The explosion spread debris and radioactive material across the surrounding area, and over the following days and weeks, most of mainland Europe was contaminated with radionuclides that emitted dangerous amounts of ionizing radiation.
Dyatlov's first concern after the explosion was that an accident in the deaerators immediately above the control room could result in boiling water raining down from the ceiling.
Realizing the magnitude of the disaster, Dyatlov suspended coolant supply to the reactor,[6] although pumping of water would be resumed by order of Chief Engineer Nikolai Fomin around dawn.
[7] Dyatlov ran to the control room of Block 3 and instructed Rogozhkin to shut down reactor 3, overriding the latter's objections that Bryukhanov's permission was needed.
[10] Dyatlov then returned to control room 4 and ordered Akimov to call the daytime shift and get people to the affected unit; namely Lelechenko, whose crew had to remove hydrogen from the generator 8 electrolyzer.
"[11] Around 5:00 a.m., already feeling ill, Dyatlov made a brief report to Bryukhanov in the Civil Defense Bunker, showing him the final printouts of reactor parameters leading up to the explosion.
At 1:23:04 a.m., the test began, and the main circulation pumps started cavitating due to the extremely high temperature of inlet water.
The coolant started boiling in the reactor, and because of a combination of a positive void coefficient and xenon burnout, the power began to increase dramatically.
By 7:45 the group made their way back to the control room of unit 4, here Akimov apologised exclaiming that they were unable to fully restart feedwater flow; before running to vomit into a bin.
Though he tried his best to explain the work to the next shift, he was unable to stop vomiting and so was ordered along with Toptunov and Nekhaev to make their way to the infirmary; they were still wearing their soaking wet clothes.
However his condition quickly worsened, his gastrointestinal system and bone marrow ravaged by the radiation, his skin now swollen and charcoal black.
He was selected for receiving a fetal liver cell transplant, however he died on the 10th May, 2 weeks after the accident due to skin and intestinal injuries.
[15] A flat and powerful thud shook the building; he and his assistant Pshenichnikov thought it was a water hammer occurring during a turbine shutdown.
He was carried out by Gorbachenko and Pyotr Palamarchuk and died at 6 a.m. in the Pripyat hospital under care of the chief physician, Vitaly Leonenko, without regaining consciousness.
[18] A report by the Associated Press at the time, citing Soviet newspaper Pravda, claimed that Shashenok was buried two days later at a village near Chernobyl.
Kurguz was shortly afterwards evacuated by an ambulance; aware of dangers of radiation contamination, Genrikh took a shower and changed his clothes.
Yuvchenko, an engineer, was in his office between reactors 3 and 4, on level 12.5; he described the event as a shock wave that buckled walls, blew doors in, and brought a cloud of milky grey radioactive dust and steam.
He met a badly burned, drenched and shocked Viktor Degtyarenko, who asked him to rescue Khodemchuk; that quickly proved impossible as that part of the building did not exist anymore.
[21][22] They returned to the building and met Valeri Perevozchenko and two junior technicians, Kudryavtsev and Proskuryakov, ordered by Dyatlov to manually lower the presumably seized control rods.
With his face already tanned by the radiation, he went to the dosimetry room and asked Gorbachenko for radiation levels; Gorbachenko left with Palamarchuk to rescue Shashenok while Perevozchenko went through the graphite and fuel containing radioactive rubble on level 10 to the remains of room 306 in an unsuccessful attempt to locate Khodemchuk, close to debris emitting over 10,000 roentgens per hour (90 μA/kg).
They witnessed fires on levels 0 and +12, broken oil and water pipes, roof debris on top of turbine 7, and scattered pieces of reactor graphite and fuel, with the linoleum on the floor burning around them.
Palamarchuk unsuccessfully attempted to contact Shashenok in room 604, then ran around the turbo generator 8, down to level 0 and urged the two men from the Kharkov mobile laboratory, assigned to record the turbine 8 vibrations, to leave; they, however, had both already received a lethal radiation dose.
They ran through the de-aerator gallery to the right to the VRSO unit elevator, found it destroyed, so climbed up the staircase instead, towards level 36; they missed Kurguz and Genrikh, who used another stairwell.
He was found guilty and sentenced to 10 years in a labor camp, but he was released early due to his poor health and mental condition.
He arrived on the north side of the reactor building, and upon seeing the destroyed power unit, called for a stage 3 alarm, summoning all available fire departments in the Kyiv Oblast.
He entered the unit 4 transport corridor and spoke with a plant worker, who suggested the roof of the turbine hall was on fire.
After hearing this radio report, Viktor Kibenok climbed to the roof of the ventilation block, possibly out of curiosity—a decision that would ultimately cost him his life.
When they reached the fire escape staircase, they were met by Ivan Shavrey and Aleksandr Petrovsky as well as Anatoly Ivanchenko, who were already on their way up to reinforce Pravik's squad.
On May 6, 1986, plant mechanical engineers Oleksiy Ananenko, Valery Bespalov, and Boris Baranov, navigated through a series of underground corridors located beneath the fourth reactor building, which had become flooded by firefighting and coolant water in the days prior, to locate and open two release valves to drain the water.
[29] During the April 2018 ceremony, with the Chernobyl New Safe Confinement structure in the background, Poroshenko noted that the three men had been quickly forgotten at the time, with the Soviet news agency still hiding many of the details of the catastrophe.