On this day in 1978 the Three Mile Island crisis began. This would be the worst nuclear power accident in American history. Three Mile Island was a nuclear power plant near Harrisburg, Pennsylvania. A pressure valve failed to close and the reactor core nearly melted down. Within days, more than 100,000 people had fled the area. The partial meltdown resulted in the release of small amounts of radioactive gases and radioactive iodine into the environment. There hasn’t been a new nuclear power plant built in the United States since the Three Mile Island incident.