Dozens of workers at Japan’s tsunami-hit nuclear power plant have come down with symptoms of stomach flu, halting a radioactive waste cleanup operation.

The operator of the Fukushima Dai-ichi nuclear power plant said on Saturday that a norovirus outbreak is suspected. Fifty-two workers assigned to the effort have been treated at a hospital over the past three days. Three of the workers have tested positive for the virus, a common form of flu.

The revelation comes a day after Japan declared stability at the plant, marking a milestone nine months after the March tsunami caused the worst nuclear crisis since Chernobyl.

The plant operator said the loss of the workers does not affect the plant’s essential reactor cooling functions.

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