SEAS Researchers Overturn Previous Theory of Irradiation Induced Instability

Researchers at Harvard’s School of Engineering and Applied Sciences have developed a new model for understanding how irradiation affects the instability of materials surfaces, which may help the nuclear industry improve structural materials for its power plants.

The team, led by Michael J. Aziz and Applied Math and Applied Physics Professor Michael P. Brenner, came across this key insight while studying collisions of energetic ions with solids in the laboratory.

“If you irradiate a solid surface with high energy particles, sometimes it’s completely stable and sometimes it spontaneously erupts into nanometer-scale patterns,” said Aziz, a professor of materials and energy technologies.

Understanding the origin of these tiny patterns has been the main goal of their research, as there is interest in harnessing the phenomenon to shape materials on scales much smaller than is possible with conventional methods.

The collisions themselves last for mere trillionths of a second—a duration that can be simulated using complex computer calculations. The researchers ran hundreds of these simulations to measure the average behavior resulting from an individual impact. They then developed a mathematical theory that took the results of the simulation and extended them over a vastly longer timescale, as they were interested in the cumulative effects of hours of steady irradiation.

To their surprise, they found that the instability of irradiated surfaces is not primarily caused by atoms being blasted away by the radiation—the conventional wisdom among materials scientists. Instead, it is the much larger number of atoms that are “knocked around” on the surface, but not actually discharged, that determines whether a surface remains stable.

“We’ve discovered that the whole field, including us, was wrong in understanding what causes the patterns,” Aziz said. This groundbreaking result in materials science also has practical implications for the nuclear industry, particularly for nuclear fusion technologies.

Fusion, if it can be successfully harnessed, has significant advantages over the fission currently used in nuclear power plants. Fusion generates much more power, produces no radioactive waste, and is renewable since it is fueled by water.

Current experiments with fusion reactors use plasma-facing fusion reactor walls made of tungsten. Tungsten atoms are so strongly bonded that the particles in nuclear fusion plasma do not have enough energy to blast them away from the wall surface. Yet, the walls surprisingly become “foamy” and degrade over time.

The recent discovery at SEAS explains how these surfaces might degrade without discharging tungsten atoms.

“It opens a very interesting path for understanding possible causes of instability, and could lead to better design criteria for the stability of materials under irradiation,” Aziz said.

Acquiring a better understanding of materials stability may bring us closer to the goal of mobilizing fusion power for commercial use.