Saturday, March 27, 2010

Advanced Materials for Safer Nuclear Reactor Vessels

The sciences of materials and nanotechnology are growing more useful and powerful. Scientists at Los Alamos have discovered a molecular mechanism that could create self-healing nanocrystalline materials for incorporating into nuclear reactor vessels, for better safety and longer life.
In a paper appearing March 26 in the journal Science, Los Alamos researchers report a surprising mechanism that allows nanocrystalline materials to heal themselves after suffering radiation-induced damage. Nanocrystalline materials are those created from nanosized particles, in this case copper particles. A single nanosized particle -- called a grain -- is the size of a virus or even smaller. Nanocrystalline materials consist of a mixture of grains and the interface between those grains, called grain boundaries.

When designing nuclear reactors or the materials that go into them, one of the key challenges is finding materials that can withstand an outrageously extreme environment. In addition to constant bombardment by radiation, reactor materials may be subjected to extremes in temperature, physical stress, and corrosive conditions. Exposure to high radiation alone produces significant damage at the nanoscale.

Radiation can cause individual atoms or groups of atoms to be jarred out of place. Each vagrant atom becomes known as an interstitial. The empty space left behind by the displaced atom is known as a vacancy. Consequently, every interstitial created also creates one vacancy. As these defects -- the interstitials and vacancies -- build up over time in a material, effects such as swelling, hardening or embrittlement can manifest in the material and lead to catastrophic failure.

Therefore, designing materials that can withstand radiation-induced damage is very important for improving the reliability, safety and lifespan of nuclear energy systems. Because nanocrystalline materials contain a large fraction of grain boundaries -- which are thought to act as sinks that absorb and remove defects -- scientists have expected that these materials should be more radiation tolerant than their larger-grain counterparts. Nevertheless, the ability to predict the performance of nanocrystalline materials in extreme environments has been severely lacking because specific details of what occurs within solids are very complex and difficult to visualize.

Recent computer simulations by the Los Alamos researchers help explain some of those details.

In the Science paper, the researchers describe the never-before-observed phenomenon of a "loading-unloading" effect at grain boundaries in nanocrystalline materials. This loading-unloading effect allows for effective self-healing of radiation-induced defects. Using three different computer simulation methods, the researchers looked at 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). _SD

Both nuclear fission and nuclear fusion create extreme local environments that cause ordinary modern materials to break down. But nuclear reactors placed inside extreme remote outposts, onboard spacecraft, or powering distant space colonies, will all need to be much sturdier and longer lived than conventional reactors of today.

Advances in nano- and materials sciences are bringing about an age of "metamaterials", unlike anything we have seen before in terms of properties and capabilities.



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