At 1:30am on April 26, 1986, an accident destroyed one the of four reactors at the Chernobyl Nuclear Power Plant in the city of Pripyat, in the Ukrainian SSR just south of the border with the Byelorussian SSR (now Belarus). Operators had temporarily shut down the reactor to recover from errors made during a safety test, and because of a flaw in the reactor design (known to top government officials but not the plant’s managers), this action triggered steam explosions that blew up the core and destroyed the reactor building, killing two engineers and severely burning two others. For several hours, managers didn’t realize the core had exploded or couldn’t bring themselves to admit it. Firefighters battling what they thought was an ordinary fire soon displayed symptoms of acute radiation syndrome, and 132 workers were hospitalized that day, of whom 28 died within four months. Although it was now clear that radiation levels were extremely high, top Party officials delayed informing or evacuating Pripyat residents until 36 hours after the explosion. A 10-km-radius “Exclusion Zone” was soon expanded to 30 km, and nearly 120,000 residents were evacuated. Hundreds of pilots flew helicopters directly over the burning reactor to drop sand, lead, clay, and boron in an unsuccessful and possibly counterproductive effort to extinguish the core fire, which continued to burn until May 4, releasing fluctuating levels of radiation.