... so that the reactor resulting in overheating, shattering fuel rods, coolant into steam
Answers (2)
en.wikipedia.org/wiki/Chernobyl_disaster#Causes
All nuclear disasters so far, except the one in Japan, have occurred during tests when safety devices were disabled specifically to create a dangerous condition. Japan has so many quakes that just building a power plant there was a major mistake.
Chernobyl Tragedy happened because the Chernobyl nuclear reactor had a number of design weaknesses that led to the fact that at one point it came out of control. These defects were not reflected in the operational documentation. I think that negligence of people is main reason. You may read more about Chernobyl disaster at tour2chernobyl.com/ChernobylDisaster