Damaged seawater pump used to cool cores of Fukushima-Daiichi reactors. Credit: IAEA Image Bank.
When Japanese officials acted out of desperation and used seawater to cool the cores at the Fukushima-Daiichi reactors last year, it looks like they made the right call. But, others that might be tempted to use seawater to cool fuel rods in the future might not be so lucky.
ACerS emeritus member and University of California, Davis professor, Alexandra Navrotsky, Notre Dame researcher Peter Burns and several of their colleagues have offered some cautionary food for thought. They say in a new paper in the Proceedings of the National Academy of Sciences that there does seem to be a mechanism for how nuclear fuel rods could be corroded by contact with sea water.
It should immediately be pointed out that the authors aren’t in any way suggesting that this type of corrosion happened during the Fukushima-Daiichi incident, and they say there is no evidence of uranium dispersion during that episode due to the seawater.
However, they say it appears there is a way for the fuel rod–seawater combination to form “uranium compounds that could potentially travel long distances, either in solution or as very small particles,” according to a UC Davis news release.
The release quotes Navrotsky, a distinguished professor of ceramic, earth and environmental materials chemistry, as saying, “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.”
The uranium compounds in the fuel rods are thought to be generally insoluble in ordinary water. Nevertheless, she says it was previously known that if some of the water is converted to peroxide (radiation has the ability to do this conversion), the peroxide can then oxidize the uranium in the rods to uranium-VI, forming spherical uranium peroxide clusters that can dissolve in water.
The new wrinkle in this is that Navrotsky et al. discovered that if alkali metal ions are present — such as the sodium that is plentiful in seawater — the uranium peroxide clusters “are stable enough to persist in solution or as small particles even when the oxidizing agent is removed.”
So, the worrisome scenario is one where seawater comes in contact with the rods and forms these clusters. The clusters dissolved in the seawater are then carried away. Because, according to Navrotsky, little is known about quickly these uranium peroxide clusters break down in the seawater, the clusters may hang around for months or years before being converted back into a common form of uranium that will precipitate out to the bottom of the ocean.