One of the key technical challenges of constructing a nuclear reactor is the choice of metal used to construct the core and to form the cladding on the fuel rods. The metal used in the reactor are subjected to high temperatures, high pressures and intense bombardment by radiation. When a neutron penetrates the metal in the core, it can displace an atom in the crystalline lattice. While the atom travels rapidly away from the point of impact, the cavity travels slowly, if at all. Repeated neutron impacts in the same area can lead to a consolidation of holes left by the impacts. These holes can form cracks that may propagate, weakening in the metal by making it more brittle and by making it less dense. Intense radiation damage can cause metals to swell to twice their original volumes.
Recent research has been aimed at reducing radiation damage by introducing micro and nano-sized particles into the metals in order to act as "absorbers" to prevent the spread of cavities caused by the neutron bombardment. Now researchers at the University of Michigan have been exploring new alloys that can help solve the radiation damage problem.
Researchers at the Oak Ridge National Laboratory in Tennessee created a number of different nickel alloys. Then researchers at the University of Tennessee bombarded the alloy samples with radiation at a temperature over nine hundred degrees Fahrenheit which would cause most popular nickel alloys to swell. Two different experimental set-ups caused radiation and temperature damage similar to a few years and a few decades of exposure.
The researchers at the University of Michigan used a transmission electron microscope at their Center for Material Characterization to examine in detail what damage was caused at the level of the crystalline lattice. The best alloys were what are referred to as solid solutions that contained crystals made of equal parts nickel, cobalt, and iron; or nickel, cobalt, iron, chromium, and manganese. Compared to pure nickel, the best alloys suffered less than one percent of the damage.
Another department at the U of M investigated the reason behind the radiation resistance in the new alloys. They created computer simulations of the alloys and found that the radiation resistance could be attributed to the way in which radiation damage propagated through the crystalline lattice of the new alloys. Because the different atoms in the lattice were different sizes, cavities which could propagate easily in the crystalline lattice of a pure metal such as nickel are blocked from easy movement in the lattice of the new alloys.
These new studies follow other studies of radiation resistant alloys. Other researchers have found that multi-metal alloys have elevated radiation resistance. Alloys with five metals are sometimes referred to as high-entropy alloys. The original article discussed in this blog post is titled "Enhancing radiation tolerance by controlling defect mobility and migration pathways in multi-component single phase alloys" and it appears in the publication Nature Communications. My earlier blog post on this subject can be found at Nuclear Reactors 345 - New High Entropy Alloys May Be Better Than Steel For Nuclear Reactor Construction
