Lethal levels of radiation were measured Monday at the bottom of a ventilation tower on the grounds between reactors No. 1 and 2, Tokyo Electric Power Company spokesman Naoki Tsunoda.
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The radiation levels measured at 10 sieverts are high enough that a single 60-minute dose would be fatal within weeks.
In addition, 10 sieverts is the highest the Geiger counters used are capable of reading, indicating the actual level could be even higher, Junichi Matsumoto, a general manager at the utility, said at a press conference.
The Fukushima Daiichi disaster occurred when a 15-meter (48-foot) tsunami inundated the coastal plant after northern Japan's historic March 11 earthquake.
The flooding knocked out the cooling systems for the three operating reactors and their associated spent fuel pools, causing the reactors to overheat and hydrogen gas explosions that blew apart the building housing reactors No. 1 and 3.
Another hydrogen blast is believed to have damaged the inside of the No. 2 reactor, while engineers are struggling to manage an estimated 100,000 tons of highly contaminated water that was used to cool the reactors during the emergency.
Tokyo Electric Power Company projects the situation won't be fully over until sometime between October and January. The disaster has caused Japan to rethink its commitment to nuclear energy, and Germany has since announced plans to abandon atomic power entirely by 2022.