On April 26th, 1986, the world was shaken by an explosion. The worst-case peacetime scenario had occurred—thermonuclear meltdown. Some 110km from the capitol of Ukraine, a nuclear power station spewed tons of deadly radioactive material across the globe. A city was laid desolate, thousands were contaminated. Even today, the long reaching effects still haunt us.
As with many accidents, this one was preventable. The three main causes: a poorly designed reactor, incompetence at the administration level leading to and during the accident, and a government system designed to hide, not fix problems.
Still under the yoke of Soviet Communism, the scientific community lived with the mentality of “Hear no evil, see no evil, speak no evil.” Under no circumstances did one want to rock the boat. Although there were inherent problems spread all over their nuclear power program, things were not discussed, accidents were covered up, information was not shared. Although the reactor had a design flaw, who was going to talk about it? In his book Chernobyl: Insights From the Inside, V. Chernousenko states, “It was the secrecy and lack of accountability of our nuclear science, and its refusal to open itself up to discussion and criticism which made possible for dangerous design faults to lead finally to a nuclear explosion of this scale.”
Chernobyl power station was a 4 reactor nuclear power facility, with 2 more reactors being added. The Soviet designed and built RBMK reactor was the most powerful reactor in the world, producing from 700 to 1250 megawatts of energy.
In a nuclear reactor, uranium 235 is used to produce energy. As neutrons bounce around, they split uranium 235 nuclei releasing power as heat. The heat is captured in water, producing steam which in turn cranks turbines, which in turn create electricity. The RBMK design was one used in early nuclear reactors. It used 4 separate components to control the reaction. Uranium is the only material known to support a nuclear chain reaction. Graphite was the moderator, used in order to slow down the neutrons, allowing them to collide with the uranium atoms. Water was used as a coolant and to produce steam. Boron was used as a neutron absorber to slow or shut down the fission reaction. So water is present in the chamber with the uranium and graphite. Neutrons bounce around and hit uranium nuclei, splitting them and releasing energy as heat. More neutrons are also released hitting more uranium atoms and releasing more neutrons, ad nauseum. The heat acts upon the water, creating steam. Once the steam is used to power the turbines, it is captured, cooled, and the water is reused. The boron can be raised or lowered around sections of the fuel thus stopping the reaction.
The problem with the RBMK reactor is called Positive Void Coefficient. This basically means “Someone really screwed up this design.” When too much steam was present in the core, things would get ugly. Too much steam meant not enough water cooling the reactor. Because the graphite rods would still be in place, the temperature would rise, causing more steam to be produced, thus causing the temperature to rise more…until the thing exploded. Newer reactor designs avoid this problem by using water as both a coolant and moderator. An excess of steam would cool the reactor, not heat it.
On the day in question, Reactor No.4 was scheduled to power down for some maintenance. While powering down, a test was scheduled to see how well the turbines would run once main power was shut off until the backups fired up. One of the main safety features was specifically wired to not work. It bypassed many of the checks and balances in the system. At the same time, some safety features, such as an emergency water system were locked down so that they would not interfere with the test.
The man in charge, Dyatlov, having experience as an electrical engineer was unfortunately not a nuclear station technician. He had lots of theory, but no practical experience. Grigory Medvedev, a nuclear physicist and engineer who was subsequently sent by Moscow to research what happened, had warned the station director years previously of Dyatlov’s unsuitability and incompetence as an administrator. He is described as having no sense of danger or caution and frequently disregarded the safety of the station and technicians. Much of the damage and death that ensued can be attributed to him.
So, with a system that hid mistakes instead of learning from them, a reactor with inherent dangerous design flaws, and a set of management that were ignorant to reactor technology but were there for their own gain, the stage was set for the nightmare history had not yet seen.
The flaws with the reactors were not only in the design, but the implementation of some important safety features. In the case of something really horrendous, the system was designed with a maximum design-basis accident button. This button, which was supposed to run through the main system and trigger several emergency systems, was rerouted not to work as designed as to avoid a heat shock in the reactor. In order to produce a “clean” test, several other mistakes were made. One of which was the fact that the test was performed while the reactor was still live. Another mistake was that the emergency water feeds were turned off and locked shut. The triggering mechanism in the emergency systems was disabled so that a second run on the test could have been made in case of failure.
The program that led to the accident was supposed to have taken place on April 25th during the day. At the request of a Kiev load controller, it was delayed until the night shift. It was begun by one team an hour before the midnight shift came on. The test included running down the reactor in order to test the efficiency of the turbines from the time the main power was shut off until the emergency generators came online. It would determine if the turbines could produce enough inertial energy to power the station until the generators kicked in.
With the reactor still functioning and the maximum design-basis accident features disabled, the reactor reached a dangerously low power level. In the RBMK reactor, when the power emissions drop below a certain level, the reaction begins to be poisoned by a buildup of certain elements, such as xenon and iodine. Where normally the emergency systems would have shut the reactor down, it was allowed to drop to 30 megawatts. At that level, it was extremely difficult—if not impossible—to control and stabilize.
When the power fell to the low levels, one of the engineers notified Dyatlov what had happened. Dyatlov told him to bring the power back up. The engineer, Tuptonov, refused and was backed up by the other senior engineer. The only safe decision was to power down the reactor completely and resume the test twenty-four hours later. Dyatlov went ballistic. He threatened them, swore at them, bullied them and threatened to replace them with others and even fire them. His manipulation finally yielded the desired result…they caved in.
In order to create additional power, it was necessary to raise some of the boron control rods. By safety specification, it was mandatory that a minimum of 28 of the 210 rods be maintained at all times in the core. At the moment of decision, that is all there were in the core. When the reactor exploded, there were only 18 left.
With the extraction of the rods, the power levels rose to about 200 megawatts before it leveled out due to the poisoning in the reactor. It was still 600 less than the minimum operational reactor reserve. The engineers were scrambling to maintain proper levels on the water circulation, the steam pressure, and the power levels. At this point, 2 minutes from the explosion, they still could have stopped it. All it would have taken was for them to carefully power down the reactor.
About 30 seconds before the explosion, the main circulation pumps began to fail. The circulating water, as part of the test, was kept close to saturation. With the increase in power and temperature, the water began to steam, killing the pumps. With that, water levels decreased and the water began to boil inside the reactor creating more steam, thus more power. The positive void coefficient was in effect.
Tuptonov reported a power surge to Akimov, the other senior engineer. Akimov, against all judgment and training, engaged the emergency power reduction system. It proved to be a fatal mistake. The emergency power reduction system involved the lowering of all boron control rods into the reactor simultaneously. You may think that this a good thing, no?
The problem was another odd design quirk. The 23 foot rods were not completely boron. The tips of the rods were actually made of graphite with an empty section before the boron. Thus, when rods were inserted into the core, they produced a power spike. Not a problem in a reactor working within normal parameters, but in a superheated reactor with fluctuating power levels, it was the end.
In his book “The Legacy of Chernobyl”, Zhores Medvedyev reveals that the turbine rundown test was to have been completed at the end of 1982, before the reactor was brought into a commercial regime. It was on a list of things, in typical Soviet fashion, that was agreed by the various ministries involved “to be completed later” notwithstanding the point that it was a requirement for the reactor to pass inspection. These kinds of oversights were typical to make sure that various projects were completed on or before deadlines, especially when there were bonuses at stake.
Z. Medvedyev also revealed that this test was written for electrical engineers, as it had to do with the electrical rundown system of the turbine generators. This could explain why Dyatlov was involved, but not why he was in charge. The test should have been performed on a fresh reactor, not one at the end of its cycle (when it is more difficult to control reactor stability). The reactor operators were unsure about the instructions for the test as they had several steps crossed out, not to mention the test plan was never approved by the upper ministry (Z. M. attributes this to the need to hide the fact that this test should have been complete 2 years previously).
When Akimov engaged the emergency power reduction system, all boron rods not already in the reactor (93 rods) began to lower. The graphite tips caused a surge in reactivity. The rods lowered only 2-3 meters and stopped. They could go no further, even after Akimov cut the power to their electric motors to allow gravity to pull them down. The fuel assemblies were already warped from the intense heat and steam.
In the reactor hall, the shift foreman watched in horror as more than 1000 fuel assembly caps weighing 770 lbs. each begin to jump up and down like popcorn. He rushed down some steps and headed for the control room. By this time the reactor had begun to release hydrogen and oxygen as the reactive process was poisoned even further. This highly explosive gas combination spread throughout the reactor. The pressure release valves on the reactor began to release the steam but were destroyed by the immense pressure. Rising at 15 atmospheres per second, the pressure destroyed the water and steamwater communication lines.
With all water to the reactor cut off, the temperature rose quickly. The hydrogen/oxygen gases ignited, causing several explosions, destroying the main circulation pump rooms where one man will be forever entombed. The upper biological shield weighing 1000 tons was blown out of place. Half of the nuclear fuel and graphite were blown out of the reactor. Some evaporated into a nuclear cloud that floated over Europe, seeding radioactive material in its wake. Pieces of burning graphite and fuel landed on the roofs of the turbine hall and adjoining buildings igniting the flammable roofing material. Other chunks fell all around, continuing to release deadly levels of radiation. Z. Medvedyev estimated around 20 million curies of radiation was released into the atmosphere in various gaseous forms and another 10 million curies were spread between 2-3 km from the Chernobyl site. One report makes it equivalent to 500 Hiroshimas.
Back in the control room, no one could understand what on earth had happened. They knew there had been at least on explosion. When the shift foreman arrived, a few seconds later, he told them what he had seen. Dyatlov sent two young (barely out of school) engineers who were there for observation and training to see what had happened to the reactor. They worked their way down into the central reactor hall. Through the rubble, they could see the sky outside and red and blue flames through the upper biological shield which was now lying crookedly across the top of the reactor core. They could see the raging inferno within. By the time they returned to the control room they were dark brown all over, even under their clothes, and their eyes smarted, they coughed and they felt a tightness in the chest. They had received fatal doses of radiation and died agonizing deaths a couple of weeks later. They reported that the reactor was gone. Dyatlov called them idiots and fools. He told them they must be mistaken, the reactor was intact. This complete ignorance and disbelief cost many people their lives. Later on, they sent a senior engineer who told them the same thing. He also died of acute radiation syndrome. They didn’t believe him either.
As soon as the explosions occurred, the fire brigades went into action. The leader of the brigade realized that the accident was bigger than he could handle and called in help from the Chernobyl city of Pripyat as well as firefighters from the Kiev region. These men were true heroes. Climbing onto the burning roofs of the nearby buildings, they saved the other reactors from damage (imagine four reactors melting down, not just the one), often tossing off pieces of burning graphite and fuel with their hands. It took nearly 5 hours to quench the burning tar. Many of the direct victims of Chernobyl were among these men. Their graves, outside of northern Moscow, are treated as heroes’ graves, tended to and honored.
When Dyatlov and Akimov reported on the accident to their boss Fomin, they reported that there had been an accident. They said that one of the water tanks or drums must have exploded. They reported that water was being fed into the reactor and that everything was fine. The only dosimeter available only read to 3.2 roentgens per hour. It was off the scale but that is what they reported as the background radiation. They seeded a lie that spread and gave everyone a sense of complacency. When another dosimeter was brought in that read up to 250 roentgens per hour, the dosimeterist was told that his machine was broken; to get rid of it. Later, when Dyatlov went to inspect the grounds around the reactor building, he still refused to accept that the reactor had exploded and tried to rationalize where the chunks of graphite and fuel (radiating 15,000-30,000 roentgens per hour, in fact) had come from.
The water, reported as being fed into the reactor, never reached it. The lines underneath had been destroyed. The water was being fed into the channels under the reactor. Had the melted reactor core collapsed into the small lake of water underneath it, it would have been the equivalent of dropping a nuclear bomb. Fortunately someone realized this and a team of engineers used artillery shells to knock a hole in the foundation to drain out the water.
The town of Pripyat was not evacuated for several days following the accident. People went to work, school, and shopping and played in the radioactive dust. The air was filled with radioactive particles. The city director was described by G. Medvedyev and a man who loved his power and prestige and refused to allow the city to be evacuated until everyone in it had received a nice healthy dose of radiation.
The population, most of them there to man the nuclear power plant, was uneducated as to the dangers of radiation or even what to do in the case of an accident. When some of them started to realize what was happening, they got drunk. They believed that vodka could disinfect the body of radiation. In the US, salt is sold with clean iodine in it. The reason behind this is the thyroid of the human body absorbs iodine. When it is kept full, any other iodine introduced into the body passes through. In Soviet Russia, iodized salt was unused. In event of an accident, iodine tablets were taken. In the hospital after being evacuated, those afflicted were irradiating others themselves. They had absorbed so much iodine-135 that their thyroids were emitting 100-150 roentgens per hours for several days.
Some victims were saved after blood transfusions and bone marrow replacement. Those who died of acute radiation syndrome died horrible deaths. Their bodies continued to turn darker and darker. They suffered from severe bowl distress, having painful, explosive bowl movements 25-30 times a day. Their mucus membranes swelled up and disintegrated. The skin on their bodies died and eventually, so did they. Akimov’s wife described him at death as being mummified: skin dark as night, his body dried up, drained of life, and weighing no more than a child. In Russia, people are generally still buried in wooden coffins. The bodies of these men were so contaminated that they were buried in lead coffins with the lids soldered on so that their disintegrating bodies would not find their way into the water table.
Reactor No. 4 was somewhat contained by a cement sarcophagus. Z. Medvedyev accurately commented that the Chernobyl sarcophagus will need to stand even longer than the pyramids. Another will need to be built over it as it succumbs to elements. Even now, there are large gaps in it. One little earthquake or terrorist act and there will be a second Chernobyl.
An estimated 5 million people were affected by the fallout. Over 2,000 cases of thyroid cancer have been reported with the numbers expected to rise. Lung cancer of the cleanup workers is on the rise as well.
The last RBMK reactor at the Chernobyl site was shut down in December 2000. There are still 14 or 15 of the same reactor type active in Russia. Theoretically, they have been corrected so that a repeat of the world’s greatest industrial accident should not happen. I sure as hell hope so. Even now I can hear in my mind Tom Lehrer’s enchanting tune…”We’ll all go together when we go…”
Note: This article originally appeared in 2005 as a three-part series. Here the parts have been combined.