From UC Davis News and Information (1/26/2012; emphasis is mine):
Japan used seawater to cool nuclear fuel at the stricken Fukushima-Daiichi nuclear plant after the tsunami in March 2011 — and that was probably the best action to take at the time, says Professor Alexandra Navrotsky of the University of California, Davis.
But Navrotsky and others have since discovered a new way in which seawater can corrode nuclear fuel, forming uranium compounds that could potentially travel long distances, either in solution or as very small particles. The research team published its work Jan. 23 in the journal Proceedings of the National Academy of Sciences.
“This is a phenomenon that has not been considered before,” said Alexandra Navrotsky, distinguished professor of ceramic, earth and environmental materials chemistry. “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.”