Testing of this nuclear safety feature in April of 1986 turned into this deadly disaster as established procedures were not followed and in the minutes leading up to the accident, emergency warnings were ignored and mistakes were made. There also proved to be several flaws within the design of the Soviet RBMK nuclear reactor and in its construction. In the early hours on the 26th of April, the experiment of testing the steam turbines to produce electricity for the reactor's water pumps began while the diesel generators began ramping up their electricity production. Where the test went wrong was when the water flow rate dropped due to the turbines losing momentum, steam voids formed within the nuclear core that caused the water coolant system to not work as effectively, which in turn drove up the power output from the nuclear reactor. As the power output increased, more heat was generated that caused more water to turn into steam and this was a cascading effect as the reactor continued outputting more power due to the rising temperature.
The rise in temperature and power was attempted to be offset by cooling down the nuclear rods via inserting them into the core, but due to technical problems, the nuclear fuel rods could not be fully inserted in time. The temperature / power output jumped greatly before the first explosion occurred due to the steam build-up as the top of the reactor blew off. Fires began on other buildings at the Chernobyl Nuclear Plant from burning graphite and other radioactive material that was shot from Reactor #4. There are many details of this accident not fully known due to, among other reasons, many of the key plant employees having died from the radiation. In the end, the Chernobyl nuclear accident is attributed to testing out a nuclear safety feature that exploited many mistakes from human error to design/construction flaws with the RBMK reactor. Worth noting is that none of the reactors at Chernobyl had a containment building, which is effectively an airtight shield made of steel and concrete that covers the nuclear reactor and presents radiation from escaping into the atmosphere. Such containments are standard in nuclear power plants found in the United States and other parts of the world, especially after the devastating nuclear accident at Chernobyl. If such a containment been in place on these reactors, the disaster should not have been nearly as devastating.
In regards to the severity of this nuclear accident, this is the only nuclear power plant disaster to ever be declared a level seven event on the International Nuclear Event Scale. A level seven nuclear event is the most severe as rated by the International Atomic Energy Agency (IAEA) with an event of this magnitude causing widespread health and environmental effects far beyond just the vicinity of the nuclear accident. For comparison, the commonly known Three Mile Island nuclear accident in Pennsylvania and the Windscale Fire in the United Kingdom were each rated only a level five event on this Richter-like scale of nuclear tragedies. There was only ever a single nuclear accident that was declared a level six event -- the second worse nuclear disaster in history -- by the IAEA and that too took place in the former Soviet Union more than 50 years ago. As another scale for comparison, the Chernobyl nuclear accident led to more than one hundred times the nuclear fallout of what was experienced during the atomic bombings of Hiroshima and Nagasaki during World War II.