Professor Kumar Sridharan and his colleagues at the University of Wisconsin-Madison helped kickstart the Nuclear Science User Facilities (NSUF), formerly known as the Advanced Test Reactor (ATR) National Scientific User Facility in 2007. The team put the organization’s first material samples into the ATR, launching a new era of research into the behaviors of fuels and materials in a nuclear reactor environment. 

His research partners included university colleagues Lizhen Tan (now at Oak Ridge National Laboratory), Yong Yang (now at the University of Florida, Gainesville), and Heather MacLean Chichester of Idaho National Laboratory, where the Advanced Test Reactor is located. 

A decade later, Sridharan remains deeply involved with NSUF, which has grown from the single user facility to a consortium led by Idaho National Laboratory with 19 partner institutions. 

“The goal is to support industry by accelerating the deployment of new materials and materials’ processing innovations into the next generation nuclear reactors, while also expanding the scientific-knowledge base on effects of radiation in materials,” Sridharan said. “NSUF helps train the next generation of scientists and engineers by providing opportunities for university students to get exposure to national labs and to other universities.”

In 2007, Todd Allen, then a University of Wisconsin-Madison professor (now at the University of Michigan) and the former deputy laboratory director for INL, led the creation of the ATR NSUF where visiting researchers could conduct experiments. Sridharan said Allen asked him to participate in the initial material tests, a project that took about five years.

The pilot project included testing about 500 samples of 20 different materials. These materials had been developed for potential use as cladding, duct and other structural components in advanced nuclear systems with greater high temperature resistance or better high dose performance. The goal was to extract initial data on the radiation response of the materials at a variety of temperatures up to 700 degrees Celsius at a range of dose accumulations.

The results helped establish the NSUF’s Nuclear Fuels and Materials Library (NFML), a catalog of irradiated samples that are available for research projects and used by researchers nationwide.

“Our goal was mainly to ensure everything went smoothly, and that paved the way,” Sridharan said. “In the last 10 years, a lot of samples have gone into the ATR. All the kinks were worked out. That was only the start.”

Alloy HT-9 is one of the 500 initial samples frequently checked out of the sample library because it is one of the most promising candidate materials for future advanced nuclear energy systems. That’s because this ferritic-martensitic steel’s body-centered cubic structure is resistant to swelling under irradiation, making it a good candidate for structural material for in-core applications. Sridharan explained that the amount of irradiation damage a material experiences is influenced by small compositional and microstructural differences. So the batches of HT-9 provided by three U.S. national lab sources were tested in ATR and are being analyzed by researchers at Pacific Northwest National Laboratory, the University of Michigan and the University of Wisconsin-Madison. Sridharan said his first NSUF project was “humongous,” taking five years, but most of the other NSUF research projects are quicker.

An example of a faster project was one he worked on using Argonne National Laboratory’s Intermediate Voltage Electron Microscopy, an NSUF partner facility. The IVEM allows researchers to view samples of materials at high magnification during irradiation, thus showing defect formation and evolution in real time. One of his projects focused on developing a more accident-tolerant coating for a light water reactor cladding. Sridharan said the IVEM allowed researchers to see how the coating responded to radiation during the experiment. 

He also oversees a project looking at oxide dispersion-strengthened steel (ODS), which has nanoparticles of oxide dispersed within it. The question is what happens under irradiation; how does the composition of the ODS change. Sridharan and his research students are looking at a different way to manufacture ODS steels. They will perform ion irradiation and atomic probe tomography (APT) as part of this NSUF project. 

Along similar lines, another of his recent research projects investigated the evolution of how certain hard chromium-rich nanoparticles can form in certain high temperature alloys. These accumulations of chromium during high radiation doses render the materials brittle. In these research studies, Sridharan’s student used APT to observe these nanoparticles (a few thousandths of the diameter of a human hair) in the material to investigate radiation-induced changes. APT involves fabricating near-atomic size tipped needles of the samples and eroding the tip atom by atom with an electric field and then recreating an image of how elements are distributed in the material. 

The NSUF materials testing projects at ATR have the goal of making future advanced reactors safer, more efficient and more cost effective. Sridharan said the materials and their characteristics equate to additional safety. The higher the temperature in a reactor, the more efficient it is, so scientists are developing advanced materials that tolerate the heat and the harsh irradiation environment. The NSUF projects help scientists create better materials. 

Scientists are also intellectually curious, Sridharan said, and want to fundamentally understand how radiation damages the materials they are developing. If damage occurs, then the idea is to figure out how to mitigate the damage by perhaps changing the composition or microstructure so the material can withstand longer times within the reactor and more doses. 

“The practical side is it opens up a lot of reactor design opportunities,” Sridharan said. “That means reactors are safer and more economical to operate.”

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Author: Erica Curless