The test being done on Reactor Number Four was to pilot it through a shutdown. There was definitely at least one human error made that night, but the system itself was the larger cause of the reactor being destabilized, leading to an explosion that completely destroyed the core of the reactor. What was left was a radioactive blaze of uranium fuel and graphite. For the better part of a day, it wasn't communicated that the reactor had been destroyed and that radiation was being released as people were afraid to deliver bad news or the destruction of the reactor was outside the realm of what they could accept. People weren't acknowledging that there even was an explosion, just saying there was an accident and it's being taken care of. Workers at the plant lived in the 50,000 person city of Priyat, a ten minute drive away and the order to evacuate the city was given at 10:00AM Sunday morning, with people receiving it at 1:10PM, a day and a half after the explosion. Around that time was when the first radiation cloud appeared over Denmark.
Initial efforts at stopping the release of radioactive contamination from the torn apart reactor, with fire still burning inside, had bags of sand and boron dropped from helicopters into the destroyed reactor. As the days went on after the explosion, there were two primary concerns, both having to do with the burning radioactive mass in the destroyed reactor, which combined with sand and boron had turned into a lava-like material burning its way down through the floor. If the mass came into contact with the water pools underneath the reactor, it could cause a steam explosion taking out the other three reactors, as Higginbotham writes generating "enough fallout into the atmosphere to render a large swath of Europe uninhabitable for a hundred years." And if that calamity was averted, there was also the possibility of the mass burning into the earth. This could have been cataclysmic if the mass got into the water tables, contaminating drinking water for millions.
To alleviate the first concern, someone had to go underneath the burning reactor and manually open the valves to pump out the water in the steam suppression pools, an extremely dangerous task that was completed successfully. The next concern was about the water tables. To alleviate this risk, they built a heat exchanger to cool the earth and stop the molten mass from continuing to melt through. This heat exchanger was never actually turned on, though, and what they eventually found was the reactor just burned itself out. All the efforts dropping sand and boron into the reactor were mostly pointless, but the lava did burn perilously close to getting through to the earth. The on the ground cleanup then involved people being drafted into service and serving as liquidators manually clearing debris, each working for a very short period of time to limit their exposure to radiation. Once this completed, a containment structure, or sarcophagus, was built around the destroyed reactor, completed seven months after the explosion. Due to the radioactive fallout, eventually every child from preschool to the seventh grade was temporarily evacuated out of Kiev, some 363,000 children, and there had to be a permanent resettlement of 116,000 people.
After this came the scapegoating, the sending to prison of the director and others involved and who lived. Mention in the investigation was made of design defects, but then swept aside in favor of the more palatable operator error assignment of blame. While it true that the operators played a small role in causing the disaster, the main fault lay with the design of the plant, the need for perfection on the part of the operators, and the aggressive Soviet bureaucracy behind everything. Higginbotham in the book provides a meticulously reported look at the disaster, its causes, the reaction to it, and the people involved.