The Cold War left behind significant amounts of low-level nuclear waste, but the future may be a bit cleaner thanks to a team of scientists at the US Department of Energy's Pacific Northwest National Laboratory (PNNL), who have vitrified low-level radioactive waste for the first time in a continuous process. In a laboratory set up, 3 gal (11.4 L) of waste taken from a tank at the decommissioned Hanford Site nuclear production complex in Washington state was turned into a form of durable glass that immobilizes the radioactive and chemical compounds inside.
Nuclear waste is one of the major environmental headaches of our time. Even if every reactor on Earth were to suddenly shut down and none built again, there is still millions of gallons of waste left over from three quarters of a century of production to deal with, along with future waste from research reactors and hospital radiology labs. Part of the problem is finding a place to store such waste long term, but another is developing a process to make it chemically inert and unable to interact with the environment.
There are a number of ways to treat nuclear waste, and one of the more promising is vitrification. That is, mixing filtered waste with glass-forming materials, then heating it in a furnace to create borosilicate glass, which remains stable for thousands of years. Developing this method has usually concentrated on high-level nuclear waste like spent fuel rods, but there's a hundred times more low-level waste. That is, waste materials that have been contaminated by radioactive elements or have been exposed to neutron radiation. These can include decayed medical isotopes, contaminated clothing, laboratory animal carcasses, and a lot of low-activity reactor residue in liquid form.
Developed by PNNL in conjunction with the US Department of Environment's Office of River Protection (ORP) and Washington River Protection Solutions (WRPS), which manages tank storage at Hanford, the new process is a pilot version of the Direct Feed Low-Activity Waste system that will one day be used to vitrify millions of gallons of low-level radioactive waste left over from the US nuclear weapons program. The test is designed to show that waste can be processed continuously instead of in batches and to better understanding how the method works to prepare for scaling it up.
For the demonstration, PNNL took liquid nuclear waste from Hanford and used filters and ion columns to remove solids and the heavy metal cesium. The processed liquid was mixed with raw material for glass making, then pumped into a five-inch-wide (12.7-cm) melter furnace at a steady, controlled rate as it was heated to 2,100°F (1,149 °C). About every 30 minutes, around 8 oz (227 g) of glass was extruded, for a total of 20 lb (9.1 kg).
Meanwhile, radioactive gases given off by the vitrification process were turned back into liquid by condensation, which will be concentrated and grouted for later vitrification. The glass and the grout will be analyzed to determine if they meet disposal standards.
According to PNNL, another laboratory vitrification test is scheduled for later this year. In this one, liquid from another Hanford waste tank will be run through a different filtration and ion exchange process before being turned into glass.
"Being able to run real tank waste instead of simulant through these tests provides valuable input for validating and refining our approach to the treatment of low-activity waste," says Kris Colosi, the WRPS project manager. "It's another important step toward the removal and disposal of a large portion of Hanford's tank waste."
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