ADVERTISEMENT

Developing Reliable Physical Models Of Oxide Scale Buildup In Nuclear Reactors

Fuel assemblies in nuclear reactors consist of bundles of fuel elements — metallic cylindrical tubes encasing fuel pellets — arranged in specific geometries in the reactor core. The material used to fabricate the tubes, know as cladding, must have a low neutron absorption propensity and a high thermal conductivity, as well as be chemically stable, strong, and corrosion-resistant. Such material is known as Zircaloy, a zirconium (Zr) alloy containing tin, iron, and chromium, added in different proportions to achieve the desired properties.

Over time, however, Zircaloy tubes may fail due to the combined action of oxidation and delayed hydride cracking of the clad, a consequence of prolonged exposure to highly corrosive environments and neutron irradiation. The long-term integrity of the fuel elements in the reactor core is highly dependent on the response of the cladding to corrosion, and it is therefore imperative that reliable models of oxide scale buildup be devised.

ADVERTISEMENT

The challenge in such undertaking, however, is to develop physical models, not based just on empiricism and experimental data correlations, but on sound physics principles that may allow scientists and engineers to understand the fundamental mechanisms behind clad oxidation. Of particular interest is the anomalous growth of the oxide layer measured in Zircaloys, which is seen experimentally to follow a cubic root power law in time, contrary to most metals, which display square root time laws.

One feature unique to Zr oxide is that it initially forms with a tetragonal crystal structure corresponding to a high-temperature phase (>1200°C). This is made possible due to the existence of interfacial stresses engendered by the size mismatch between the metallic alloy and the zirconium oxide. These stresses maintain the oxide under a state of compression, stabilizing the tetragonal phase even at temperatures as low as 300°C. However, tetragonal ZrO2 admits small deviations from stoichiometry that favor the accumulation of anionic vacancies. The concentration of vacancies decreases with distance from the interface (as does the volume fraction of the tetragonal phase, in favor of the intrinsically stable monoclinic structure), creating a charge gradient that has a significant effect on oxygen diffusion.

In the work described here, a model of the evolution of the Zr oxide layer has been developed, accounting for thermo-migration (diffusion in the presence of a thermal gradient) and electro-migration (diffusion due to a charge gradient) of oxygen transport in the clad, in addition to standard thermally-activated diffusion. As well, the model is guided by quantum mechanical calculations of the stability of zirconium-oxygen compounds, which have been shown to emerge within the oxide layer in several forms, some of them —such as the ZrO phase— unknown to researchers until very recently. This points to the need to consider multiple layers representing different Zr-O compounds at the same time, as multiple experimental studies have shown.

The model assembles all these new insights and successfully predicts the cubic root-of-time law observed in Zircaloys under service. The results point to electro-migration as the main driving force responsible for the deviations of Zircaloy from the general oxidation behavior of most metallic alloys. The figure shows the evolution of the oxide layer as predicted by the model compared to several experimental data (Motta et al., “Microstructural characterization of oxides formed on model Zr alloys using synchrotron radiation”, in: Zirconium in the Nuclear Industry: 15th International Symposium, ASTM International, 2009). The power law that best fits the model curves displays an exponent of 0.34, in agreement with the ⅓ expected evolution.

ADVERTISEMENT
Credit: Jaime Marian

These findings are described in the article entitled Multilayer interface tracking model of zirconium clad oxidation, recently published in the Journal of Nuclear Materials. This work was conducted by researchers in the Jaime Marian group at the University of California Los Angeles.

Comments

READ THIS NEXT

Treating Pigs With A Gene Therapy For Huntington’s Disease

Back in 1993, a gene was discovered to be responsible for the heritable brain disorder called Huntington’s disease. It was […]

Hypertonic Solution: Definition And Role In Cell Biology

A hypertonic solution refers to a solution that has a greater concentration of solute than another solution. In the context […]

Real-Time Classification Of Sound Type To Improve Hearing Devices In Various Noise Environments

Over 5% of the world’s population or 360 million people have some sort of hearing loss [1]. One way that […]

Iterative Design Of Solar Reactor For Hot Spot Reduction And Enhanced Temperature Uniformity  

Published by Nesrin Ozalp and Hamed Abedini Najafabadi Mechanical and Industrial Engineering Department, University of Minnesota Duluth, 55812, Duluth, MN; School of […]

Molar Mass Of Glucose (C₆H₁₂O₆)

Glucose (C6H12O6) is an organic macromolecule that is essential for the metabolism of essentially all eukaryotic organisms. Glucose is a […]

Shoulder Ligaments, Bones And Tendons

The human shoulder is a complex structure that must be stable enough to support the actions of the arm and […]

The Caspian Sea–Hindu Kush Index (CasHKI): A Climatic Index That Affects Dust Activity Over Southwest Asia

A comprehensive investigation of the dust-storm characteristics over the Sistan Basin in eastern Iran revealed that the dust-storm days during […]

Science Trends is a popular source of science news and education around the world. We cover everything from solar power cell technology to climate change to cancer research. We help hundreds of thousands of people every month learn about the world we live in and the latest scientific breakthroughs. Want to know more?