During which construction project did 250,000 workers reach their lifetime radiation limits ?
The worst of the radioactive debris was collected inside what was left of the reactor , much of it shoveled in by liquidators wearing heavy protective gear ( dubbed " bio - robots " by the military ) ; these workers could only spend a maximum of 40 seconds at a time working on the rooftops of the surrounding buildings because of the extremely high doses of radiation given off by the blocks of graphite and other debris .