The research team published its work Jan. 23 in the journal Proceedings of the National Academy of Sciences.

Uranium in nuclear fuel rods is in a chemical form that is “pretty insoluble” in water, said Professor Alexandra Navrotsky, UC Davis, corresponding author on the paper, unless the uranium is oxidized to uranium-VI—a process that can be facilitated when radiation converts water into peroxide, a powerful oxidizing agent.

Peter Burns, professor of civil engineering and geological sciences at the University of Notre Dame and a co-author of the new paper, had previously made spherical uranium peroxide clusters, rather like carbon “buckyballs,” that can dissolve or exist as solids.

In the new paper, the researchers show that in the presence of alkali metal ions such as sodium—for example, in seawater—these clusters are stable enough to persist in solution or as small particles even when the oxidizing agent is removed.

The Fukushima-Daiichi nuclear accident brought together compromised irradiated fuel and large amounts of seawater in a high radiation field. Based on newly acquired thermochemical data for a series of uranyl peroxide compounds containing charge-balancing alkali cations, here we show that nanoscale cage clusters containing as many as 60 uranyl ions, bonded through peroxide and hydroxide bridges, are likely to form in solution or as precipitates under such conditions. These species will enhance the corrosion of the damaged fuel and, being thermodynamically stable and kinetically persistent in the absence of peroxide, they can potentially transport uranium over long distances.

—Armstrong et al.

In other words, these clusters could form on the surface of a fuel rod exposed to seawater and then be transported away, surviving in the environment for months or years before reverting to more common forms of uranium, without peroxide, and settling to the bottom of the ocean. There is no data yet on how fast these uranium peroxide clusters will break down in the environment, Navrotsky said.

This is a phenomenon that has not been considered before. We don’t know how much this will increase the rate of corrosion, but it is something that will have to be considered in future.

—Alexandra Navrotsky

Japan used seawater to avoid a much more serious accident at the Fukushima-Daiichi plant, and Navrotsky said, to her knowledge, there is no evidence of long-distance uranium contamination from the plant.

Navrotsky and Burns worked with the following co-authors: postdoctoral researcher Christopher Armstrong and project scientist Tatiana Shvareva, UC Davis; May Nyman, Sandia National Laboratories; and Ginger Sigmon, University of Notre Dame. The US Department of Energy supported the project.

Resources

• Christopher R. Armstrong, May Nyman, Tatiana Shvareva, Ginger E. Sigmon, Peter C. Burns, and Alexandra Navrotsky (2012) Uranyl peroxide enhanced nuclear fuel corrosion in seawater PNAS doi: 10.1073/pnas.1119758109