In Reactors, Radiation Is Trapped and Contained in Several Ways:

Small amounts of radioactivity can be released into the environment but only under controlled and monitored conditions. The only major accident in a nuclear power plant in this country was at Three Mile Island near Harrisburg, Pennsylvania, in March 1979. At Three Mile Island, there was major fuel damage, and radioactive gases and contaminated cooling water filled the containment building. Some radioactivity was released into the atmosphere, but it didn’t hurt people or the environment.

A much more serious accident happened in 1986 at Chernobyl in the former Soviet Union. That reactor was built differently than those in the U.S. Most importantly, it had no containment system. The reactor core was severely damaged and a large amount of radioactivity was released into the environment.