It started when Exelon engineers found heat-transfer fill packs in the pool of cool water 45 feet below their normal perch at the Byron Generating Station nuclear power plant in Byron, Ill. Over years, the fill packs, which weigh less than 100 pounds new, had bulked up to about five times their original weight, due to the accumulation of silt and microorganisms. Occasionally, the fill packs became so heavy that they would break off an

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