The Chernobyl nuclear reactor after the disaster. Reactor 4 (center). Turbine building (lower left).
Reactor 3 (center right).

http://en.wikipedia.org/wiki/Chernobyl_disaster

Kiev - The Soviet-era reactor used at the Chernobyl nuclear power plant contained several design flaws that contributed directly to the 1986 accident, studies have shown.
Unlike modern reactors, the one employed at Chernobyl, known as a high-power channel-type reactor (RBMK), was not designed to contain unexpected radiation leaks.

Engineers at Lithuania's Ignalina plant, who also operated RMBK reactors, had uncovered this shortcoming before the Chernobyl accident.

The Soviet government made the Ignalina findings secret, and Chernobyl engineers were not aware of it when they conducted a low- power reactor test in the early morning hours of 26 April 1986.

Most RMBK reactors were taken out of commission after Chernobyl. Russia continues to operate six, which have received upgrades in control systems, housing and safety equipment.
Sources: International Atomic Energy Agency, Ukraine's Emergency Situations and Chernobyl Ministry.

Source:
http://www.monstersandcritics.com

Radioactive fallout after Chernobyl.