<human>:  When did the exclusion zone begin?
<bot>: The exclusion zone was established on April 26, 1986. The Chernobyl nuclear reactor exploded on April 26, 1986, which released a large amount of radiation into the environment. The accident contaminated the surrounding land, air, and water. The exclusion zone was established to keep people away from the contaminated areas, so they could not be exposed to any additional radiation.