The science of radiation chemistry flourished from the 1940s through the 1960s as the United States weighed the benefits of several different reactor technologies to power an energy-hungry planet.
During that time, researchers needed to understand how chemical reactions occurred in high-radiation environments to optimize the performance of materials, fuels and coolants in these various reactor types, including gas- and water-cooled technologies.
But in the 1970s, due in part to the emergence of light-water-cooled reactors as the technology of choice for the nuclear power industry, the science of radiation chemistry was relegated to a few, specific niches.
Now, as a new generation of nuclear reactor designers develop advanced molten salt reactor concepts as an alternative for providing reliable, sustainable, carbon-free power, the need for radiation chemistry has never been greater.
To meet that need, Idaho National Laboratory’s Center for Radiation Chemistry Research has developed a capability that supports the nuclear energy industry by researching radiation-induced effects in advanced reactors, fuels, coolants, materials and fuel recycling technologies while also training the next generation of radiation chemists.
“Currently, there are few institutions in the world that train radiation chemists or perform radiation chemistry research,” said Gregory Horne, director of the INL Center for Radiation Chemistry Research. “With chemistry, radiation can shove your way out into the unknown.”
That’s because high-energy radiation can alter the ways atoms interact with each other.
“Not only do we provide radiation chemistry expertise and capabilities, we have links with other world-leading facilities,” Horne said. “Our problem has been making sure people know this discipline exists.”
The center was born out of the need to support the back end of the nuclear fuel cycle — what happens to nuclear fuel when it’s done making electricity. These needs include the reprocessing of nuclear fuel for the Department of Energy’s Office of Nuclear Energy and ensuring safe storage and final disposal of fuel and nuclear waste for DOE’s Office of Environmental Management.
But now, its researchers have expanded their efforts into other realms, especially fundamental science.
“We’re looking into the basic chemistry of actinides like californium and berkelium,” Horne said. “There are only a handful of people in the world who have handled these elements.”
All said, the center is trying to bring together a group of subject matter experts who can help meet the most significant scientific challenges facing the nuclear energy industry.
Molten salts in extreme environments
One key challenge is understanding the chemistry of molten salts — imagine compounds like table salt liquified by melting instead of being dissolved in water — as they endure intense heat and radiation under envisioned reactor conditions.
The research need was identified as a priority by a Department of Energy Office of Basic Energy Sciences (DOE-BES) sponsored Basic Research Needs workshop held in 2017.
The workshop prompted the establishment of an Energy Frontier Research Center (EFRC) focused on molten salts in extreme environments, that includes INL, Brookhaven National Laboratory, Oak Ridge National Laboratory, Iowa State University, Stony Brook University and the University of Notre Dame. These research centers are funded through DOE-BES and bring together creative, multidisciplinary scientific teams to tackle the toughest scientific challenges preventing significant advances in energy technologies.
As a part of the molten salt team, Center for Radiation Chemistry Research scientists are working to understand the fundamental processes in molten salts, especially the structures, properties and radiation chemistry of molten salts and how they interact with other materials.
“While it’s fundamentally science driven, molten salt chemistry has all sorts of ties to INL’s core mission,” said Simon Pimblott, an INL Laboratory Fellow and deputy director of the Molten Salts in Extreme Environments (MSEE) EFRC. “Now we’re seeing more and more applications of molten salts in nuclear and other net-zero (carbon) energy technologies.”
By itself, molten salt chemistry is tricky because it’s extremely hot and highly corrosive. In the next generation of molten salt reactors, the science of molten salts will need to account for the intense radiation field in the reactor core.
“The biggest challenge we have for molten salts is that previous research had very limited scope,” Horne said. “There’s a major knowledge gap. For instance, MSEE researchers have observed that, for certain dissolved metal ions, as soon as you apply a radiation field to molten salt, you have metal precipitating out of it.”
This chemistry is particularly important in some molten salt reactor concepts where the fuel is mixed into the molten salt.
“Putting impurities — like actinide fuels — in molten salts changes their properties,” Pimblott said. “It might enhance or inhibit the corrosive properties. It changes the oxidation-reduction chemistry. These things are affected by the temperature and the radiation field. It’s a complex system. That’s why this is a hard challenge.”
Building the right capabilities for radiation chemistry
Building a laboratory with the right capabilities to study radiation chemistry is a challenge for which Idaho National Laboratory, with its long history of nuclear science, is especially well suited.
“Molten salts cannot be handled and studied without an inert atmosphere and high temperatures,” said Ruchi Gakhar, an INL research scientist within the Molten Salt Systems and Pyrochemistry Department. “Developing a system for an experiment is not easy. Everything has to be done inside a glove box.”
(A laboratory glove box is a workstation that allows a technician to manipulate sensitive or hazardous materials without the materials contacting the technician or the outside environment.)
Right now, the laboratory has equipment that allows Gakhar and her colleagues to work with chloride salts, but the goal is to expand this capability. “Fluoride salts are even more corrosive than chlorides, and we are developing some specialized sample holders for the handling of fluorides and other corrosive materials at high temperatures,” she said.
Other capabilities include a host of spectroscopic techniques that allow the researchers to study how molten salts interact chemically with metals and graphite, which is used to moderate neutron speed in several types of reactors, as well as changes in the oxidative states of molten salts, which can impact corrosivity.
These and other instruments are built to withstand the extreme environments of irradiated molten salt systems. The center’s cobalt-60 gamma irradiators allow researchers to perform radiation experiments quickly without the need for space in a reactor like INL’s Advanced Test Reactor.
“There are all sorts of interesting questions here,” Pimblott said. “It’s fundamental science, but it’s directly applicable to advanced nuclear reactors and other applications such as in concentrated solar power systems.”