information:highlighting, structural:summarization, linguistic:paraphrasing
Debris removal

In the months after the explosion, attention turned to removing the radioactive debris from the roof. While the worst of the radioactive debris had remained inside what was left of the reactor, an estimated 100 tons of debris on that roof had to be removed to enable the safe construction of the "sarcophagus"—a concrete structure that would entomb the reactor and reduce radioactive dust being released. The initial plan was to use robots to clear the roof. The Soviets used approximately 60 remote-controlled robots, most of them built in the Soviet Union. Many failed due to the difficult terrain, combined with the effect of high radiation fields on their batteries and electronic controls. In 1987, Valery Legasov, first deputy director of the Kurchatov Institute of Atomic Energy in Moscow, said: "We learned that robots are not the great remedy for everything. Where there was very high radiation, the robot ceased to be a robot—the electronics quit working."

Consequently, the most highly radioactive materials were shoveled by Chernobyl liquidators from the military, wearing protective gear (dubbed "bio-robots"). These soldiers could only spend a maximum of 40–90 seconds working on the rooftops of the surrounding buildings because of the extremely high radiation levels. Only 10% of the debris cleared from the roof was performed by robots; the other 90% was removed by 3,828 men who absorbed, on average, an estimated dose of 25 rem (250 mSv) of radiation each.