Powerplants: when bigger isn't better.

Biocorrosion of cooling-water systems in large electricity generating plants appears to be a growing problem. Materials engineers are now finding that contrary to good engineering practice, pipes and tanks that had been filled with water to test for leaks are not always drained and thoroughly dried at the completion of those tests.

Meryl Bibb of the Electricity Supply Commission in Johannesburg, South Africa, reported on one case where an inch-thick pipe developed a 20-millimeter diameter perforation only six months after a powerplant's startup. Though the pipe had been lined with polyamide epoxy to protect the metal from the corrosive ravages of water and 'bugs,' an inspection showed bacteria ate right through the epoxy to set up hard mounds protecting their colony on the metal surface below. A pigment in the epoxy appears to have provided a gourmet meal for those bacteria. Once the lining was breached, other microbes moved in, including the infamous, corrosion-fostering Desulfovibrio. (see p. 43).

Bibb says biocorrosion is more of a problem in newer plants constructed since 1975. And she attributes that to their size; they are up to 9 times larger than their predecessors. The bigger the plant, the longer the lag--and Microbial-incubation period--between leak testing and plant startup.

Daniel Pope of Renselaer Polytechnic Institute in Troy, N.Y., noted that U.S. plants appear to be suffering similar problems, and for the same reasons. Ironically, he notes, the stainless steels used extensively throughout U.S. nuclear plants are among the most susceptible to this biocorrosion. Based on this finding, he and colleague David Duquette are now conducting a biocorrosion survey of nuclear plants for the Electric Power Research Institute in Palo Alto, Calif.