A nuclear reactor is a tough place for metals. All those neutrons bouncing around wreak havoc with the crystalline structure of steel, tungsten and other metals used in fuel rods and other parts. Over time, the metals can swell and become brittle. (They become radioactive, too, but that’s another story.)
Now researchers at Los Alamos National Laboratory in New Mexico have shown that by altering the microstructure of metals, metallurgists may be able to make reactor parts that are self healing.
Blas P. Uberuaga, Xian-Ming Bai and colleagues conducted computer simulations of the long-term impact of neutron emissions on copper — not because much copper is used in nuclear plants, but because it is a relatively well-modeled metal. Their findings are published in Science.
When a neutron hits metal, it displaces atoms within the crystal lattice. In a metal with a largely uniform structure, these atoms move to the surface, eventually causing the metal to swell, and the vacancies they leave behind can lead to voids that further weaken the material.
But it is possible to fabricate metals that have a nonuniform structure, with very small crystal grains, or regions of different phase or orientation. When atoms are displaced in this nanocrystalline material, rather than traveling to the surface they migrate to the boundaries between the grains. In their simulations, the researchers found that these atoms can then travel back away from the boundary and, if they find a vacancy, fill it, in effect healing the defect.
Dr. Uberuaga said that the basic principle should apply to other metals and complex alloys, and that if self-healing metals were used in the cladding around nuclear fuel, the fuel might be able to be “burned” at higher rates, with less damage to the metal. But there are many technological hurdles to overcome. “It’s not going to change how we design reactors tomorrow,” he said.
Henry Fountain, New York Times