A systems test was scheduled for the night of the 25th of April 1986. The test was designed to ascertain whether the reactor could maintain adequate cooling water circulation after a SCRAM (emergency shutdown), when the reactor itself loses power. After a SCRAM, a reactor continues to produce approximately 7% of its total thermal output. With a typical operating efficiency of around 32%, a 1GW reactor would continue to generate some 200MW (thermal) of energy after shutting down.
In order to continue to provide cooling for the reactor, the turbines that were running down were intended to continually run the pumps until the backup turbo generators could take over running the cooling pumps. There was a well known design flaw with the RBMK type reactors that meant that after a SCRAM the reactor would lack flowing cooling water for up to 75 seconds, during which time enough heat could build up in the reactor core to cause a catastrophic failure. By using the turbines to run the pumps while the turbo generators started, this time it could be reduced to approximately 15 seconds, which was considered acceptable.
Due to the nature of the test, the automatic shutdown systems were required to be deactivated, as the test itself would have been perceived as a failure by the control systems. The test was due to have been carried out earlier, but due to logistical and staffing problems it did not take place until after the shift changeover. This meant that the staff carrying out the test had not been properly briefed and were not completely in the picture as to the test procedure. In order to safely carry out the test, the power was required to be reduced to approximately 20% of its normal operating power. However, unknown to the staff, a design flaw within the reactor caused the reactor to become unstable when operated at low energy outputs, a phenomenon known as reactor poisoning. When the power was reduced, a dangerous dip in the output occurred due to an engineer mistakenly inserting the control rods too far. By this point, between 00:35 and 00:45 on 26th April, hot spots were growing within the reactor core, but as the emergency shutoff system had been disabled, alarms were ignored and the test was continued. Auxiliary water pumps were activated as part of the test, which increased the flow of water to the turbines, which caused an increase in the water temperature. The presence of more water caused the reactor power output to drop further, and the water flow exceeded the safe limit at 01:19. In an effort to increase reactor power, two of the circulation pumps were switched off, and control rods were withdrawn from the core. All but nine of the control rods were withdrawn in an attempt to stabilize the reactor, the whole time the hotspots in the reactor core were increasing. The rods were manually operated, with control taken away from the reactor’s automated control system.
At 01:23 the test was begun. The main turbine was allowed to run down, but produced much less torque than expected and this caused a lack of cooling water within the core, with steam bubbles being created. This caused the reactor output to increase, which the control system attempted to counteract by the controlled insertion of rods into the core. With the increase in power, more water flashed into steam within the core, causing a positive feedback loop. The automated control system successfully contained this effect, until it had used all 12 of the control rods that had not been taken under manual control.
At 01:23:40, the emergency SCRAM button was pressed. The reason for this is not known, although it is believed to have been a response to the rapid unexpected power increase caused by the positive feedback loop. This caused the insertion of all of the control rods, including those that had been taken under manual control. The control rods took approximately 18 seconds to reach full insertion.
The control rods were of a design which is now known to be dangerously flawed. During their travel, the graphite tip initially displaced neutron absorbing coolant before replacing it with neutron absorbing boron. This caused the power output of the reactor to spike dangerously. The core overheated, causing the fracture of some of the fuel rods, blocking the control rods and jamming them at 1/3 insertion. The power output rose within three seconds to over 530MW. This caused a massive increase in steam pressure, releasing radioactive material into the coolant, rupturing the fuel channels. Suddenly the reactor power jumped to 30GW (thermal), 10 times the normal power output. The last reading registered on the control panel is 33GW.
This sudden rise in power output caused a massive increase in steam pressure, with catastrophic results. A steam explosion tore off the 2000 ton reactor cover, sending it through the roof of the reactor building. This caused the rupturing of further fuel channels, and severed most of the coolant channels within the core. This caused a further jump in thermal power.
About 3 seconds later, a second, more powerful explosion tore through the reactor, caused by the exposure of superheated reactor core elements to air, spreading parts of the reactor over a wide area and leaving the devastation that was evident as the sun rose. Highly radioactive reactor parts were scattered over a wide area. Due to the intense heat, remaining components of the reactor caught fire, contributing to the spread of radioactive material and contamination of the area. This second explosion was probably a nuclear power transient, the absence of coolant and any form of control causing the core to undergo runaway prompt criticality similar to that of a nuclear weapon, with a yield of approximately 0.3kT. According to witnesses the first explosion was followed by a red blaze and the second explosion had a light-blue blaze, after which a mushroom cloud rose above the reactor.
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