Thomas Hueckel is a professor of civil and environmental engineering at Duke University. He is an expert in the study of the mechanics of earthen materials and the way that they react to everything else including water, gases, fossil fuels, and structures. This field of research is called multi-physics geomechanics. Hueckel has teamed with Manolis Veveakis, another Duke University researcher with similar expertise. The two of them have just received an eight hundred thousand dollar grant from the U.S. Department of Energy to study how physical and chemical processes underground might interacted with or degrade nuclear waste storage facilities. They will also try to find ways of dealing with such potential problems.
When a spent nuclear fuel rod is removed from a reactor core, it is still highly radioactive. Rods are left in a cooling pool filled with water for ten years to allow them to dissipate some of their radioactivity. Even after that, a typical fuel rod will have a temperature of about two hundred and fifteen degrees Fahrenheit. It will still require at least ten thousand years before it is no dangerous.
Currently, most of these spent fuel rods are stored at the site where they were used. Cooling pools are rapidly filling up at many nuclear power stations and, if some of the spent fuel rods are not removed, the reactors at the site may have to be shut down. Some spent fuel rods which have cooled for ten years are removed from pools and stored in concreate and steel cylinders called dry casks.
The disaster at Fukushima clearly illustrated that storage of spent fuel rods in cooling pools is dangerous. When the cooling pool lost water in one of the reactor building, the rods burst into flame and burned down the building. Hueckel says that spent nuclear fuel should be stored at least a quarter of a mile beneath the Earth’s surface.
Countries that use nuclear power have been working on designs for decades for deep underground repositories for spent nuclear fuel. Many of the features of such designs are similar. In these designs, a vertical shaft opens on an underground cavern full of separated chambers. The walls of the chambers are lined with bentonite clay or other types of crushed rocks. Cracks between the materials in the linings are then filled in with clay and spayed with water to form a sealed lining. The waste is placed in dry casks and each cask is then inserted into a single chamber which is sealed with multiple barriers to prevent access by human beings or leaks that could allow the radioactive material to migrate out of the storage facility. While some test facilities have been dug, there is no major functioning facility anywhere in the world yet for spent nuclear fuel. The U.S. will not have such a national geological repository until at least 2050.
While this sounds like an excellent system to isolate spent nuclear fuel for thousands of years, Hueckel says that the steady emission of heat by the spent nuclear fuel rods has a deleterious effect on the rock around the repository. The rock may dry, crack and disintegrate, removing the solid barrier around the waste and allowing it to migrate into the ecosystem. Other geological processes might adversely impact the repository such as corroding the material used to construct the dry casks or changing the chemical composition of the clay in the walls.
Hueckel said, “With the new grant, we are charged with understanding how temperature and pressure contribute to drying and cracking in the planned repository, and what kinds of remedies we can.”
One way to improve the integrity of the walls of the repositories is to incorporate fibers into the clay used to seal the walls. This is an ancient technique which utilized animal hair to improve the strength of construction materials. Hueckel will experiment with nano- or micro-sized fibers of various materials. He will also investigate designs that increase the probability of the formation of shear cracking as opposed to tensile cracking. He says, “In many places where you see inclined cracks in materials, like in an iceberg, the two surfaces slide past one another—but they don't open up. It could reduce the potential for radionuclide migration.”