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Material interactions research plays small role in large-scale reactor development

microscopic image of the metal-helium interaction

This image shows a tungsten (110) surface after approximately 6 microseconds of plasma exposure. White colors mean above the original surface by about 10 nm; black means below the original surface by the same amount. The dark gray block that covers the upper left corner is the original surface’s height. Image courtesy of Karl Hammond

Nuclear energy has the capability to produce vast amounts of electricity to power millions of homes and businesses. Traditional nuclear energy involves the splitting of atoms, a process called nuclear fission, to release the energy in the nucleus.

Conversely, when two or more low atomic weight nuclei collide at high speed, they join to form a higher atomic weight nucleus, a process called nuclear fusion. However, unlike the relative ease of nuclear fission, initiating nuclear fusion requires extreme conditions, especially high atomic velocities consistent with temperatures of millions of degrees. The sun is a natural fusion reactor due to its extremely high temperature and density found in its core. Achieving nuclear fusion without the help of gravity — i.e. outside the core of a star — is extremely challenging. As such, commercially-viable fusion reactors are, for now, non-existent.

An international initiative that began during the Reagan administration resulted in what’s now known as the International Thermonuclear Experimental Reactor (ITER), a large-scale nuclear fusion test reactor that will use the last 50 years of advancements in plasma physics research to build an actual fusion facility that produces more power than it consumes for several minutes at a time. The ITER project is a seven-member collaboration currently building what will become the world’s largest tokamak — doughnut-shaped — nuclear fusion reactor in the south of France. Its members include the U.S., the European Union, China, India, Japan, South Korea and Russia.

Research from MU Engineering Assistant Professor Karl Hammond is one small part of the world’s fusion research. His current research examines the interactions of plasma on a tungsten metal surface, something that will occur when ITER goes online in five years or so.

“The bottom part of the ITER reactor, called the divertor, is intended to be the ‘strike point’ for high-energy helium and hydrogen atoms as they exit the magnetic field,” Hammond said.

The helium and hydrogen atoms colliding with the divertor are expected to damage it. “Helium, in particular, collects below the surface, forming microscopic bubbles of helium gas that may eventually destroy the material,” he said.

Karl Hammond

Karl Hammond

Researchers in Hammond’s lab perform simple simulations of the helium bubble–tungsten interactions in order to assess and quantify potential damage to the divertor.

The interactions are difficult to measure because of their size and the length of time over which they form. The simulations are relatively small, usually only 1-100 nanometers. Large-scale simulations, consisting of millions or perhaps tens of millions of atoms, take weeks, even years, to attain only a microsecond of actual time. Furthermore, simulations may take months of work to prepare before “production” runs are ready.

Hammond, who came to the Chemical Engineering Department last fall, brought the project with him from his previous appointment as a post-doctoral researcher at the University of Tennessee. He worked on the project last year with two engineering undergraduate researchers, freshmen Kaylee Libbert and Gabe Ort. Ort, who will be a sophomore next fall, has continued his work on Hammond’s research through the summer.

Ort said his role in the research is visualization, or crafting the illustrations, of the simulations.

“We’ve run simulations that had more than a million atoms,” Ort said.

The team looks at the simulations to “estimate the nature and rate of damage to the metal surface, the extent of gas retention and the extent of damage to the surface,” Hammond said.

Currently, they’re working on creating a different kind of simulation that could eventually be used to simulate fusion-relevant materials at more realistic time scales.

Academically, papers from the research have received international recognition. EuroVis 2015, the EG VGTC conference on visualization, accepted a paper co-authored by Hammond on the visualizations of these simulations, and EuroPhysics Letters recently accepted Hammond’s manuscript on helium diffusion near defects.

“These simulations provide valuable information about the mechanisms of damage that will be involved in fusion reactors and set the stage for even larger but less-detailed simulations of radiation damage,” Hammond said.