May 24, 2024

Ceramic Metal Composites: Possibilities for Immobilizing Nuclear Wastes

PNNL researchers review ceramic-metal composite technology for immobilizing radioactive wastes from nuclear reactors, including advanced reactors

This illustration shows the concept of how a ceramic-metallic waste form could appear in a canister.

A conceptual design of how a cermet waste form could appear in a canister along with potential candidate waste streams that could be incorporated into such a waste form.

(Composite image by Cortland Johnson | Pacific Northwest National Laboratory)

Carbon-free nuclear energy is an integral part of our nation’s energy portfolio, contributing nearly 20% of electricity generated in the United States. Researchers at Pacific Northwest National Laboratory (PNNL) are exploring ceramic-metal (cermet) composites as one possible solution to immobilize radioactive wastes from nuclear processes.

Their research was featured on the cover of Industrial & Engineering Chemistry Research, an American Chemical Society for Applied Chemistry and Chemical Engineering publication.

“When fuel rods are removed from a reactor, you can recover and reprocess the fuel that is still usable, but there would still be waste that is left over from that fuel cycle,” said PNNL Materials Scientist Brian Riley. “The question we’re trying to help answer is, are cermet composites a viable pathway to immobilize some of the leftover wastes and help close the nuclear fuel cycle.”

Research in the publication sets a foundation of knowledge and understanding about cermet technology, abilities, and limitations as a waste form for different types of existing and future nuclear wastes.

Currently, the Department of Energy has selected borosilicate glass technology for immobilizing high-level waste from legacy nuclear plutonium production efforts.

“Spent nuclear fuel from generating electricity contains a large fraction of the elements on the periodic table and many of those may require a unique immobilization method,” said Jonathan Evarts, PNNL materials scientist and lead author of the publication. “Borosilicate glass is a great waste form, but it does have weaknesses just like everything else. The cermet waste form can help fill in those gaps.”

Cermet complements glass as it can immobilize waste streams containing metals, salts, ceramics, and high-carbon streams like graphite or silicon carbide.  

“One thing glass can’t accommodate is metal,” Riley said. “Cermet waste forms are technically designed to accommodate just that.”

Similar to how water exists in different phases such as ice in a solid phase or humidity in a gas phase, cermet’s two common phases are a ceramic phase and metallic phase. The phases can be tailored by modifying the crystal structure to optimize their properties and performance.

“As you change or add different ingredients, we’re interested in knowing what happens to overall properties of the composite waste form,” said Riley.

Understanding the properties of each phase can help researchers design cermets for specific use cases and even plan for waste management, using cermet composites, at different steps in the fuel cycle.

“The phases provide flexibility because not only can you combine many different types of metals and ceramics, but the metals can come from reactor components,” Evarts said. “For example, you can melt down steel reactor components and use that as the metal phase.”

As the nuclear energy industry explores advanced reactor concepts, and continues to provide clean energy with the existing fleet of nuclear power plants, Riley and Evarts’ paper revisits the existing literature and outlines multiple avenues for cermets as a future waste management option.

“We’re reinvigorating the current toolbox of waste forms to support advanced reactors that are on the horizon,” said Riley. “While interest is currently on the front end of building a new reactor, we’re preparing to deal with the back end of the nuclear fuel cycle and making sure that the country is prepared for treating waste to create safe, environmentally friendly, and long-term storage options. It is important to me that we help prepare the way for future generations to come with cradle-to-grave options for reducing the effects of nuclear wastes on the environment.”

This work was funded by the Department of Energy Office of Nuclear Energy (DOE-NE) under the Material Recovery and Waste Form Development (MRWFD) Campaign.