Taking out the Nuclear Trash

UNC-Chapel Hill Solves 50-Year-Old Nuclear Waste Puzzle
April 22, 2016

The topic of nuclear power as an energy source is one that can stir up emotions. Some see it as a clean energy source for the future, which as more research and a few major discoveries have shown, provides potentially limitless greenhouse gas-free energy. Others see it as a danger, with the potential for large-scale disasters like the ones at Fukushima and Chernobyl, and the potential to create toxic, radioactive waste. 

Americium Metal

Both sides make valid points, but the problem of radioactive waste is significantly closer to being solved, thanks to new research from UNC-Chapel Hill chemists. UNC chemistry professor Dr. Tom Meyer and his colleagues devised a way to remove the radioactive element americium from nuclear wastes, making the wastes far less radioactive and far less toxic.

The problem of toxic waste, and americium, begins at the nuclear power process. Currently, nuclear power comes from a nuclear reaction process called fission, in which the nucleus, or center of a large atom, will break apart, creating two smaller atoms and an enormous amount of energy. Large, heavy elements at the bottom of the periodic table like uranium and its big brother plutonium are the big names in fission power, and the nuclear reactions from these two elements power nuclear plants around the globe. 

The trouble is that with all the energy and subatomic particles flying around in a nuclear reactor, uranium and plutonium don’t always do what they’re supposed to, that is, break apart. Sometimes they go the other direction, picking up neutrons and rearranging their nuclei to give themselves more protons through a process called beta decay. The end result is bigger elements, including americium, the next element up from plutonium.

Americium is certainly not the most abundant product left over in used nuclear fuel. In fact, the roughly 30 tons of nuclear fuel it takes to power a large reactor for a year will produce less than seven pounds of americium. The problem is when fuel is taken out of the reactor for recycling and disposal, scientists did not have a way to pull the radioactive americium out of the waste material. 

Americium’s most common forms have radioactive half-lives of hundreds or even thousands of years (a property that makes them good for their most common use: the smoke detector). So they will be radioactive for a very long time, and if you can’t separate the americium out, everything it's mixed with has to be considered radioactive as well. Then you have a lot of waste piling up that will continue to be radioactive for thousands of years, with nothing you can do but store it.

Nuclear Waste PitSome elements can be easily removed from the waste. Uranium and plutonium can be pulled out of the waste material by dissolving them in acid. These elements can then be stored for a few years before being recycled into new fuel through a process called reprocessing. But americium does not have its electrons in an orientation that allows acid to dissolve it, so the americium stays trapped in the non-recyclable waste where scientists have not been able to extract it. 

To solve the americium problem, Dr. Meyer looked at the element's electrons. Dr. Meyer then adapted technology from another alternative energy source: solar energy. Specifically, he borrowed from a technology developed at UNC’s Energy Frontier Research Center for Solar Fuels that uses sunlight to split water into hydrogen and oxygen gas, which burn together like fossil fuels but only emit water.

The solar process anchors a chemical catalyst embedded in a dye molecule called a chromophore to a titanium dioxide nanoparticle. When sunlight activates the chromophore-catalyst assembly, it grabs onto water molecules and rips away some electrons. The titanium dioxide nanoparticle scoops those electrons away to prevent water from reforming.

Dr. Meyer adapted this system to attack americium instead of water, and though it takes about twice as much energy to oxidize—or steal electrons from—americium as it does to oxidize water, this system reliably takes three electrons from americium, creating a state in which acid can successfully dissolve it.

Much of this study was conducted at and in conjunction with Idaho National Lab, which has facilities built to safely handle radioactive material. Dr. Meyer is currently working the lab to design future experiments that would scale-up his method. A successful scale-up would enable this technique to handle the large amounts of nuclear waste that come from today’s reactors, and would make the waste of the future much more safe to handle and store.

—Daniel Lane

Daniel Lane covers science, medicine, engineering and the environment in North Carolina.