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Hydrogen generated from the damaged fuel in the reactors accumulated in the reactor buildingseither during venting operations or from other leaksand ignited, producing explosions in the Unit 1 and Unit 3 reactor buildings and significantly complicating the response. The hydrogen generated in Unit 3 may have migrated into the Unit 4 reactor building, resulting in a subsequent explosion and damage. The loss of primary and secondary containment integrity resulted in ground-level releases of radioactive material. Following the explosion in Unit 4 and the abnormal indications on Unit 2 on the fourth day of the event, the site superintendent directed that all nonessential personnel temporarily evacuate, leaving approximately 70 people on site to manage the event.
During releases, dose rates as high as 1,193 millirem per hour (mrem/hr) (11.93 mSv/hr) were measured at the site boundary, approximately 0.6 miles (1 km) from units 14.

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windows for the emergency response center had to be covered with lead shielding to reduce dose rates in the center. Organized off-site radiation surveys began on March 16. Radiation levels off site at that time ranged from 0.1 mrem/hr (1 μSv/hr) to 20 mrem/hr (200 μSv/hr). Thirty-seven miles (60 km) northwest of the station, the dose rate was 0.8 mrem/hr (8 μSv/hr). Water and soil samples indicated the presence of strontium, iodine, and cesium. Food and water restrictions were implemented in some areas as a result of radioactivity. People within the 12.4 miles (20 km) surrounding the station were evacuated, and those living up to 18.6 miles (30 km) away were directed to shelter inside their homes as the releases of radioactive gases and materials increased as the event progressed and more fuel damage occurred. Potassium iodide tablets and powder were distributed to local governments beginning March 21. Because the evacuations had already been completed, however, the potassium iodide was not issued to the population.
Radiation surveys of the on-site areas surrounding units 13 showed dose rates as high as 13 rem/hr (0.13 Sv/hr) in areas around units 2 and 3. More detailed surveys performed over the following weeks discovered localized dose rates greater than 1,000 rem/hr (10 Sv/hr) around equipment and debris outside units 1 and 3.
Some personnel who responded to the event received high doses of radiation. Two control room operators received the highest dosesa calculated internal and external dose of 67.8 rem (0.678 Sv) and 64.3 rem (0.643 Sv). The majority of dose received by these workers was internal (85-87 percent). Potassium iodide was provided to some station personnel on March 13. As of the end of March, approximately 100 workers had received doses of greater than 10 rem (0.1 Sv).
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