Scientists have been grappling over how to safely store nuclear waste since before the first nuclear power plants were built in the 1950s.
One strategy for high-level waste is vitrification, widely used in the United States. It involves mixing it with silica sand and other chemicals to form glass and heating it until molten so that it can be stored in specialized containers where it cools and solidifies.
While glass can accommodate small, light radionuclides, it can only store so many larger, heavier actinide waste elements. Ceramic materials on the other hand, can accommodate more of those actinide elements.
That’s why researchers have spent the last few decades testing glass-ceramic composite materials for storing high-level nuclear waste, making sure it resists corrosion from groundwater at least as well as glass alone.
But most of that research has measured corrosion resistance over a few months at most. No one has looked at whether it might be robust over the long term.
Enter chemistry PhD student Mehrnaz Mikhchian and her supervisor, University of Saskatchewan chemist Andrew Grosvenor.
“If we want to propose a new material, we need to know how it behaves over the long-term. How the surface reacts after exposure to water,” says Mikhchian. So using the VLS-PGM beamline at the university’s Canadian Light Source, along with a beamline at the Advanced Photon Source, she examined the glass-ceramic composite’s corrosion over the course of a year, longer than any other such study.
“I know it’s not a geological time scale,” admits Grosvenor. “But I want Mehrnaz to graduate.”
Mikhchian established that the composite material – composed of crystalline ceramic materials mixed into an amorphous glass matrix – resisted corrosion at least as well as glass.
Western University chemist James Noel – who studies the longevity of nuclear fuel waste containers with respect to corrosion and the degradation and dissolution of the used nuclear fuel – calls the paper “a nice bit of fundamental scientific work.”
He says it represents an advance in the design of these types of materials and the understanding of their degradation mechanisms. But as Noel points out, Mikhchian’s research “explored only the degradation behaviour of the virgin composite material, not that of the material charged with actinides and/or fission product ions. Once the material is loaded with a variety of elements that are foreign to it, its degradation behaviour could be different.”
Noel also cautions that “the host material may maintain its integrity, but the radionuclides it contains might be able to leak out anyway.”
Mikhchian and Grosvenor agree much more research will be necessary. Grosvenor, who likens the composite material to a chocolate chip cookie, where “the dough represents the glass matrix and the chocolate chips represent the crystalline crystallites of the ceramic oxide,” says this paper is just a first step.
The next phase of testing will involve putting simulated non-radioactive waste in the glass-ceramic composite material, followed by a small laboratory-scale study using real nuclear waste.
“When you’re developing a new material for a new nuclear wasteform, it’s a decades-long investigation from idea to proof of concept to actual scale up and use,” says Grosvenor.
While the work may eventually have applications in countries using vitrification, like the US, sequestering high-level nuclear waste in glass or glass-ceramic composites is not a strategy being considered in Canada.
“In Canada, the approach is to leave the nuclear fuel pellets exactly as they are when removed from the reactor and place them into strong, corrosion resistant containers in their original form,” says Noel.
“Immobilization of radionuclides by incorporation into a glass or glass-ceramic matrix is currently being considered by certain other countries, generally to handle wastes from reprocessing of used fuel and from weapons production, neither of which happens in Canada.”
