The safety and efficiency of a large, complex nuclear reactor can be enhanced by hardware as simple as a tiny sensor that monitors a cooling system. That’s why researchers at the Department of Energy’s Oak Ridge National Laboratory are working to make those basic sensors more accurate by pairing them with electronics that can withstand the intense radiation inside a reactor.
The ORNL research team recently met with unexpectedly high success using a gallium nitride semiconductor for sensor electronics. A transistor made with the material maintained operations near the core of a nuclear reactor operated by research partner The Ohio State University.
Gallium nitride, a wide-bandgap semiconductor, had previously been tested against the ionizing radiation encountered when rockets hurtle through space. Devices with wide-bandgap semiconductors can operate at much higher frequencies, temperatures and irradiation rates. But gallium nitride had not faced the even more intense radiation of neutron bombardment.
That could offer a big boost for equipment monitoring in nuclear facilities. The information gathered by sensors provides early warnings about wear and tear on equipment, allowing timely maintenance to avoid broader equipment failures that cause reactor downtime. Currently, this sensing data is processed from a distance, through yards of cable connected to electronics with silicon-based transistors.
Researchers irradiated gallium nitride transistors for three days at temperatures up to 125 degrees Celsius close to the core of The Ohio State University Research Reactor. The team pushed the transistors all the way to the reactor’s safety threshold: Seven hours at 90% power.
The gallium nitride transistors were able to handle at least 100 times higher accumulated dose of radiation than a standard silicon device, said researcher Dianne Ezell, leader of ORNL’s Nuclear and Extreme Environment Measurements group and a member of the transistor research team.
She said the transistor material needs to be capable of surviving at least five years, the normal maintenance window, in the pool of a nuclear reactor. After the research team exposed the gallium nitride device to days of much higher radiation levels within the core itself, they concluded that the transistors would exceed that requirement.
This is an important technical advance as attention turns from the large-scale existing fleet of nuclear energy plants to microreactors that could generate from tens to hundreds of megawatts of power. Although these novel reactor designs are still in the development and licensing stage, their potential portability could allow them to be deployed on the back of a truck to a military or disaster zone.
Advanced reactors are being designed to operate at higher temperatures using different forms of fuel. Because microreactors will be so compact, all the operating components, including the sensors, will have to be able to function in the radiation field, Ezell said. Gallium nitride transistors could be the key.
Ohio State researchers built devices of different designs and sizes to meet specifications set by ORNL, and then the team compared their responses to radiation, finding that larger devices seemed less susceptible to radiation damage. Ohio State is now developing computer models to project how various circuit designs will perform under different temperatures and radiation levels.
In the long run, researchers would like to demonstrate that gallium nitride circuits could be used to transmit data from sensors wirelessly. The material is already used for devices that support radio frequency applications, like cell phones, and for power electronics.
