According to the International Atomic Energy Agency, the disaster at Chernobyl was caused by a faulty reactor design and negligence on behalf of the workers operating the Unit 4 reactor. This reactor did not have a containment structure to prevent radiation leakage in the event of an accident. During the test that resulted in the explosion, workers ignored safety measures.
Reactor operators were conducting a low energy test when the reactor malfunctioned, causing an explosion. Greenpeace notes that the resulting fire burned for nine days, releasing large amounts of radiation into the surrounding area. The World Nuclear Association estimates that the explosion and fires released approximately 5 percent of the radioactive core into the environment. Two workers died the night of the explosion. Approximately 28 members of the clean-up crew died within three months from acute radiation poisoning.
Greenpeace notes that approximately 300,000 of the estimated 600,000 to 800,000 people involved in the clean up received more than 500 times the radiation dose limit for the public over one year. After the incident, a concrete sarcophagus structure was built around the damaged reactor to protect the rest of the plant. The entire plant was closed on Dec. 15, 2000, according to the IAEA.