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On April 25, 1986, the Unit 4 reactor was scheduled to be shut down for a test. It had been decided to use this occasion as an opportunity to test the ability of the reactor's turbine generator to generate sufficient electricity to power the reactor's safety systems (in particular, the water pumps) in the event of a loss of external electric power. Reactors such as Chernobyl have a pair of diesel generators available as standby, but these do not activate instantaneously—the reactor was, therefore, to be used to spin up the turbine, at which point the turbine would be disconnected from the reactor and allowed to spin under the force of its own rotational inertia, and the aim of the test was to determine whether the turbines in the rundown phase could sufficiently power the pumps while the generators were starting up. The test was successfully carried out previously on another unit (with all safety provisions active) and the outcome was negative (that is, the turbines generated insufficient power in the rundown phase to power the pumps), but additional improvements were made to the turbines which prompted the need for another test. The power output of the Chernobyl-4 reactor was to be reduced from its normal capacity of 3200 MW thermal to 1000 MW thermal in order to conduct the test at a safer, lower level of power. However, due to a delay in beginning the experiment the reactor operators reduced the power level too rapidly, and the actual power output fell to 30 MW thermal. As a result, the concentration of the nuclear poison product xenon-135 increased (this product is typically consumed in a reactor under higher power conditions). Though the scale of the power drop was close to the maximum allowed by safety regulations, the crew's management chose not to shut down the reactor and to continue the experiment. Further, it was decided to 'shortcut' the experiment and raise power output only to 200 MW. In order to overcome the neutron absorption of the excess xenon-135, the control rods were pulled out of the reactor somewhat farther than normally allowed under safety regulations. As part of the experiment, at 1:05 AM on April 26, the water pumps which were to be driven by the turbine generator were turned on; the water flow generated by this action exceeded that specified by safety regulations. The water flow increased at 1:19 A.M.—since water also absorbs neutrons, this further increase in the water flow necessitated the removal of the manual control rods, producing a very unstable and dangerous operating condition. At 1:23:04 A.M., the experiment began. The unstable state of the reactor was not reflected in any way on the control panel, and it does not appear that anyone in the reactor crew was fully aware of danger. Electricity to the water pumps was shut off, and as they were driven by the inertia of the turbine generator the water flow rate decreased. The turbine was disconnected from the reactor, increasing the level of steam in the reactor core. As the coolant heated, pockets of steam formed in the coolant lines. The particular design of the RBMK graphite moderated reactor at Chernobyl has a large positive void coefficient, which means that the power of the reactor increases rapidly in the absence of the neutron-absorbing effect of water, and in this case, the reactor operation becomes progressively less stable and more dangerous. At 1:23:40 A.M. the operators pressed the AZ-5 ("Rapid Emergency Defense 5") button that ordered a "SCRAM" — a shutdown of the nuclear reactor, fully inserting all control rods, including the manual control rods that had been incautiously withdrawn earlier, into the reactor. It is unclear whether it was done as an emergency measure, or simply as a routine method of shutting down the reactor upon the completion of an experiment (the reactor was scheduled to be shut down for routine maintenance). It is usually suggested that the SCRAM was ordered as a response to the unexpected rapid power increase. On the other hand, Anatoly Dyatlov, chief engineer on Chernobyl nuclear station at the time of the accident, writes in his book: "Prior to 01:23:40, systems of centralized control ... didn't register any parameter changes that could justify the SCRAM. Commission ... gathered and analyzed large amount of materials and, as stated in its report, failed to determine the reason why the SCRAM was ordered. There was no need to look for the reason. The reactor was simply being shut down upon the completion of the experiment." [6] Due to the slow speed of the control rod insertion mechanism (18–20 seconds to complete), the hollow tips of the rods and the temporary displacement of coolant, the SCRAM caused the reaction rate to increase. Increased energy output caused the deformation of control rod channels. The rods became stuck after being inserted only one-third of the way, and were therefore unable to stop the reaction. By 1:23:47 the reactor jumped to around 30 GW, ten times the normal operational output. The fuel rods began to melt and the steam pressure rapidly increased causing a large steam explosion, displacing and destroying the reactor lid, rupturing the coolant tubes and then blowing a hole in the roof. To reduce costs, and because of its large size, the reactor was constructed with only partial containment. This allowed the radioactive contaminants to escape into the atmosphere after the steam explosion burst the primary pressure vessel. After part of the roof blew off, the inrush of oxygen—combined with the extremely high temperature of the reactor fuel and graphite moderator—sparked a graphite fire. This fire greatly contributed to the spread of radioactive material and the ultimate contamination of outlying areas. There is some controversy surrounding the exact sequence of events after 1:22:30 local time due to the inconsistencies between eyewitness accounts and station records. The version that is most commonly agreed upon is described above. According to this theory, the first explosion happened at approximately 1:23:47, seven seconds after the operators ordered the "SCRAM". It is sometimes claimed that the explosion happened 'before' or immediately following the SCRAM (this was the working version of the Soviet committee studying the accident). This distinction is important, because, if the reactor went critical several seconds after the SCRAM, its failure would have to be attributed to the design of the control rods, whereas the explosion at the time of the SCRAM would place the blame on the operators. Indeed, a weak seismic event, similar to a magnitude-2.5 earthquake, was registered at 1:23:39 in the Chernobyl area. The situation is complicated by the fact that the "SCRAM" button was pressed more than once, and the person who actually pressed it died two weeks after the accident from radiation poisoning. In January 1993, the IAEA issued a revised analysis of the Chernobyl accident, attributing the main root cause to the reactor's design and not to operator error. The IAEA's 1986 analysis had cited the operators' actions as the principal cause of the accident.

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