CLEMSON – From the mining of uranium ore to the storage of used fuel, radioactive waste is generated at every stage of the nuclear fuel cycle, and a Clemson University scientist is pursuing research that could help in handling it.
In a collaboration with the Savannah River National Laboratory (SRNL) in Aiken, College of Science professor Stephen Creager of the department of chemistry is working on ways to clean water contaminated by radioactive tritium.
Tritium is an isotope, or form, of hydrogen with a nucleus that has one proton and two neutrons. It differs from hydrogen in that the element’s most common form – protium – has a lone proton and no neutrons. The other isotope of hydrogen – deuterium – falls in the middle of the three, having one proton and one neutron.
Because tritium is an unstable, radioactive isotope, it’s the rarest form of hydrogen, produced mostly as a byproduct of nuclear reactors rather than as a naturally occurring isotope. The Savannah River National Laboratory is one of two facilities in the United States that stores the majority of the country’s nuclear waste, prompting the desire for SRNL to better understand how to dispose of or repurpose tritium. Creager might have a way to do it using the world’s thinnest material: graphene.
“In 2010, the Nobel Prize in physics was awarded for studies on graphene,” Creager said. “At the time, I thought it was trivial because graphene is just a layer of graphite – the lead of pencils – but just that single layer has all of these interesting properties, one of which is that it’s an excellent barrier. It’s thin, but it’s also very impenetrable, and people are doing things like putting it in batteries because it’s got a very high surface area and it’s electronically conductive.”
The researchers who won the Nobel Prize continued to work with graphene and discovered in 2014 that under certain conditions the material is permeable to hydrogen ions, also called protons.
“It was a shocking result because everyone had assumed up to that point, and all the measurements had confirmed, that even something as small as a helium atom – two protons and two electrons – couldn’t move through graphene. And yet, a proton could,” Creager said. “That was a cool discovery and there are all kinds of things you can do with that knowledge.”
Two years later, in the publication that caught Creager’s attention, the same researchers found that not only did graphene allow protons to flow through it, but it let them through 10 times faster than deuterons, the nuclei of deuterium atoms. In other words, graphene allows 10 times more protons to be separated from deuterium in a single step, an enrichment factor that trumps conventional methods.
One of those conventional methods – electrochemical hydrogen pumping – provides a way to separate mixtures of gases composed of hydrogen isotopes. Schematically, a hydrogen pump is an electrochemical cell that has a cathode on one end, an anode at the other and is connected to a power source.
“You take a mixture of hydrogen and deuterium gas and it comes in one side. You oxidize the hydrogen into protons and deuterons, which go across the cell. The deuterons are blocked and only pure hydrogen comes out the other side,” Creager said. “It’s a boring device because hydrogen comes in and hydrogen goes out, but it’s also very interesting because it provides a means of separating hydrogen ions from deuterium and tritium ions.”
At the time the Nobel Prize researchers released their latest findings, Creager was already working with the Savannah River National Laboratory on a different, less successful project. He was a perfect match to reimagine the hydrogen pump, given his development of a miniature fuel cell that requires only a few micrograms of catalyst to stimulate a reaction. In Creager’s miniature cell, a 2-by-2-inch piece of graphene – with a market value of $250 – can provide for 20 experiments, whereas a typical cell would need 20 times that amount.
The team’s idea is to build an electrochemical cell that can clean tritium out of contaminated water by means of water electrolysis – useful to SRNL in the event that its storage facilities ever leak tritium into the groundwater and useful to Creager for his miniature cell specialty.
In the cell, contaminated water would flow in one side and a reaction will convert the water into its components: hydrogen – or deuterium or tritium – and oxygen. The isotope would then arrive at the second end of the cell and be reduced into hydrogen. Adding a graphene layer could selectively collect deuterium and tritium on the water side of the cell, allowing only depleted pure hydrogen on the other side, thereby cleaning the water.
“The place with the greatest need for cleanup in the world right now is at the Fukushima plant in Japan that was damaged by the 2011 tsunami,” Creager said. “There are millions of gallons of contaminated water there and there’s not really any viable way to clean it up.”
While nuclear cleanup is a primary interest of the project, being able to concentrate tritium can have other applications, too.
“The whole reason that the Savannah River Lab exists is because they produced the hydrogen and tritium used in hydrogen bombs in the 1950s,” Creager said. “They still produce the tritium that is used to make nuclear weapons, the point being they have lots of experience handling the stuff.”
But tritium isn’t all sinister, Creager pointed out. The isotope has a half-life of 12 years, meaning that it will last for 12 years before it loses half of its radioactivity. That makes it an ideal substance to use in a long-lasting battery, though it’s also known for its use in glowing “exit” signs above doorways.
The team’s method – technically called hydrogen isotope fractionation using graphene – could also have applications in the world’s ever-changing energy landscape.
“What if you could couple a deuterium isolation plant to one of these water electrolysis plants that is associated with a renewable energy source? If you can’t dump the electricity on the grid, then you have to convert it to some useful product. Maybe that product could be hydrogen,” Creager said. “I don’t know the answer to that – we provide the knowledge for a design engineer or maybe a utility company who can then find the most optimal ways to use energy.”
Creager’s portion of the research reported in this publication is supported by a $331,942, two-year grant from the U.S. Department of Energy under Award Number DE-SC0018151. The content is solely the responsibility of the authors and does not necessarily represent the official views of the DoE.
Hannah Halusker is with the College of Science at Clemson University.