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 aluminium and alkaline, or caustic sodium compounds, mixing with high-level radioactive material, said Cliff Johnston, lead author and Purdue University environmental scientist. It is also said to document the metamorphosis of the high aluminium 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 US Department of Energy’s Hanford Nuclear Reservation. Congress has ordered removal of the waste to permanent storage.
The radioactive waste is primarily a combination of aluminium-clad nuclear fuel rods and caustic solutions added to the storage tanks to break down the rods and minimise corrosion of the tanks. This created compounds high in aluminium but with unknown chemistry. In studying how the different aluminium compounds in the tanks transform from a soluble liquid to a solid form, the scientists are also said to be learning more about how to handle the highly toxic waste.
‘We’ve gained new information about the chemistry of aluminium in these very concentrated waste solutions,’ said Johnston, a Purdue agronomy professor. ‘The significance of that is related to two different areas: minimising 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 aluminium in waste material samples. Next, they used a graph, called a phase diagram, to determine at what points the aluminium 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 fibreoptic 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 could potentially save US taxpayers millions of 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 US 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 aluminium compounds inside or outside the tanks, to predict when that change occurs, and to aid in dealing with spilled waste.
The combination of the aluminium 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 colour-coded pH sensor, to detect undesirable chemical composition, Johnston said.
‘We know that with caustic waste under very highly alkaline conditions, the aluminium is stable in solution,’ he said. ‘This is important because the tanks are leaking with a high concentration of aluminium 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 aluminium 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 aluminium itself is not. It is an unstable metal and so changes forms relatively easily. When an aluminium compound precipitates and becomes a solid, it can take the radioactive material with it.