Karthik Chinnathambi, a materials scientist in the Micron School of Materials Science and Engineering at Boise State, is working toward a stronger, safer graphite material for use in nuclear environments.
His research is funded by a three-year, $510,989 award from the Department of Energy titled “Irradiation-Induced Defect Evolution in Nuclear Graphite.” Materials scientist Rick Ubic is the co-principal investigator.
Nuclear energy is being touted in some quarters as the best sustainable solution to our growing energy needs. There are currently 440 commercial nuclear reactors in the world, which collectively provide about 11 percent of the world’s electricity needs.
Most of these reactors are of the Generation II (water-cooled) type developed in the 1970s; the next-generation (Generation IV) reactors must have enhanced power conversion efficiencies and the ability to produce hydrogen, best accomplished with high-temperature, gas-cooled systems. The principal challenge to the success of these advanced reactor concepts is the development of high-performance materials.
Graphite, which is a crystalline form of carbon, will be used as a structural material as well as neutron moderator and fuel component in the Gen IV systems. During operation, the reactor environment presents extreme challenges in terms of irradiation damage from fast neutrons. This creates lattice defects leading to changes in physical and mechanical properties of the materials, especially in the case of graphite. This damage leads to component failure and limits the lifespan of a reactor.
Although graphite has been used in nuclear applications since the first critical reactor, there is a lack of complete understanding of irradiation-induced property changes, mainly due to the difficulties associated with neutron-irradiation experiments. Traditionally, graphite is examined for defects after being removed from the neutron-irradiation environment, but materials don’t become defective at one specific moment. It occurs over time, with multiple processes contributing to the final outcome.
To overcome these difficulties Chinnathambi will use the electron beam (as a substitute for neutrons) in a transmission electron microscope (TEM) to simulate the reactor environment.
In a TEM, a high-energy beam (typically 200 keV) of electrons are transmitted through a thin specimen, creating images that contain microstructural and crystallographic information about it. The resolving power of a TEM is about 1,000 times better than that of an optical microscope, enabling researchers to see the arrangement of atoms in a material. The electron beam in the TEM will be used to simultaneously irradiate the graphite samples and image them.
“The process of electron imaging itself damages the sample and we are going to use that to our advantage,” said Chinnathambi. This will enable real-time monitoring of defects created by the irradiation, which is not possible in the case of neutron irradiation in a reactor. “Changes in the graphite lattice and the bonding environment of carbon will be captured at different dose rates and temperatures.”
Chinnathambi hopes that the results will help advance the understanding of the mechanism of irradiation-induced dimensional change and other related phenomena. In-situ TEM analyses will be conducted at the Boise State Center for Materials Characterization (BSCMC).
The project is part of the Department of Energy’s EPSCoR State-National Lab Partnership Program, the main goal of which is to advance fundamental energy oriented scientific and engineering research collaborations with the DOE Federally Funded Research and Development Centers. Boise State researchers will collaborate with the Idaho National Laboratory for this project.