Inside an operating nuclear reactor, the environment is extreme, as reactor components are exposed to a combination of intense radiation and heat as well as chemically reactive coolant. That’s why, in order to operate reactors safely, scientists need to design their components with materials that can withstand these harsh conditions.

Researchers at the U.S. Department of Energy’s (DOE) Argonne National Laboratory have made a pivotal discovery by taking a technique originally developed for another industry and using it as a way to coat nuclear materials. This technique, called atomic layer deposition (ALD), forms the basis of new methods to protect nuclear fuels and materials from direct exposure to the reactor’s hostile environment.

Nuclear materials are radioactive elements. When we look at the periodic table, there are multiple elements that are radioactive, but uranium is the one that is used the most. Uranium is used as the nuclear fuel in both nuclear power plants and nuclear research reactors. Basically, reactors work by using uranium in very small pellets sealed into metal tubes. Free-floating neutrons (which aren’t part of any atom) hit the pellets and break the nuclei (protons and neutrons in the center of an atom) of the uranium atoms (this is called fission).  When these nuclei break, more neutrons are released, smaller atoms are formed, and energy is released in the form kinetic energy (moving energy). Energy can be transformed into many things and one of those is heat. In a nuclear reactor that produces electricity for our homes, that heat boils water to make steam. That steam is then used to spin a turbine which produces electricity. Nuclear research reactors may be designed in such a way that releases high levels of neutrons that can be used to test various materials, as the goal there is not to produce electricity. 

Uranium can be mined from the ground, but when it is mined it is not at a concentration that can be used in nuclear power plants or nuclear research reactors, so it needs to be concentrated.  If concentrated at a low level (<20%) it is considered low-enriched uranium.  If concentrated above 20% it is considered high-enriched.  Low-enriched uranium is preferred because it is generally safer and cannot be used in weapons, but low-enriched uranium cannot be directly substituted for high-enriched in research reactors without changing other components. So, researchers first figured out how to make low-enriched uranium work as well as high-enriched, but then it created another set of issues. That’s where the new research into coatings comes in!

Atomic layer deposition ― like its name suggests ― allows researchers to layer extremely thin films, just a few atoms thick, of a specific material on a surface. By building up these layers, Argonne scientists can form chemically accurate coatings designed to have a set of particular properties.

“We are pioneering the use of ALD for nuclear applications,” said Argonne nuclear engineer Abdellatif Yacout. Argonne experts in the technique, led by Michael Pellin, were influential in those advancements.

Fuel coatings support efforts to minimize high-enriched uranium

In one set of experiments, Argonne scientists have used ALD to layer zirconium nitride (ZrN), a compound that is very hard and known to be used for coating materials. The coating is thin enough to allow neutrons to go through, while protecting the fuel (low-enriched uranium) from breaking down. This break down usually occurs from an interaction with aluminum, a major part of a research reactor system.

In order to study the stability of the newly developed ZrN coating and how it interacts with aluminum, scientists performed multiple studies using heavy ions (particles with one or more units of electric charge) to simulate damage from fission fragments (nuclei that have been split by fission).

Coating claddings to hold up to reactor environments

Two other sets of experiments involving ALD revolve around claddings, which are structural materials that enclose the fuel parts inside of a nuclear reactor.

One project used ALD to design cladding materials that would resist fretting wear, a behavior in reactor assemblies that contributes to mechanical wear. ​“A way to resist fretting is to coat the surface of the cladding to increase its hardness,” Yacout said. ​“Cladding surfaces modified with an ALD coating (for example, aluminum oxide [Al₂O₃]) and followed by other treatments, increases the surface hardness by almost 100-fold.

The second project revolved around developing a coating for claddings to improve their ability to better withstand the high temperatures inside a reactor during accident conditions. The team developed a unique ceramic compound material, which can be created at low temperature but with a significantly compact microstructure (very small-scale structure of a material).

Developing this ceramic-based compound coating is a two-step process. It involves combining electrophoretic deposition (EPD), a fast and low-temperature deposition method, with ALD. In this way, the Argonne researchers were able to quickly create a thick ceramic-ceramic compound coating that both adheres and conforms to the cladding surface.

The power of a joint technique

Neither EPD nor ALD as a deposition process by itself would have created a coating good enough to protect the cladding, said Argonne researcher Sumit Bhattacharya. ​“Even though ALD generates a pinhole-free, dense and adherent coating, the deposition rate is relatively slow. In order to deposit the thickness you need, it will take days or even in some cases weeks,” he said.

“Meanwhile, if you only use EPD, the deposited layer is sponge-like, with many small spaces and it requires high temperatures and pressure to become dense and stick to the substance. This is not ideal, as the cladding material is temperature-sensitive, and it will lose all of its mechanical properties.”

One key advantage of using the dual deposition techniques consists in the ability to greatly reduce the temperature needed to produce a sticky coating. Generally, to develop a dense ceramic compound, a high-temperature and pressure step is necessary. However, because the cladding is made of metal, the typical high-temperature and pressure would cause the substance to melt or lose its strength.

The combination of the EPD/ALD technique achieves a sticky coating at a temperature of only around 300 degrees Celsius, far lower than the typical pressures and temperatures required for such compounds.

In order to test how the coating can tolerate the reactor environment, the researchers bombarded it with heavy ions at various temperatures in Argonne’s Intermediate Voltage Electron Microscope facility (IVEM). Afterward, the sample stayed intact and scientists found no apparent changes in the nano-powder and the overlaying ALD coating.

Original article published on September 3, 2019