Sequence of Events—Chernobyl Accident Appendix 1.” World Nuclear Association.
Updated June, 2019.

https://world-nuclear.org/information-library/safety-and-security/safety-of-plants/appendices/chernobyl-accident-appendix-1-sequence-of-events.aspx

Sequence of Events – Chernobyl Accident Appendix 1

(Updated June 2019)

During the course of a safety system test being carried out just before a routine maintenance outage, Chernobyl 4 was destroyed as a result of a power transient on 26 April 1986.

The accident at Chernobyl was the product of a lack of safety culture. The RBMK reactor design was poor from the point of view of safety and unforgiving for the operators, both of which provoked a dangerous operating state. The operators were not informed of this and were not aware that the test performed could have brought the reactor into an explosive condition. In addition, they did not comply with operational procedures^a. The combination of these factors provoked a nuclear accident of maximum severity in which the reactor was totally destroyed within a few seconds.

A full description of the accident, its environmental and health effects, as well as lessons learned, can be found in the information paper on the Chernobyl Accident.