One of the key challenges when designing nuclear reactors is finding materials that can withstand the massive temperatures, radiation, physical stress and corrosive conditions of these extreme environments. Exposure to high radiation alone produces significant damage at the nanoscale, so scientists at Los Alamos National Laboratory, New Mexico, have been working on a mechanism that allows nanocrystalline materials to heal themselves after suffering radiation-induced damage. This gives hope for materials that will improve the reliability, safety and lifespan of nuclear energy systems.

Radiation can cause individual atoms or groups of atoms to be jarred out of place, leaving behind an empty space known as a vacancy. The displaced atoms are called interstitials and they, and the vacancies they cause, build up over time in a material, causing effects such as swelling, hardening or brittleness in the material and lead to catastrophic failure.

The nanocrystalline materials the scientists have been working on are those created from nanosized particles, in this case from copper. A single nanosized grain is the size of a virus or even smaller. Nanocrystalline materials comprise a mixture of grains and the interface between those grains, called grain boundaries.

Nanocrystalline materials contain a large amount of grain boundaries, which are thought to be able to absorb and remove defects like interstitials and vacancies, so scientists have thought that these materials would be more tolerant to radiation than their larger-grain counterparts.

But until conducting recent computer simulations, scientists lacked the ability to predict the performance of nanocrystalline materials in extreme environments. That’s because specific details of what occurs within solids are extremely complex and therefore difficult to visualize.

First-seen phenomenon

In a paper appearing in the journal Science, the researchers describe a newly discovered “loading-unloading” phenomenon at grain boundaries in nanocrystalline materials, which allows for effective self-healing of radiation-induced defects.

Using three different computer simulation methods, the researchers studied the interaction between defects and grain boundaries on time scales ranging from picoseconds to microseconds (one-trillionth of a second to one-millionth of a second).

On the shorter timescales, radiation-damaged materials underwent a “loading” process at the grain boundaries, in which interstitial atoms became trapped — or loaded — into the grain boundary. After trapping interstitials, the grain boundary later “unloaded” interstitials back into vacancies near the grain boundary, therefore healing the damaged material.

The scientists say this unloading process was totally unexpected because grain boundaries traditionally have been regarded as places that accumulate interstitials, but not as places that release them. The healing process does require some energy to operate but at lower levels than conventional mechanisms.

It is hoped that this new could eventually assist or accelerate the design of highly radiation-tolerant materials for the next generation of nuclear energy applications.

The Los Alamos National Laboratory research team includes: Xian-Ming Bai, Richard G. Hoagland and Blas P. Uberuaga of the Materials Science and Technology Division; Arthur F. Voter, of the Theoretical Division; and Michael Nastasi of the Materials Physics and Applications Division.