By far the worst nuclear accident in history started shortly after midnight on April 26, 1986, at the Chernobyl power plant in the Soviet republic of Ukraine.

The facility had been built during the 1970s using a design that lacked some of the safety features in many other nuclear power plants. Most notably, Chernobyl's reactors were not enclosed in sturdy containment buildings, like the massive concrete domes seen at other nuclear plants. Such shells are able to trap any radiation that accidentally escapes from the reactor core, the site of the nuclear fuel.

The accident at Chernobyl was due to multiple operating mistakes that compounded the serious design flaws. The operators were shutting down one of the plant's four nuclear reactors, something that had to be done every year to perform routine maintenance. But plant managers decided this would be a good time to...