Location of Chernobyl nuclear power plant The abandoned city of Pripyat with the Chernobyl facility visible in the distance The disaster began during a systems test on 26 April at reactor 4 of the Chernobyl plant near Pripyat and in proximity to the administrative border with Belarus and the Dnieper River.
There was a sudden and unexpected power surge. When operators attempted an emergency shutdown, a much larger spike in power output occurred. This second spike led to a reactor vessel rupture and a series of steam explosions. These events exposed the graphite moderator of the reactor to air, causing it to ignite. The plumes drifted over large parts of the western Soviet Union and Europe.
Thirty-six hours after the accident, Soviet officials enacted a kilometre exclusion zone , which resulted in the rapid evacuation of 49, people primarily from Pripyat, the nearest large population centre. Initially, the town itself was comparatively safe due to the favourable wind direction.
Until the winds began to change direction, shelter in place was considered the best safety measure for the town. A further 68, persons were evacuated, including from the town of Chernobyl itself. The rate of new construction builds for civilian fission-electric reactors dropped in the late s, with the effects of accidents having a chilling effect. The World Association of Nuclear Operators was formed as a direct result of the accident with the aim of creating a greater exchange of information on safety and on techniques to increase the capacity of energy production.
The accident raised the already heightened concerns about fission reactors worldwide, and while most concern was focused on those of the same unusual design, hundreds of disparate electric-power reactor proposals, including those under construction at Chernobyl, reactor No. There was a precipitous drop in the prior rate of new startups after In , thirty-one deaths were directly attributed to the accident, all among the reactor staff and emergency workers.
The Chernobyl Forum predicts that the eventual death toll could reach 4, among those exposed to the highest levels of radiation , emergency workers, , evacuees and , residents of the most contaminated areas ; this figure is a total causal death toll prediction, combining the deaths of approximately 50 emergency workers who died soon after the accident from acute radiation syndrome , 15 children who have died of thyroid cancer and a future predicted total of deaths from radiation-induced cancer and leukaemia.
Models predict that by about 16, cases of thyroid cancer and 25, cases of other cancers may be expected due to radiation from the accident, whereas several hundred million cancer cases are expected from other causes. Two anti-nuclear advocacy groups have publicized non-peer-reviewed estimates that include mortality estimates for those who were exposed to even smaller amounts of radiation.
The Union of Concerned Scientists UCS calculated that, among the hundreds of millions of people exposed worldwide, there will be an eventual 50, excess cancer cases, resulting in 25, excess cancer deaths, excluding thyroid cancer. Consequences of the Catastrophe for People and the Environment , which suggests that among the billions of people worldwide who were exposed to radioactive contamination from the disaster, nearly a million premature cancer deaths occurred between and The review by M.
Balonov published by the New York Academy of Sciences concludes that the report is of negative value because it has very little scientific merit while being highly misleading to the lay reader.
It characterized the estimate of nearly a million deaths as more in the realm of science fiction than science. As the reactor had not been encased by any kind of hard containment vessel , this dispersed large quantities of radioactive isotopes into the atmosphere : The accident occurred during an experiment scheduled to test the viability of a potential safety emergency core cooling feature, which required a normal reactor shutdown procedure.
This heat continues for some time after the chain reaction is stopped e. Analysis indicated that this residual momentum and steam pressure might be sufficient to run the coolant pumps for 45 seconds, : An initial test carried out in indicated that the excitation voltage of the turbine-generator was insufficient; it did not maintain the desired magnetic field after the turbine trip.
The system was modified, and the test was repeated in but again proved unsuccessful. In , the tests were attempted a third time but also yielded negative results. The test procedure would be repeated in , and it was scheduled to take place during the maintenance shutdown of Reactor Four. The test procedure was expected to begin with an automatic emergency shutdown. No detrimental effect on the safety of the reactor was anticipated, so the test programme was not formally coordinated with either the chief designer of the reactor NIKIET or the scientific manager.
Instead, it was approved only by the director of the plant and even this approval was not consistent with established procedures. If test conditions had been as planned, the procedure would almost certainly have been carried out safely; the eventual disaster resulted from attempts to boost the reactor output once the experiment had been started, which was inconsistent with approved procedure.
The station managers presumably wished to correct this at the first opportunity, which may explain why they continued the test even when serious problems arose, and why the requisite approval for the test had not been sought from the Soviet nuclear oversight regulator even though there was a representative at the complex of 4 reactors.
The reactor was to be running at a low power level, between MW and MW. The steam-turbine generator was to be run up to full speed. When these conditions were achieved, the steam supply for the turbine generator was to be closed off.
Turbine generator performance was to be recorded to determine whether it could provide the bridging power for coolant pumps until the emergency diesel generators were sequenced to start and provide power to the cooling pumps automatically. After the emergency generators reached normal operating speed and voltage, the turbine generator would be allowed to continue to freewheel down. Conditions before the accident The conditions to run the test were established before the day shift of 25 April The day-shift workers had been instructed in advance and were familiar with the established procedures.
A special team of electrical engineers was present to test the new voltage regulating system. A schematic diagram of the reactor At this point, another regional power station unexpectedly went offline, and the Kiev electrical grid controller requested that the further reduction of Chernobyl's output be postponed, as power was needed to satisfy the peak evening demand. The Chernobyl plant director agreed, and postponed the test. Given the other events that unfolded, the system would have been of limited use, but its disabling as a "routine" step of the test is an illustration of the inherent lack of attention to safety for this test.
This delay had some serious consequences: According to plan, the test should have been finished during the day shift, and the night shift would only have had to maintain decay heat cooling systems in an otherwise shut-down plant. Alexander Akimov was chief of the night shift, and Leonid Toptunov was the operator responsible for the reactor's operational regimen, including the movement of the control rods.
Toptunov was a young engineer who had worked independently as a senior engineer for approximately three months. Due to the reactor's production of a fission byproduct, xenon , which is a reaction-inhibiting neutron absorber , core power continued to decrease without further operator action—a process known as reactor poisoning. This continuing decrease in power occurred because in steady state operation, xenon is "burned off" as quickly as it is created from decaying iodine by absorbing neutrons from the ongoing chain reaction to become highly stable xenon When the reactor power was lowered, previously produced high quantities of iodine decayed into the neutron-absorbing xenon faster than the reduced neutron flux could burn it off.
The operation of the reactor at the low power level and high poisoning level was accompanied by unstable core temperature and coolant flow, and possibly by instability of neutron flux, which triggered alarms. As part of the test plan, extra water pumps were activated at The increased coolant flow rate through the reactor produced an increase in the inlet coolant temperature of the reactor core the coolant no longer having sufficient time to release its heat in the turbine and cooling towers , which now more closely approached the nucleate boiling temperature of water, reducing the safety margin.
The flow exceeded the allowed limit at At the same time, the extra water flow lowered the overall core temperature and reduced the existing steam voids in the core and the steam separators. The crew responded by turning off two of the circulation pumps to reduce feedwater flow, in an effort to increase steam pressure, and by removing more manual control rods to maintain power. Nearly all of the control rods were removed manually, including all but 18 of the "fail-safe" manually operated rods of the minimal 28 which were intended to remain fully inserted to control the reactor even in the event of a loss of coolant, out of a total control rods.
Further, the reactor coolant pumping had been reduced, which had limited margin so any power excursion would produce boiling, thereby reducing neutron absorption by the water.
The reactor was in an unstable configuration that was outside the safe operating envelope established by the designers. If anything pushed it into supercriticality, it was unable to recover automatically. Experiment and explosion This section needs additional citations for verification. Please help improve this article by adding citations to reliable sources.
Unsourced material may be challenged and removed. April Learn how and when to remove this template message Radioactive steam plumes continued to be generated days after the initial explosion, as evidenced here on 3 May due to decay heat. The roof of the turbine hall is damaged image centre. Roof of the adjacent reactor 3 image lower left shows minor fire damage. Igor Kostin would take some of the clearer pictures of the roof of the buildings when he was physically present on the roof of reactor 3, in June of that year.
Four of the main circulating pumps MCP were active; of the eight total, six are normally active during regular operation. The steam to the turbines was shut off, beginning a run-down of the turbine generator.
The diesel generators started and sequentially picked up loads; the generators were to have completely picked up the MCPs' power needs by In the interim, the power for the MCPs was to be supplied by the turbine generator as it coasted down. As the momentum of the turbine generator decreased, so did the power it produced for the pumps. The water flow rate decreased, leading to increased formation of steam voids bubbles in the core. Because of the positive void coefficient of the RBMK reactor at low reactor power levels, it was now primed to embark on a positive feedback loop, in which the formation of steam voids reduced the ability of the liquid water coolant to absorb neutrons, which in turn increased the reactor's power output.
This caused yet more water to flash into steam, giving a further power increase. During almost the entire period of the experiment the automatic control system successfully counteracted this positive feedback, inserting control rods into the reactor core to limit the power rise.
This system had control of only 12 rods, and nearly all others had been manually retracted. The reason why the EPS-5 button was pressed is not known, whether it was done as an emergency measure in response to rising temperatures, or simply as a routine method of shutting down the reactor upon completion of the experiment. There is a view that the SCRAM may have been ordered as a response to the unexpected rapid power increase, although there is no recorded data proving this.
Despite this, the question as to when or even whether the EPS-5 button was pressed has been the subject of debate. There have been assertions that the manual SCRAM was initiated due to the initial rapid power acceleration.
Others have suggested that the button was not pressed until the reactor began to self-destruct, while others believe that it happened earlier and in calm conditions. The control rod insertion mechanism moved the rods at 0. A bigger problem was the design of the RBMK control rods , each of which had a graphite neutron moderator rod attached to the end to boost reactor output by displacing water when the control rod section had been fully withdrawn from the reactor.
Thus, when a control rod was at maximum extraction, a neutron-moderating graphite extension was centered in the core with a 1. Therefore, injecting a control rod downward into the reactor during a SCRAM initially displaced neutron-absorbing water in the lower portion of the reactor with neutron-moderating graphite on its way out of the core. As a result, an emergency SCRAM initially increased the reaction rate in the lower part of the core as the graphite section of rods moving out of the reactor displaced water coolant.
This behaviour was revealed when the initial insertion of control rods in another RBMK reactor at Ignalina Nuclear Power Plant in induced a power spike, but since the subsequent SCRAM of that reactor was successful, the information was disseminated but deemed of little importance.
A few seconds into the SCRAM, a power spike occurred, and the core overheated, causing some of the fuel rods to fracture, blocking the control rod columns and jamming the control rods at one-third insertion, with the graphite displacers still in the lower part of the core. Apparently, the power spike caused an increase in fuel temperature and steam buildup, leading to a rapid increase in steam pressure.
This caused the fuel cladding to fail, releasing the fuel elements into the coolant, and rupturing the channels in which these elements were located. It was not possible to reconstruct the precise sequence of the processes that led to the destruction of the reactor and the power unit building, but a steam explosion , like the explosion of a steam boiler from excess vapour pressure, appears to have been the next event.
There is a general understanding that it was explosive steam pressure from the damaged fuel channels escaping into the reactor's exterior cooling structure that caused the explosion that destroyed the reactor casing, tearing off and blasting the upper plate, to which the entire reactor assembly is fastened, through the roof of the reactor building. This is believed to be the first explosion that many heard. The total water loss in combination with a high positive void coefficient further increased the reactor's thermal power.