What is the Chernobyl nuclear accident?

A steam explosion followed by graphite fire that destroyed an RBMK nuclear power reactor, dispersing most of the radioactive material in the reactor's core across the Ukraine and much of Europe.
The firefighters and many other emergency workers called to the site died from radiation poisoning.
The causes of the disaster are complex including an unsafe design, improperly performing a test, repeated failure to follow safe operating procedures, having a manager in charge of the test with no nuclear reactor experience (his training was in hydroelectric power), etc.