Chernobyl disaster was biggest nuclear accident, but not the biggest disaster related to energy production. In 1975 the failure of the Banqiao Reservoir Dam and other dams in Henan Province, China caused more casualties than any other dam failure in history. The disaster killed an estimated 171,000 people and 11 million people lost their homes. It's not much known in the West because various elements of the Chinese government concealed the details of the disaster until the 1990s.

https://en.wikipedia.org/wiki/1975_Banqiao_Dam_failure

In terms of loss of life Chernobyl disaster is comparable to Bhopal disaster (at least 3,787 over 16,000 claimed deaths), caused by Methyl isocyanate leak from the storage tank. A combination of poorly-maintained or non-functional facilities, a disregard of safety standards, and an under-trained workforce made it possible for water to leak into the MIC tanks in the absence of properly working safety systems.

https://en.wikipedia.org/wiki/Bhopal_disaster

Chernobyl disaster was caused by a combination of reactor design error and operator error. After this errors have been fixed (different nuclear fuel composition, modern monitoring and controlling equipment, improved operator training and procedures) there are still 7 operational RBMK in Russia. (The same reactor type as Chernobyl)