What Happened on April 26, 1986?
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Before its meltdown, the Chernobyl nuclear power plant generated approximately 10% of Ukraine’s energy supply. Each of the four RBMK-1000 reactors was capable of producing 1 gigawatt of electricity, and by the late 1970’s all four reactors were coming online. It’s interesting to note that two more RBMK-1000 reactors were under construction when the accident occurred.
At 1:23 a.m., on April 26, 1986, reactor number four experienced a nuclear meltdown as a result of a series of explosions and fires. While the exact actions that caused the accident may never be known (there are some differences among various sources about the details relating to the accident), we do know that the meltdown was the result of a botched safety test.
The meltdown caused roughly 5% of the radioactive material underneath the containment shell to be thrown into the atmosphere, and of that 5% nearly 60% of it landed in Belarus. The accident resulted in the evacuation and resettlement of over 336,000 people.
Events Leading Up to the Accident
On April 25, 1986 the reactor in question (reactor number four) was actually scheduled to be shut down for maintenance. This scheduled shut down provided a great opportunity to test the reactors ability to generate sufficient electricity to power its internal safety systems in the event of a loss of electrical power. The RBMK-1000 reactor requires that, so long as nuclear fuel is present, water must be continuously circulated through the core (via water pumps which, obviously, rely on electricity to operate).
Previous tests had failed, and the reactors had not generated enough power during shut down to power the water pumps, but since improvements had been made to the turbines the operators thought they would fare better the second time around. What’s interesting to note is that Chernobyl’s reactors had a pair of backup diesel generators, but there was a 40 second delay before they were spinning at full speed. Because of that (short) delay, the reactor was going to be used to spin the turbine.
During the day conditions were optimal, and so the reactor was gradually reduced to 50% output. However, a regional power station unexpectedly went offline, and it was requested that the reactor return to normal operational status in order to satisfy the evening demand in power. The test was postponed and scheduled to be conducted by the night crew.
Of course, the night crew had little to no experience with a nuclear power plant, as most of the crew had been brought over from coal-fired power plants. This inexperience, combined with the fact that the night crew was essentially a skeleton crew, laid the foundation for what would result in the single greatest nuclear accident in the world.
At 11:00 pm it was deemed that the power output could be reduced as the evening spike in power consumption had passed. Hence, power of reactor number four was reduced from its nominal 3.2GW thermal to 1.0GW thermal. The new crew was unaware of the proper procedure for slowdown of the reactor, however, and reduced power too quickly. This created what is called a “positive feedback situation”, and is the result of the properties of the nuclear material present in the RBMK-1000 reactor. A product of nuclear fission is the isotope iodine-135 (I-135). I-135 decays with a half life of 6.7 hours into xenon-135. Xe-135 is a potent reactor poison.
Reactor poison refers to an element/isotope that impedes a nuclear reactors ability to maintain critical mass. In other words, it impedes the reaction and lowers its output.
All of this means that the positive feedback situation confused the inexperienced operators who, when seeing power drop to 5% of what was expected, removed the automatic control rods from the reactor as they believed they were malfunctioning. As you can now see, this created a highly volatile situation as the reactor was now being operated far beyond its original design and allowable safety regulations.
Removing the rods increased power to approximately 33% of what was needed to conduct the experiment. However, the crew continued on with the experiment, and at 1:05 am the water pumps were turned on. At 1:19 am the water flow increased significantly. Water also absorbs neutrons, thus decreasing the power output even further, and the decision was made to remove the manual control rods as well. At this point the reactor was operating without any kind of control infrastructure in place, meaning that there was nothing to prevent the reaction from expanding beyond the control of the operators.
The only thing preventing the reactor from melting down at that point was the poisoning effect of the Xe-135.
The Reactor Melts Down
At 1:23:04 a.m. the experiment began. To the operators, everything was operating normally at this point as they were unable to see any signs of danger via their instrumentation. The steam to the turbines was shut off, reducing the amount of water being pumped into the core (the momentum of the steam turbines powered the water pumps). Since the water also acted as an inhibitor to the reaction the reaction began to increase in volatility as less water was present in the core.
With no control rods in place, and with the reaction beginning to surpass Xe-135’s ability to impede it, the stage was set for a nuclear catastrophe. All of this was going on while the operators were completely unaware. Then, at 1:23:40 a.m. the operators ordered a “SCRAM”, a complete and total shutdown of the reactor. A SCRAM fully inserts all control rods, including the manual control rods that had been removed earlier. The reason that this is significant is that inserting the rods briefly elevates the reaction on the RBMK-1000 reactor, and the resulting spike in output contributed to the end result.
By 1:23:47 the reactor had jumped to 30GW output- 10 times its design capabilities. The fuel rods melted and steam pressure increased, causing a large explosion. Steam was displaced, travelling vertically and eventually destroying the reactor lid, coolant tubes, and other pieces of equipment. The sudden influx of oxygen, combined with the obscenely high temperatures of the reaction, caused the graphite reaction moderator to catch fire. Disaster had struck. In less than a minute all hell had broken loose.
What Just Happened?
Immediately following the disaster the staff was unaware of the true radiation levels, causing them to make poor judgements and gross assessments of the actual situation. Radiation levels in some parts of the building were in excess of 20,000 roentgen per hour, causing some workers to receive fatal doses within minutes.
Low readings on measuring equipment (the proper equipment was inaccessible due to the explosion) caused operators to believe the reactor was intact. Evidence of a severe malfunction and impending crisis (reactor fuel and graphite strewn across the building, for example) was ignored. The crew operating reactor number four stayed in the building until daylight, trying to pump water into the reactor. None of them wore protective gear, and almost all of them were dead within three weeks due to radiation poisoning.
Firefighters arrived shortly after the accident in an attempt to contain the fires. None of the firefighters were informed of how lethal the radioactive smoke and debris truly was, and it is likely that they thought the fire was the result of an electrical fire. To quote Lieutenant Vladimir Pravik, who diead on May 9, 1986, “We didn’t know it was the reactor. Nobody told us.”
The fire inside reactor number four could not be extinguished by water, and it was not until helicopters dropped sand, lead, clay, and boron onto the fires that they finally extinguished.