Tokyo Electric Power Co., operator of Japan’s crippled Fukushima Dai-Ichi nuclear plant, said it detected the highest radiation to date at the site.

Geiger counters, used to detect radioactivity, registered more than 10 sieverts an hour, the highest reading the devices are able to record, Junichi Matsumoto, a general manager at the utility, said today. The measurements were taken at the base of the main ventilation stack for reactors No. 1 and No. 2.

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