Experiments ✚ Simulations = Better Nuclear Power Research

An international collaboration of physicists is working to improve the safety and economics of nuclear power by studying how various cladding materials and fuels used in reactors respond to radiation damage.

In a study published May 14, 2015 in the Journal of Materials Research (JMR), the research team—which includes representatives from Los Alamos, Pacific Northwest and Idaho National Laboratories as well as institutions in Germany and Belgium—used molecular dynamics simulations run at NERSC to identify atomic-level details of early-stage damage production in cerium dioxide (CeO₂), a surrogate material used in nuclear research to understand the performance of uranium dioxide (UO₂). The study was featured on the cover of a JMR Focus Issue on Characterization and Modeling of Radiation Damage on Materials.

“People have been implanting swift heavy ions into CeO₂ to simulate fission fragment damage to nuclear fuel in a reactor,” said co-author Ram Devanathan, a scientist at Pacific Northwest National Laboratory (PNNL). “The actual material of interest is UO₂, but when it comes out of the reactor it is extremely radioactive and you have to take a lot of precautions to study it. So in addition to performing limited studies on actual nuclear fuel, you do surrogate studies of nonradioactive materials that share similar structure and chemistry to understand how nuclear fuel responds to radiation damage.”

For the JMR study, the researchers coupled swift heavy ion experiments and electron microscopy analysis with parallel simulations—one of the few instances of combining physical experiments and simulations for this kind of research. The result, Devanathan emphasized, was the ability to characterize material defects at the atomic level—something that is difficult to do accurately using experiment alone.

“In this work, we had physical experiments that involved implanting CeO₂ with 940 MeV gold ions and performing very careful high-resolution electron microscopy studies to understand the structure of the damaged material,” Devanathan explained. “The problem is that the process of radiation damage creates defects at the atomic level, and characterizing them accurately using only experiment is difficult.”

In addition, the evolution of radiation damage is controlled by events that take place on the nanosecond scale, he noted.

“This is where computer simulations come in,” Devanathan said. “They help us fill gaps in our experimental understanding by simulating these transient events that take place on very small time and length scales. So what we have difficulty capturing in the experiments we can simulate using resources like those at NERSC.”

The research team was also able to glean detailed information about the density changes in the material that were caused by the passage of a swift heavy ion, he noted.

“It has been speculated experimentally that the passage of swift heavy ions (through a material) would create density changes in the material,” he said. “But what these simulations were able to reveal was quantitative information about these density changes and defect clusters, and the level of detail was a pleasant surprise.”

While the molecular dynamics code (DL_POLY) Devanathan and his colleagues used is fairly common, the simulations themselves were very computationally intensive; the team simulated a system of 11 million atoms for a fairly long time, something that couldn’t have been done without massively parallel computing resources, according to Devanathan.

“This size of simulation is at the frontier of this field in terms of radiation damage characterization,” he said. “To study the damage by swift heavy ions you need to perform these large-scale simulations because the amount of energy imparted to the material is so large that you need a very large simulation cell of millions of atoms to simulate the process.”

Going forward, Devanathan and his colleagues are investigating radiation damage in a number of other ceramic materials and extending their study to look at accident-tolerant nuclear fuel cladding.

“This marriage of experiment and simulation conveys information that is more than what either of these techniques could have produced individually,” he said.