A fatal dose of radiation is estimated at around 500 rem—roentgen equivalent man—or the amount absorbed by the average human body when exposed to a field of 500 roentgen per hour for sixty minutes. In some places on the roof of Unit Three, lumps of uranium fuel and graphite were emitting gamma and neutron radiation at a rate of 3,000 roentgen an hour. In others, levels may have reached more than 8,000 roentgen an hour: there, a man would absorb a lethal dose in less than four minutes.
