It’s a story of questionable reactor design, abysmal safety practices/culture, heroic responders, and government denials to try to save face on the international stage.

Chernobyl #4 was an RBMK-1000 reactor, which is a graphite moderated light-water cooled design. This is an important distinction to make, compared to water moderated/water cooled designs. This page (specifically the void coefficient section) explains the relevant science, but essentially, the RBMK-1000 design does not include one of the critical design safety features of most Western reactor designs.

Couple this missing design safety feature with the absence of safety culture and a lack of adherence to thoroughly written and reviewed procedures, and it’s unfortunately no real surprise that there was such a disaster.

The immediate response is full of stories of the bravery of workers and first responders, some of whom made the ultimate sacrifice to ensure the disaster did not get worse.

[Disclaimer: this is the part I’m least familiar with] And the government did all they could to sweep the incident under the rug and avoid the West learning about it, as is their MO (this attitude has persisted into the 21st Century, even. See: The Kursk disaster in 2000