97 (±0.04) at ~25ºC. This study represents the first spectral-based estimation of a thermodynamic equilibrium constant for the Al2O3-Na2O-H2O system.
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