The Chernobyl Power plant located in Chernobyl(about 130km above Kiev, Ukraine) was a fission powered nuclear power plant but on April 26th 1986, Chernobyl became the city that hosted the worlds worst nuclear reactor catastrophe known as the Chernobyl Disaster.
This disaster killed over 30 residents and emergency workers with several more dying in later months. Subsequently all people living within a 30km radius of the power plant were evacuated within a few days (over 200000). However, the areas contaminated contained about 5 million people and more than 200000 live in areas of strict control due to the radiation's severity. The explosions would release a total of 5% of the nuclear reactor's radioactive core into the atmosphere.
The accident was the direct result of a failed reactor test causing a single reactor to explode due to an obsolete reactor design accompanied by insufficiently trained operating personnel who made critical errors and mistakes.
Initially the radiation exposure was extremely devastating because of the radioactive iodine-131 released which only had a short half-life of 8 days. This basically meant that it let out butt loads of radiation in a VERY short amount of time.