A single particle, such as an atom or a neutron, when fired into a piece of copper, causes a fountainlike cascade of disturbance, knocking countless copper atoms out of their positions in the metal's crystalline structure. A few trillionths of a second later, most of the atoms settle back into the crystal's lineup, but a handful are permanently displaced, misaligned and unable to fit back in anywhere. If that material is in an environment with radiation, such as part of a nuclear reactor, over time those wayward atoms migrate and build up on the part's surface, leaving behind voids that can make the material brittle. "After irradiation the size can increase up to 10 percent because of the atoms moving to the surface," says Blas Uberuaga, a materials scientist at Los Alamos National Laboratory. "And that's bad because if you make parts that all fit together, and then they swell, nothing fits together like it's designed to."
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