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The Chernobyl disaster was a nuclear accident that occurred on 26 April at the No. The accident occurred during a safety test meant to measure the ability of the steam turbine to power the emergency feedwater pumps of an RBMK-type nuclear reactor in the event of a simultaneous loss of external power and major coolant leak.

During a planned decrease of reactor power in preparation for the test, the operators accidentally dropped power output to near-zero, due partially to xenon poisoning. In an attempt to restore the power level specified by the test program, the operators removed a number of control rods which exceeded limits set by the operating procedures. Upon test completion, the operators triggered a reactor shutdown. Due to a design flaw, this action resulted in localized increases in reactivity within the reactor i.

This resulted in rupture of fuel channels, leading to a rapid decrease in pressure which caused the coolant to flash to steam. This decreased neutron absorption, leading to an increase in reactor activity, which further increased coolant temperatures a positive feedback loop.

This process resulted in steam explosions and melting of the reactor core. The meltdown and explosions ruptured the reactor core and destroyed the reactor building.

This was immediately followed by an open-air reactor core fire which lasted until 4 May , during which airborne radioactive contaminants were released which were deposited onto other parts of the USSR and Europe.

Following the reactor explosion, which killed two engineers and severely burned two more, a massive emergency operation to put out the fire, stabilize the reactor, and clean up the ejected radioactive material began. During the immediate emergency response, workers were hospitalized, of which exhibited symptoms of acute radiation syndrome.

Among those hospitalized, 28 died within the following three months, all of whom were hospitalized for ARS. In the following 10 years, 14 more workers 9 who had been hospitalized with ARS died of various causes mostly unrelated to radiation exposure. Chernobyl’s health effects to the general population are uncertain. An excess of 15 childhood thyroid cancer deaths were documented as of [update].

The most widely cited studies by the World Health Organization predict an eventual 4, fatalities in Ukraine, Belarus and Russia. Following the disaster, Pripyat was replaced by the new purpose-built city of Slavutych. It reduced the spread of radioactive contamination from the wreckage and protected it from weathering. The confinement shelter also provided radiological protection for the crews of the undamaged reactors at the site, which were restarted in late and However, this containment structure was only intended to last for 30 years, and required considerable reinforcement in the early s.

The Shelter was supplemented in by the Chernobyl New Safe Confinement which was constructed around the old structure. This larger enclosure aims to enable the removal of both the sarcophagus and the reactor debris while containing the radioactive materials inside. Clean-up is scheduled for completion by This decay heat continues for some time after the fission chain reaction has been stopped, such as following a reactor shutdown, either emergency or planned, and continued pumped circulation of coolant is essential to prevent core overheating, or in the worst case, core meltdown.

In this scenario the emergency core cooling system ECCS needed to pump additional water into the core, replacing coolant lost to evaporation. The turbine’s speed would run down as energy was taken from it, but analysis indicated that there might be sufficient energy to provide electrical power to run the coolant pumps for 45 seconds. The turbine run-down energy capability still needed to be confirmed experimentally, and previous tests had ended unsuccessfully.

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 electrical system was modified, and the test was repeated in but again proved unsuccessful.

In , the test was conducted a third time but also yielded no results due to a problem with the recording equipment. The test procedure was to be run again in and was scheduled to take place during a controlled power-down of reactor No. A test procedure had been written, but the authors were not aware of the unusual RBMK reactor behaviour under the planned operating conditions.

According to the regulations in place at the time, such a test did not require approval by either the chief design authority for the reactor NIKIET or the Soviet nuclear safety regulator. The test was to be conducted during the day-shift of 25 April as part of a scheduled reactor shut down. The day shift crew had been instructed in advance on the reactor operating conditions to run the test and in addition, a special team of electrical engineers was present to conduct the one-minute test of the new voltage regulating system once the correct conditions had been reached.

Soon, the day shift was replaced by the evening shift. At , the Kyiv grid controller allowed the reactor shutdown to resume. This delay had some serious consequences: the day shift had long since departed, the evening shift was also preparing to leave, and the night shift would not take over until midnight, well into the job. 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.

The night shift had very limited time to prepare for and carry out the experiment. Anatoly Dyatlov , deputy chief-engineer of the entire Chernobyl Nuclear Power Plant , was present to supervise and direct the test as one of its chief authors and the highest-ranking individual present.

Unit Shift Supervisor Aleksandr Akimov was in charge of the Unit 4 night shift, and Leonid Toptunov was the Senior Reactor Control Engineer responsible for the reactor’s operational regimen, including the movement of the control rods. The test plan called for a gradual decrease in reactor power to a thermal level of — MW [25] and an output of MW was reached at on 26 April. In steady-state operation, this is avoided because xenon is “burned off” as quickly as it is created from decaying iodine by the absorption of neutrons from the ongoing chain reaction, becoming highly stable xenon With the reactor power reduced, high quantities of previously produced iodine were decaying into the neutron-absorbing xenon faster than the reduced neutron flux could “burn it off.

When the reactor power had decreased to approximately MW, the reactor power control was switched from LAR Local Automatic Regulator to the Automatic Regulators, in order to manually maintain the required power level. In response, Toptunov reduced power to stabilize the Automatic Regulators’ ionization sensors.

The result was a sudden power drop to an unintended near- shutdown state, with a power output of 30 MW thermal or less.

The exact circumstances that caused the power drop are unknown. Most reports attribute the power drop to Toptunov’s error, but Dyatlov reported that it was due to a fault in the AR-2 system. To increase power, control-room personnel had to remove numerous control rods from the reactor. Over the next twenty minutes, reactor power would be increased further to MW. The operation of the reactor at the low power level and high poisoning level was accompanied by unstable core temperatures and coolant flow, and, possibly, by instability of neutron flux.

In response, personnel triggered several rapid influxes of feedwater. Relief valves opened to relieve excess steam into a turbine condenser. When a power level of MW was reattained, preparation for the experiment continued, although the power level was much lower than the prescribed MW. As part of the test program, two additional main circulating coolant pumps were activated at The increased coolant flow lowered the overall core temperature and reduced the existing steam voids in the core.

Because water absorbs neutrons better than steam, the neutron flux and reactivity decreased. The operators responded by removing more manual control rods to maintain power. This was not apparent to the operators because the RBMK did not have any instruments capable of calculating the inserted rod worth in real time.

The combined effect of these various actions was an extremely unstable reactor configuration. Nearly all of the control rods had been extracted manually, and excessively high coolant flow rates through the core meant that the coolant was entering the reactor very close to the boiling point.

Unlike other light-water reactor designs, the RBMK design at that time had a positive void coefficient of reactivity at low power levels. This meant that the formation of steam bubbles voids from boiling cooling water intensified the nuclear chain reaction owing to voids having lower neutron absorption than water. Unbeknownst to the operators, the void coefficient was not counterbalanced by other reactivity effects in the given operating regime, meaning that any increase in boiling would produce more steam voids which further intensified the chain reaction, leading to a positive feedback loop.

Given this characteristic, reactor No. The reactor was now very sensitive to the regenerative effect of steam voids on reactor power. At , the test began. 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 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 in the coolant flowing up through the fuel pressure tubes. At , as recorded by the SKALA centralized control system, a scram emergency shutdown of the reactor was initiated [32] as the experiment was wrapping up.

The personnel had already intended to shut down using the AZ-5 button in preparation for scheduled maintenance [33] and the scram likely preceded the sharp increase in power. When the AZ-5 button was pressed, the insertion of control rods into the reactor core began. 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 section attached to its end to boost reactor output by displacing water when the control rod section had been fully withdrawn from the reactor.

That is, when a control rod was at maximum extraction, a neutron-moderating graphite extension was centered in the core with 1. Consequently, injecting a control rod downward into the reactor in a scram initially displaced [neutron-absorbing] water in the lower portion of the reactor with [neutron-moderating] graphite.

Thus, an emergency scram could initially increase the reaction rate in the lower part of the core. Procedural countermeasures were not implemented in response to Ignalina. However, they did appear in almost every detail in the course of the actions leading to the [Chernobyl] accident. A few seconds into the scram, a power spike did occur, and the core overheated, causing some of the fuel rods to fracture. Some have speculated that this also blocked the control rod columns, jamming them at one-third insertion.

Within three seconds the reactor output rose above MW. Instruments did not register the subsequent course of events; they were reconstructed through mathematical simulation. Per the simulation, the power spike would have 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. As the scram continued, the reactor output jumped to around 30, MW thermal, 10 times its normal operational output, the indicated last reading on the power meter on the control panel. Some estimate the power spike may have gone 10 times higher than that.

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 called the upper biological shield, [39] 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. This explosion ruptured further fuel channels, as well as severing most of the coolant lines feeding the reactor chamber, and as a result, the remaining coolant flashed to steam and escaped the reactor core.

The total water loss combined with a high positive void coefficient further increased the reactor’s thermal power. A second, more powerful explosion occurred about two or three seconds after the first; this explosion dispersed the damaged core and effectively terminated the nuclear chain reaction.

Chernobyl disaster – Wikipedia.Synths : Reaktor 6 | Komplete