A next-generation nuclear fuel could soon be cleaning up last generation's nuclear mess.
Scientists from the University of Notre Dame have created grains of thorium, a proposed fuel for future nuclear power plants, that soak up highly radioactive materials like technetium produced during the reprocessing of spent nuclear fuel.
The tiny grains of the chemical element can be tailor-made to absorb any ion, not just harmful radioactive ions, and could be used to clean up a wide variety of environmental contaminants while dropping the cost to store the most dangerous and radioactive waste.
"When plutonium is extracted [from used nuclear fuel] you end up with millions of gallons of high energy radioactive waste that contains some very bad things," said Thomas Albrecht-Schmitt, a scientist at the University of Notre Dame and co-author of a recent paper in the journal Angewandte Chemie International Edition. "This is one of the very few materials that can trap and remove those things."
Right now the best material for removing radioactive ions is clay. The harmful ions have a slight positive charge. The clay has a slight negative charge, which draws ions out of the solution and locks them in place. The clay, along with all the radioactive material it has absorbed, can then be safely stored in a high level nuclear waste containment site, such as an old salt mine.
Clay does an great job at removing these harmful ions. Almost too good of a job in fact. Besides removing powerful beta emitters like uranium, clay also removes less harmful ions like nitrates as well. If the dangerous and the not so dangerous could be stored separately, instead of together, the government and nuclear industry could save money.
The exact amount the potential savings is uncertain. So far the thorium cubes have only been tested in controlled laboratory conditions, where they removed about 70 percent of the technetium in a sample. By optimizing the size, shape and charges of the thorium cubes Albrecht-Schmitt estimates at least 90 percent of the technetium could be removed from a sample.
Field testing will soon begin at the Savannah River National Laboratory in South Carolina that will hopefully answer the money question soon.
"The material could be used as filler to prevent or reduce the chance release of radioactive material from that repository for thousands or even hundreds of thousands of years," said David Hobbs, from the Savannah River National Laboratory, who will be testing the new thorium crystals soon.
To the human eye, each crystal resembles a grain of white, ionized salt. When a cup of the crystals is dropped into into solution, however, the crystals change color depending on which ions they absorb. Chromate turns the crystals yellowish orange. The prettiest is the toxic pertechtinate, says Albrecht-Schmitt, which turns the crystals a nice shade of purple.
Albrecht-Schmitt refers to the thorium borate grains as cubes, but at their tiniest they actually have eight sides each, like two pyramids stuck together at the base. All eight sides of the thorium borate are riddled with billions of tiny, oval-shaped pores less than one nanometer across.
The tiny size of the pores are ideal for soaking up the harmful positive ions while leaving the less harmful ones behind — and so are their charges. The inside of each pore carries a slight negative charge that attracts and traps the slightly positive metal ions, removing them from solution.
Its the electrical charge the makes this material so unique. Of all the series of elements in the periodic table the Notre Dame scientists tested, thorium was the only material that had negatively charge pores. Every other metal or combination of metals had positively charged pores.
To create the thorium cubes, the Notre dame scientists first heated a combination of thorium and boric acid to 220 degrees Celsius (428 degree Fahrenheit). Once the mixture hardened, the scientists poured water over the mix, dissolving the excess boric acid and leaving the crystals on the bottom of the tank, where they can then be scooped up and used.
The crystals only remove ions suspended in liquids; the actual solid materials or sludge is formed into glass bars and stored separately. Once removed however, the thorium cubes could be added to the sludge, storing the most radioactive components of nuclear fission together in one place.
Besides its ability to remove nuclear waste, thorium is also a contender for next-generation nuclear fuel reactors that would run cleaner and with less waste and cheaper raw materials (due to a greater amount of available thorium) than current uranium-based reactors.