The Chernobyl Nuclear Accident

On the 26th of April, 1986, a series of events caused the fourth reactor of the Chernobyl Nuclear Power Plant to explode and send a radioactive ash cloud into the atmosphere and across Europe. This was the worst nuclear disaster the world had seen, but the world was yet to realise that it had happened.

The events leading up to the accident are complex, intertwined with both human and engineering design error. When a nuclear reactor has been shut down, it still generates a small amount of residual heat, thus requiring cooling to be supplied. Before the accident occurred at the plant, the operators were planning to carry out an emergency test to ensure that the backup systems would be able to provide sufficient cooling to the plant in the event of a power loss. The emergency generators within reactor four would take approximately 60-75 seconds to start up and begin supplying cooling to the reactor. This was an unacceptable gap, and was an inherent design flaw with the RBMK reactors. The plant operators had devised a test that was to explore whether the turbines associated with reactor four could provide enough energy to keep the coolant pumps running whilst the standby generators came up to speed.

The test had been scheduled to run during the day shift on the 25th of April, however another regional power generating station went offline and a request from Kiev came through to keep power supplied to the city. The plant operators agreed to postpone the test. Later that evening at 23:04hrs, the Kiev Grid Controller allowed the shutdown of the reactor to resume. This delay had some serious consequences, the day shift had already departed, and the night shift would not start for another 50 minutes, well into the test. This left little time to prepare. The test called for the gradual shutdown of the reactor, to approximately 700MW, to take place. Due to a build up of the neutron absorbing xenon-135 in the core, the reactor power continued to decrease below the test parameters. As the reactor neared 500MW of power, the plant operator, Leonid Toptunov, accidently inserted the control rods. This caused the reactor to come to a near shutdown.

The decision was made to extract the control rods to try and restart the reactor. Due to the build up of the Xenon within the core, only 200MW power was achieved. This low power made it essential that the control rods be removed further, causing a large number of alarms to start sounding. Between 00:35hrs – 00:45hrs, alarms signalling a critical fault were ignored and the emergency protection system turned off the turbine generators. After a short period, a stable power level was reached and the test continued. At 01:23hrs, the experiment began, with the steam to the turbine generators being shut off, with four coolant pumps being supplied by power from the turbines as they were coasting down to a stop. As the momentum of the turbines generators decreased, the coolant supplied to the reactor slowed, causing steam pockets to building up within the core. This caused the reactor power to increase, due to the inability to absorb neutrons, and with the decreased water flow and increase in steam, the reactor power began to rise dramatically. During the test period, and this dangerous rise in power, the automatic control rod system was unable to counteract the effect by inserting the control rods into the core. At 01:23:40hrs an emergency shutdown of the core was triggered. A few seconds later, a massive power spike was recorded, the core overheated and the explosion occurred. Some of the fuel rods fracture and blocked the control rods from being inserted any further than one third of their full depth. The first explosion was caused by a build up of steam, the second explosion moments later was caused by a build up of hydrogen. This sent the roof of the reactor, radioactive components and a huge dust cloud into the local area.

Initially, the nearby city of Pripyat was not informed, nor evacuated and locals even gathered on a nearby bridge to watch the burning of the reactor. The rest of the world was also not informed; the Soviet Government sat on their hands and kept it quiet. Immediately after the explosion, fire fighters arrived at the plant to deal with the situation, they did not know they were dealing with a nuclear emergency and thought it a regular fire. The first man to arrive was Volodymyr Pravik. He received a fatal dose of radiation and died of radiation sickness within two weeks. The immediate priority of the fire-fighters was to deal with a small scale fires and protect Reactor 3, which was in the same building and backed onto Reactor 4. All the fires, except the one within the main reactor hall, had been extinguished by 05:00hrs. The fire within reactor four smouldered until the 10th of May, and only after 5000 tonnes of sand, lead, clay and boron had been dropped on to it from the air. A huge concrete sarcophagus was constructed around the core to stop any more radiation from leaking into the atmosphere. Teams of ‘Liquidators’ worked to shovel any radioactive debris from the roof, down into the reactor housing. Due to the high amount of radiation, they could only work for 40 seconds at a time. The liquidators were mainly conscript soldiers and were only provided with very basic protection.

The evacuation of Pripyat started at 11:00hrs on the 27th of April 1986, some 36 hours after the initial explosion. The evacuees were told that it would only last three days and were ordered to only take essential items with them. The entire city had been completely evacuated by 15:00hrs. Europe was still unaware what was happening, it was only when a Finnish Nuclear Power Plant detected a high spike in background radiation, did they realise something was up. It took 28 days for the Soviet Government to admit to the world the accident at the Chernobyl Nuclear Power Plant.

A 30km exclusion zone was put in place around the plant, which has since been redefined with the accurate measurements of the fall out. The Zone is a controlled military area, with only special, authorised visitors being permitted to enter. At the present, approximately 3000 workers are located within the zone, working on decommissioning the other three reactors, servicing the area, maintaining the sarcophagus and manning essential services, such as the fire brigade. A large number of these are foreign workers and scientists. A small amount of people returned to their homes within the zone, and refused to leave. The whole zone has recently been graded a nature reserve and is teeming with elk, deer, wolves, bear, a multitude of birds and insects and domestic pets now turned feral. It has also been a large scale study of the long term effects of radiation on animals and plants.

Modified: 6th Nov 2017