The irony is that in both cases the risk of accident actually was very remote - assuming that established safety protocols were followed.
Culture of Complacency
In both nuclear accidents there was an entrenched culture of complacency. Corners were often cut and safety procedures ignored. At Chernobyl the danger of a nuclear meltdown was systematically downplayed and rules became lax. Igor Kazachkov, one of the shift operators at Chernobyl, stated "We didn't have any foolproof safeguards against this particular thing happening... There are lots of safeguards but nothing that controls the number of rods. We have often had less than the required number of rods [controlling the reaction] and nothing happened. No explosion, everything proceeded normally."
HOWSTUFFWORKS: How a Nuclear Reactor Works In other words, the plant had operated safely and things turned out okay when safety rules were ignored, so operators became complacent. This is human nature, and can be seen in the psychology of drunk drivers who think, "Well, the last few times I drove home safely, so I can do it again." Getting away with breaking the rules - especially repeatedly - makes the action seem less dangerous.