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The amount of radioactivity given off by substances such as uranium was measured in roentgens. These units of measurement are named after Wilhelm Roentgen, the man who discovered X-rays—radioactive rays similar to gamma rays. As scientists investigated further, they discovered radioactivity’s harmful effects on humans. At that point, scientists developed a measurement known as a rad (radiation absorbed dose). More research revealed the different types of radioactive rays, as well as the fact that each ray had different biological effects on humans.

Such a response would cause the reactor to be flooded. But shutting off the alert system also disabled the pumps that sent water into the reactor’s cooling system. By shortly after midnight, the water level in the cooling system had fallen to less than 1 percent capacity. This caused the reactor to overheat. , Eastern European Summer Time (EEST), Toptunov saw that the energy level in the reactor was suddenly rising. He alerted Akimov, who immediately moved to completely shut down the reactor by reinserting the control rods to stop the fission process.

Before anyone at Pripyat had to make a final decision, Brukhanov received word from Moscow that scientists and party ministers would arrive at the plant by noon. No action was to be taken until then. In Pripyat, townspeople began the day—Saturday—as usual. At about noon, less than 12 hours after the blast, one resident came inside after sunning himself on his apartment roof. He cheerfully noted that he had never tanned so quickly. Within hours, he was taken to the hospital, vomiting uncontrollably.