One of the major considerations of creating nuclear reactors is the question of what materials to use for what purposes. A variety of elements in a variety of combinations have been found to be useful. The materials near the core of nuclear reactors are subjected to intense heat and bombardment with neutrons which causes some of them to become porous and brittle with age. This is a major reason that nuclear power plants are licensed for limited life spans. Forty years has been a common license period with twenty year renewals if the reactor is found to be sound enough to keep operating.
A team of researchers at MIT has found that adding small quantities of carbon nanotubes (CNT) to aluminum can slow down the process of embrittlement in a nuclear reactor. At this time, the new additive can only be used in lower temperature environments like research reactors. It is hoped that, in time, the same additive might be used in other metals and in the higher temperatures of nuclear power reactors. In addition to dealing with radiation damage, it has been found that addition of the carbon nanotubes can increase the strength of the composite material by up to fifty percent. It also increases the tensile ductility of the composite which means that it can withstand more deformation before it breaks.
Metals such as aluminum have microscopic grains or zones. When subjected to conditions in reactor cores, radiation transmutation can generate helium which forms tiny bubbles along the boundaries of the grains. This porosity makes the metal brittle. When about two percent of the volume of the metal is occupied by carbon nanotubes, the nanotubes form a one dimensional transport network along which the helium can percolate. The helium is able to leak out of the metal and does not remain to cause further damage. Although the carbon nanotubes are converted to carbides after being exposed to radiation, the carbides retain the long narrow shape which provides an avenue of escape for the helium. The tubular carbides have a huge combined surface area while allows point defects in the metal to recombine which also prevents embrittlement. The carbide nanotubes can withstand a lot of radiation. The reduction in embrittlement is between five and ten times that of untreated metal.
The current research is focused on aluminum but the researchers are going to move on to test the process on zirconium. Zirconium is used extensively in the nuclear industry for the coating layer or cladding on the outside of long thin cylinders or rods of nuclear fuel. They hope to find that the beneficial results of adding carbon nanotubes to aluminum will be replicated when they are added to other metals. If this proves to be true, the new process could be extremely useful in a number of applications in nuclear power reactors.
The amount of carbon nanotubes added to the metal constitute only about one percent by weight. Carbon nanotubes are inexpensive to produce. The composite resulting from the addition of carbon nanotubes to metal can be created inexpensively in standard industrial processes. Korea is already manufacturing tons of the new material for use in automobile manufacture.
