Too hot to handle

By Andy Coghlan MICROORGANISMS that corrode metal have been discovered in the ponds where spent nuclear fuel rods are stored. Safety experts fear that the bacteria could cause leaks in the metal cladding around the fuel rods. The Westinghouse Savannah River Company, which manages the Savannah River nuclear repository in Aiken, South Carolina, presented the findings last week in Atlanta. “If you get large amounts of microbial growth, it could become a problem,” says Carl Fliermans, who heads a team investigating the phenomenon at the Savannah River Technology Center. The water which shields stored fuel rods is supposed to be sterile and free of the nutrients that bacteria need to grow. “We’ve had these systems going since the mid-1950s and we’ve not seen a problem with corrosion,” says Fliermans. The finding that metal-munching bacteria can thrive in the barren environment of the storage ponds is ill-timed: a worldwide downturn in nuclear reprocessing means that in some countries the rods are being stored for years, rather than the few months that was intended. Fliermans and his colleagues discovered the problem when they examined samples of cladding materials that had been dunked in seven of the eight basins at Savannah River for up to a year. The samples were tiny rods, each 1 centimetre long, imbedded in Teflon and lowered to four different depths beside the fuel rods. “We used them as surrogates,” says Fliermans. “They go down there right beside the rods, so you get the same radiation levels and chemistry.” The researchers tested two types of chromium-nickel stainless steel and two types of aluminium-based alloys. The aluminium alloys came off worst. After just three weeks, bacterial cells stuck to the rods’ surfaces. After 12 months, they were caked with a biofilm of bacteria, and showed cracks and pitting typical of corrosion. Many of the bacteria reduce sulphates and produce corrosive acids. The biofilms contained radioactive alpha and beta particles, as well as traces of radioactive isotopes of caesium and europium—suggesting that some material from the neighbouring fuel rods had seeped through their cladding. “We are concerned about exposure of workers and the environment to anything that would breach this cladding,” says Fliermans. He suspects that the bacteria reached the ponds on rods brought in from abroad. Algae growing at the edges of the ponds may have provided the necessary nutrients, he says. Adding biocides such as chlorine to the ponds is not the answer, says Fliermans, as these could contribute to the corrosion. So contaminated rods will have to be isolated and treated separately. British Nuclear Fuels has also found biofilms on fuel rods stored at the Sellafield nuclear reprocessing plant in Cumbria. However, the bacteria in these films do not cause corrosion. More on these topics:
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