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April 2012

Seawater Enhances the Corrosion of Nuclear Fuel Rods

Seawater, used to cool overheating nuclear reactors in Japan, increases corrosion of nuclear fuel rods

Anne-Marie Carey

Peroxide linkage of uranyl ions gives a bent molecule that causes nanoscale cage clusters of uranyl peroxide to self-assemble. Uranium and oxygen atoms are shown as yellow and red spheres, respectively. Adapted from Armstrong et al. (2012). DOI: 10.1073/pnas.1119758109

The Fukushima-Daiichi nuclear accident, which followed the March 2011 Tōhoku earthquake and tsunami, is the largest nuclear disaster since the 1986 Chornobyl accident. Substantial volumes of seawater were used in attempts to cool the overheating nuclear reactors. Now, a new study conducted by a team of researchers at the Materials Science of Actinides Energy Frontier Research Center shows that seawater can potentially accelerate the corrosion of nuclear fuel rods, producing uranium compounds that could travel long distances and persist in the environment.

Corrosion of Nuclear Fuel Rods

The radioactive decay of nuclear fuel rods releases large amounts of energy that breaks water molecules apart, forming peroxide, a highly reactive oxygen species. This peroxide oxidizes uranium dioxide, or UO2, the main component of the nuclear fuel in the Fukushima-Daiichi plant, to a uranyl ion (UO2)2+ that is significantly more soluble in water than the fuel itself. In this study, the researchers investigated the formation and stability of the resulting uranium compounds by examining the movement of heat and energy through thermodynamic measurements and calculations.

Nanoscale Cages of Uranium

Peroxide strongly complexed uranyl ions and enhanced the solubility of the ions in alkaline conditions. Simple uranyl peroxide complexes contain a single uranyl ion; however, peroxide linkage of uranyl ions gives a bent molecule, which causes nanoscale cage clusters of uranyl peroxide, known as U60, housing up to 60 uranyl ions, to self-assemble. Under alkaline conditions, alkali metal ions balanced the negative charges of uranyl peroxide clusters, increasing their stability.

Persistence of Uranium in the Environment

The persistence of U60 clusters in the absence of peroxide was measured by dissolving crystallized U60 in water and observing the concentrations over time. After 294 days, concentration levels of the uranium-containing clusters had not changed. So, in the presence of sodium ions, plentiful in seawater, or other alkali metal ions, stable uranyl peroxide clusters are likely to form; their stability means that they could persist in the environment.

At the Fukushima-Daiichi nuclear plant, the authors believe that peroxide will accumulate in the stagnant pools of seawater within spent-fuel cooling ponds and reactor cores, increasing corrosion of nuclear fuel rods. Stable uranyl peroxide complexes are likely to form in the alkaline seawater and, given that the U60 clusters are thermodynamically stable even in the absence of free peroxide, may remain intact even beyond the radioactive environment and be transported long distances.

Peter Burns, one of the authors of the study and Director of the Materials Science of Actinides EFRC, does however caution that the situation in the reactor cores and cooling pools at Fukushima is extremely complex and poorly defined. “Although we have demonstrated in laboratory experiments that uranyl peroxides, including nanoscale cage clusters, form under conditions that may be similar to those at Fukushima, there is a great deal of uncertainty concerning their importance at Fukushima.” 

He further elaborates, “We lack a scientific basis to predict mechanisms of release of radioactive materials from damaged nuclear fuel in contact with water in general, and especially where the added components in seawater are present.”

There is clearly a need to understand the fate and persistence of radioactive compounds in the environment to comprehend and adequately address the legacy presented by the Fukushima-Daiichi nuclear accident. This is one of the goals of the Materials Science of Actinides EFRC.

More Information

Armstrong CR, M Nyman, T Shvareva, GE Sigmon, PC Burns and A Navrotsky. 2012. “Uranyl Peroxide Enhanced Nuclear Fuel Corrosion in Seawater.”  Proceedings of the National Academy of Sciences. DOI: 10.1073/pnas.1119758109.

Acknowledgments

This work was conducted by researchers at the University of Notre Dame, University of California and Sandia National Laboratories, as part of the Materials Science of Actinides EFRC and was funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences.

About the author(s):

  • Anne-Marie Carey recently joined the Photosynthetic Antenna Research Center working as a postdoctoral researcher in Professor Richard Cogdell’s group at the University of Glasgow. Her research is focused on the structure and function of light-harvesting complexes in purple bacteria, in the pursuit of optimized, tunable systems for artificial photosynthesis.

Tempest in a Flask

Highly active molecules and uranium clusters seen from using seawater to cool reactors

Peroxide linkage of uranyl ions gives a bent molecule that causes nanoscale cage clusters of uranyl peroxide to self-assemble. Uranium and oxygen atoms are shown as yellow and red spheres, respectively. Adapted from Armstrong et al. (2012). DOI: 10.1073/pnas.1119758109

When nuclear reactor cooling systems failed after the March 2011 earthquake and tsunami that battered Japan, substantial volumes of seawater were used to cool the radioactive fuel. The combination of damaged fuel, seawater and high levels of radiation created a unique system. Scientists re-created the conditions in a laboratory half a world away  to better understand at the molecular level what may be happening. The scientists factored in high levels of radiation by adding hydrogen peroxide, as well as uranium and other atoms found in the reactor fuel and chemically complex seawater. They determined that fractured water molecules form a highly reactive, oxygen-rich molecule that can degrade the nuclear fuel. In addition, they found that uranium ions join together to make clusters that that are more stable than their constituents. This research provides fundamental insights that could assist those doing the hard work of cleanup. This work was done by the Materials Science of Actinides Energy Frontier Research Center, led by the University of Notre Dame.

Written by Anne-Marie Carey and Kristin Manke

More Information

Armstrong CR, M Nyman, T Shvareva, GE Sigmon, PC Burns and A Navrotsky. 2012. “Uranyl Peroxide Enhanced Nuclear Fuel Corrosion in Seawater.”  Proceedings of the National Academy of Sciences. DOI: 10.1073/pnas.1119758109.

Disclaimer: The opinions in this newsletter are those of the individual authors and do not represent the views or position of the Department of Energy.