As a Nuclear Science User Facilities instrument scientist, Lingfeng He works with more than 20 NSUF users each year on the microstructural characterization of irradiated nuclear fuels and materials at the Irradiated Materials and Characterization Laboratory at Idaho National Laboratory. This work ultimately helps discover and create more efficient and cost-effective improvements for the next generation nuclear reactors.
He assists users, often students, develop NSUF proposals ranging from studying irradiated structural materials to nuclear fuels and nuclear waste associated materials. Once awarded, He works with the users on experimental characterization at IMCL. Most recently, He helped prepare nine NSUF proposals with eight accepted.
“I like to work with the users, especially students,” He said.
In March, a Massachusetts Institute of Technology student was awarded a NSUF project that He is excited about. This new project fits with He’s personal interest of researching the corrosion and irradiation of a nickel chrome model alloy in extreme environments. He said the nickel-based alloy is a leading candidate for material in new reactors, but scientists are still pursuing the basic corrosion mechanisms under irradiation. This project will add more data to a limited data set.
The MIT student is also looking at additional funding, so he can work as an INL summer intern. He said the NSUF projects with students foster an important pipeline of talented early career research scientists, the same pipeline that landed him a job with INL in 2014. As a postdoctoral associate at the University of Wisconsin-Madison, He received his first NSUF award in 2011 that focused on radiation damage of oxide materials with structures similar to the common nuclear fuel uranium dioxide. This project exposed him to INL and the cutting-edge instruments that he now works with daily, often with other NSUF users.
The goal of his initial NSUF project was to understand the performance of nuclear fuels so they can engineer more robust fuels in the future with longer, even more efficient lifetimes.
“It’s very important for scientific engineering to find damage,” He said. “If we can understand the evolution of damage in a reactor, we can predict performance of nuclear fuel.”
After irradiation, He uses a dual beam focused ion beam to cut the samples of material into microscopic pieces, undetectable to the naked eye. Then He often uses transmission electron microscopy (TEM) to view the sample. Sometimes researchers will opt for other methods like jet polishing when preparing metallic TEM materials. TEM transmits a beam of electrons through the sample to form an image, which is then magnified and focused onto a screen for viewing and detecting damage to the sample.
He spends 20 percent of his time developing methodology for material characterization and maximizing the use of the cutting-edge instruments while improving their capabilities. The goal is to help instrument scientists and NSUF users work more efficiently. He also spends time maintaining the instruments and working with vendors to perform upgrades.
He is the TEM group leader whose main role is managing the IMCL’s two transmission electron microscopy instruments. INL has nearly a dozen different types of instruments to help prepare and view irradiated material and fuel samples, looking for damage at the micro, nano and atomic levels. By year’s end, the lab will receive a new generation atom probe tomograph, which will allow for atomic resolution in 3D and provide a better view of the composition at the microscopic level. Atom probe tomography (APT) involves fabricating nanometer size tipped needles of the samples and eroding the tip atom by atom through an electric field and then recreating an image of how the elements were distributed in the material.
“We want to make sure our expertise is cutting edge,” He said.
To fulfill his own research goals, He also applies for his own NSUF projects such as a current award to study the microstructure of nickel-based alloy X-750 that is often used in reactors. He is using advanced electron microscopy and spectroscopy in his most recent NSUF research project to view the microstructure damage after neutron irradiation of the X-750 and austenitic stainless steel type 304. The project uses legacy specimens from INL’s Experimental Breeder Reactor-II (EBR-II) that operated between 1961 and 1994. At that time, little microstructure information was provided, information that’s needed to understand the damage changes after neutron irradiation. This is the first attempt to investigate the microstructure of these two high-dose specimens.
X-750 is often used as core material in pressurized water reactors (PWRs) and boiling water reactors (BWRs) and also as spacer material in CANDU (Canadian Deuterium Uranium) reactors. Type 304 stainless steel is used as core structure and piping in PWRs.
“Half a century ago, we didn’t have this capability,” He said.