Hello, friends! 26th April, 1986 at 1:23 AM, the Reactor Number 4 of the Chernobyl Nuclear Power Plant in the Soviet Union. The lid on top of the reactor starts shaking, And the shockwaves are felt all throughout the building. The workers present realised that the nuclear reaction in the reactor was completely out of control by then. And that reactor had to be stopped quickly. So one of those workers acted fast and pressed the Emergency Shut Down button. By pressing the button, the control rods were designed to enter the reactors, and prevent the reactions. But they didn't work as designed. After the button was pressed, the control rods entered the reactor, but as soon as they went in, there was a huge blast. A huge explosion. After this blast, the reactor was engulfed in flames. The harmful radioactive material present there rose up into the air with the flames. The amount of harmful radioactive material released in this disaster, was equal to 400 Hiroshima atomic bomb. This disaster is still considered to be the World's Worst Nuclear Disaster. The radiation spreading through the air, had an effect on not only Ukraine but all over Europre from Spain to Sweden also. There was radioactive rain in the United Kingdom. The radioactive dust which settled on the grass in the hills, were consumed by cows and the amount of radiation in the milk of those cows spiked. Due to this, thousands of children got Thyroid Cancer. Come, in this video, let's understand why this Chernobyl disaster took place. The reasons behind it, and the effects that were seen all over the world. "In a nuclear accident near the Soviet Union..." "...once Reactor 4, the scene of a total meltdown..." "The explosion at a nuclear plant in the Soviet Union." "Near the reactor at Ukraine's Chernobyl Power Plant." "...the ecplosion's powerful enough to blow up the steel and concrete lid." Friends, after World War II, the Sovient Union invested a lot of money in nuclear power plants. It built many nuclear power plants on of which was the Chernobyl Nuclear Power Plant. It's official name was It was built in the early 1970s, even though it is named the Chernobyl Nuclear Power Plant, it wasn't built in the city of Chernobyl. Chernobyl was a small city approx 16 km away from this nuclear power plant. But friends, this Chernobyl Plant was among one of the most advanced nuclear power plants in the Sovient Union. There were four nuclear reactors in this, in the RBMK 1000 design. RBMK 1000 is a type of nuclear reactor. The operations of the first 2 reactors had begun in 1977, the third in 1981, and the fourth in 1983. The purpose of power plants is to generate electricity. In this case, each reactor could produce 1,000 Megawatts electricity. The four reactors could together supply electricity to meet 10% of Ukaraine's electricity demand. So you can imagine how powerful these reactors were. Coming to the Chernobyl disaster, this disaster took place in the Reactor Number 4, during a routine safety test. To understand exactly what had happened that day, we first need to understand how a nuclear reactor works. In most of the methods of generating electricity in the simplest terms, you need a rotating wheel. You need movement for kinetic energy. In hydroelectricity, when the water falls from above, it moves the wheels, and creates energy. In wind energy, the wind moves the turbine, which produces energy once it starts rotating. In thermal powerplants, that uses coal to generate electricity steam is released by buring the coals, and the steam later moves the wheels. The kinetic energy is thus generated. Even in case of nuclear energy, the wheel is moved by steam. But to make that steam, water has to be heated up first, whereas in thermal energy, coal heats up the water, in nuclear energy, the nuclear reactions taking place heats up the water. Look at this diagram from a school book. You don't need to get into the details, you can see a turbine that is rotated by steam. And the steam is generated because the nuclear reaction heats up the water. This water acts as a coolant as well. To prevent overheating around the reactor, the constant supply of water is a must. In the Chernobyl plant, the water came from a nearby man-made water reserve. There was a river beside it too. It is important to have a consistent supply of water at all times. For this, the water pumps pushing the water into the pipes, should be working non-stop. But what happens in case of a power cut? To ensure that the water pumps keep working at all times, There were some backup diesel generators in the Chernobyl plant. So that they could provide uninterrupted electricity to the pumps. But to start these generators, it took about 1 or 2 minutes. Within those 1-2 minutes, there was a need for an additional power source to ensure that the water pumps keep working. The builders of Chernobyl reactor had said that the electricity needed for these crucial 1-2 minutes, could be sourced from the nuclear reactor itself. Since the turbines would be moving because of the steam, and even after a reactor shutdown, there would be some amount of steam, that would last for a couple of minutes, would be enough to power those plants. So friends, in the wee hours of 26 April, they were testing this. If there was a power cut, would the turbines be able to produce enough electricity to power the water pumps? Tihs test was run earlier as well, but this test was never successful in reactor number 4. The workers hoped that the test would be successful that ill-fated night. One day before this, on 25th April, they had tried to conduct a test, but it couldn't be carried out. And so the test was delayed by a day. The workers working in this plant didn't even get full sleep, At 11:10 PM on 25th April, they began the preparations for this test. To understand the rest of the story, we need to understand the science of reactors for a bit, The process of the nuclear reaction in the nuclear reactor. The core of the nuclear reactors, the main part, is mainly built of three things. Fure rods are basically nuclear fuel, which causes the nuclear reactions. In this case, Uranium Dioxide enriched with Uranium 235 Isotope. We know that an atom is built of electrons, protons, and neutrons, and each element has a different number of electrons, protons, and neutrons. Several elements have isotopes as well. Isotopes are basically a type of variations in the elements. Take the element Uranium, which has three isotopes, Uranium-238, Uranium-235, and Uranium-234. In different isotopes, the number of protons remains the same, but they have different number of neutrons. But there are some rare isotopes, of some elements, that can undergo the nuclear fission reactions. Uranium-235 is an isotope of Uranium which permits nuclear fission reaction and create nuclear energy. This Uranium isotope is unstable due to which it falls apart and released radiations. This keeps on disintegrating on its own but it can also be broken apart if a neutron collides with it. Both cases are known as a nuclear fission reaction. In this process, one atom of this element splits into two and a few extra lonesome neutrons are formed. Additionally, it releases kinetic energy as well that converts into thermal energy which later goes on to move the turbine. But the extra neutrons that are realeased, go and collide with more atoms and such disintegration continues, and just like that, a chain reaction begins. It is known as Nuclear Chain Reaction. To slow down this reaction, Control rods are used. In this case, the control rods were made of Boron Carbide. Boron is an element which absorbs neutrons quite well. The extra neutrons formed as a result of the nuclear fission reaction get absorbed by Boron, and do not collide with the other atoms, and the speed of the nuclear fission reaction slows down. In the core of a nuclear reactor, when the fuel rods are inserted, the control rods are inserted beside it, to control the nuclear reaction. The more the control rods are inserted, the reaction will be proportionately slowed down. Apart from this, the third thing that I told you about, the Moderator. In this case, graphite blocks were the moderators. These help to speed up the reaction. Graphite slows down the energy of the neutrons, increasing the chances of neutrons colliding more with Uranium atoms. Thus, increasing the speed of the reaction. Today, most of the nuclear plants water is used as moderator. But in the RBMK 1000 reactors used in Chernobyl back then, graphite was used as Moderator. To summarised the science of it, imagine a car on a road If you hit the gas pedal, the speed of the car will increase. And if you hit the brakes, it will slow down and eventually stop. In case of the nuclear reactor, the car moving on the road is the fuel rods of Uranium where the reaction takes place. The gas pedal works are the Moderator, The deeper you insert it, the reaction would be sped up and the control rods act as the brakes. when you hit the brakes, it'll slow down the reaction. The after effect of the Chernobyl disaster was worse that that caused by the bombs dropped on Hiroshima and Nagasaki. Today, you can see the live examples of it. The disaster zone of Chernobyl is so dangerous that people cannot go there. It is a restricted area. But the cities of Hiroshima and Nagasaki, are populated by so many people. on that ill-fated night, 26th April, the workers were testing the unit. Workers were tasked with reducing the output of the nuclear power plant from 1,600 MW of electricity to 700 MW. But as soon as the workers inserted the control rods, the power dropped even more. It fell below their expectations. And reached 30 MW. The workers working there couldn't comprehend why it happened. The reason for this was that in this nuclear fission reaction, a byproduct was created Xenon 135. Similar to Boron, Xenon 135 could absorb the neutrons quite efficiently. Because this byproduct was being created in this reaction, it was absorbing the excess neutrons, and the brakes were being hit quite hard. That's why the electricity produced was lower than expected. Normally when Xenon is produced in a nuclear reaction, it burns out on its own. Or decays within a few hours. But in this case, it kept getting accumulated in the nuclear core, because the power output was already so low. When high amounts of Xenon starts accumulating in the reactor core, it is known as Xenon poisoning. It kept slowing down the reaction. Seeing that very little electricity was produced, the shift supervisor, Anatoly, ordered the workers to take out some of the control rods. He reasoned that taking out the control rods could speed up the reactions a bit. At 1:00 AM on 26th April, the power output reached 200 MW after taking out some of the control rods. But they still couldn't run their tests, Because they aimed to take the output to 700 MW. They had to speed up the reaction further. Anatoly gave the next set of instructions. He wanted to speed up the reactions by taking out more control rods. There are normally 211 control rods in a reactor, All but 8 of them were taken out. Only 8 control rods remained in the reaction chamber. This was a violation of the safety protocols, because it was clearly written in the rule book that under any circumstances, there should not be fewer than 15 control rods in the reactor. But here, there were only 8 control rods in the reactor. Due to this, the power output spiked up. At 01:19 AM, because the control rods were abruptly taken out, the reaction spiked suddenly. The power output increased rapidly, and whatever little water was present in the core, turned into steam. It also meant that the amount of water in the reactor core kept decreasing. Remember, water was supposed to work as the coolant. A positive feedback loop was created here. As the water turned into steam, it couldn't act as the coolant, resulting in the reaction gaining more speed. A fun fact here, to avoid this positive feedback loop, in the present-day nuclear reactors, water is used as a coolant and as a moderator both. So even if the amount of water decreases, the moderator will also decrease, so this will be the same as taking your foot off of the accelerator. And it would result in a negative feedback loop. But in this RBMK reactor, graphite was the moderator, and the water was turning into steam rapidly. The Xenon present in the reactor till then which was working to slow down the reaction, burned out when the reaction sped up. So there were no more Xenons to absorb the neutrons. This was the point of Extreme Power Surge. The speed of the nuclear reaction increased exponentially. A lot of steam was being produced here, and the lid of the containment structure, started shaking. Shockwaves were felt throughout the building. Workers realised that they have to execute an emergency stop. So they pressed the Emergency Stop button. It was at 01:23 AM. Pressing this button meant that the control rods would be reinserted into the reactor, to slow down the reaction. At a glance, it would feel like a logical step, since our car is going at a high speed, we need to slam the brakes to slow it down. But there was a design flaw in this reactor. A design flaw related to the control rods. A control rod is actually made up of two parts. The main part of the control rod is made of Boron, it slows down the reaction by absorbing the Neutrons. But the tip of the control rods, were made of graphite. The same graphite that was used as a moderator in the reaction. That worked by speeding up the reaction. As soon as the button was pressed, and the remaining control rods were inserted into the reactor, the control rods with their graphite tips. This caused a blast. The graphite infinitely sped up the already speeding reaction. The power generation of the reactor reached 33,000 MW. The first blast was 6 to 8 seconds after the emergency button was pressed. Because of this blast, the core of the reactor started melting. After 2-3 seconds, there was an even more powerful blast. The 1,000-tonne lid on top of the structure, was blown into the air. The radioactive material started spreading through the atmosphere. The reason behind the second blast is said to be the graphite present in the reactor started burning. It combusted as well. Two people were instantly killed in this blast. But more than 100 radioactive elements and 5% Uranium fuel, there were 192 tonnes of Uranium fuel in the reactor, went into the atmosphere. At 01:26 AM the fire alarm began ringing. Firefighters reached the spot. Initially, the firefighters didn't have an idea of what was happening there. When they reached the place, they assumed it was a normal fire, and started trying to put it out with water. But this fire was caused due to graphite. It wasn't about to die down easily. It took more than 10 days to put out this fire. Helicopters were called in to drop thousands of tonnes of clay, sand, Boron, and lead from the sky. These elements can reduce the spread of radiation. But it was extremely difficult to do this. Because to drop the elements on the fire, the helicopter would need to fly right on top of it. A truly dangerous task. One of the helicopters crashed due to this. Killing four people. The firefighters who had reached to put out the fire, experienced fatigue and nauseous after they went home. Symptoms of Radiation Sickness. When you are exposed to an extreme high dose of radiation, these symptoms begin appearing indicating that you will die in a few months. The thing about radiation is that the more you are exposed to radiation, the deadlier it is. About 28 of these firefighters passed away within a few months. People who were exposed to comparatively less radiation, died in a couple of years, and those who were exposed even less, developed diseases like cancer within 10 years. Coming back to the reactor, the fire had died down after 10 days, But this reactor was producing high levels of heat even after the fire died down. It couldn't be controlled. Due to this heat, the base of the reactor started cracking. It was highly problematic. Because there was a water tank under the reactor. Filled with radioactive water. If the reactor producing high levels of heat came in contact with the water, the water would have instantly converted into steam. When temperatures are so high, that water is instantly converted into steam, this leads to an explosion. If you'd recall your Chemistry lessons, liquids occupy lesser space than gas. Since the atoms are more spread out when in a gaseous form. They require more space. When the water converts into steam suddenly, there is often no space for the steam to exit. Because there isn't one, it leads to an explosion. They were wary of a third blast. One that'd be bigger than the previous two. This blast would mean even more radioactive material being spread. There was only one way to prevent this from happening. A person had to dive into the radioactive water tank, and drain the radioactive water out of the reactor. To do this, a real-life hero was needed. The water was radioactive. Anyone diving into the water, would be in a danger throughout their remaining life. They may die within a few years. Or even within a few months. They had to literally risk their lives to carry out the task. We had three such heroes. On 4th May, 1986, wearing simple diving equipment, they dived into the water tank. They swam in the dark guided by only one lamp. They located the valves that had to be opened to drain the water. These three heroes are so important in our story that you'd find it difficult to believe. Because it is said that had the third explosion taken place, it would've been so dangerous that it might have killed millions of people. For the next 500,000 years, almost the entirety of the European continent would've become uninhabitable. The good news is that all three divers survived. Boris passed away in 2005 due to a heart attack. Alexei and Valeri are still alive. Once this risk was dealt with, the next step was to clean up the radioactive waste lying there. Initially, the officers of the Soviet Union used remote-controlled robots to do this. But the robots started breaking down near the radioactive waste. Thousands of people had to be sent in to clean up. They were known as the liquidators. During 1986-1987, 200,000 liquidators were dispatched to actively clean up this area. When the fire caused by the blast was being put out, The Soviet Union government tried to cover up the whole incident. The Cold War was underway. So the Soviet Union didn't want the world to know what had transpired there. But it is something that couldn't be hushed up for long. Because the radioactive dust, had reached all the way to Sweden. Swedish monitoring stations detected that the levels of radioactivity were unreasonably high. They carried out an analysis of the direction of the wind, and estimated the source of it. This forced the Soviet authorities to put the information about the disaster to the public. On 28th April 1986, the Soviet Union admitted that a disaster had indeed taken place. Two days after the actual disaster. The people living around this power plant, were told about what had happened after quite some time. This is another event for which the Soviet government is blamed. By 2nd May 1986, a 30 km radius was set up all around this nuclear power plant, It was declared an exclusion zone. This became a restricted area where no one could enter except for scientists and government officials. To contain the radioactive waste, another structure was built on top of it, known as the Sarcophagus, It was completed by November 1986, a concrete and metal structure to control the radiation coming off of it. But this structure wasn't built for the long term. About 28 years after it was constructed, rust and cracks started forming. This is why in 2010, the construction of the new safe confinement began. This new structure was fit atop the existing sarcophagus. 3 billion dollars were spent on building this new structure. It took 9 years to build it. Being completed only in 2019. But since it was built with a plan for the long term, it is said that this can keep the reactor closed off for the next 100 years. In 2018, the United Nations Scientific Community reported that 20,000 cases of thyroid cancer were seen in children below the age of 18 years. Who lived through the disaster. The main reason for this is said to be the radioactive dust settling on the grass on the pastures where cattle were fed. After the cows ate the grass, their milk contained a high level of Iodine 131. It gets absorbed in our thyroid gland, leading to thyroid cancer in children. The trees around this area, had a red ginger-coloured tint to them. This led to the area being called the Red Forest. In terms of the economics of it, the cost of the disaster to the Soviet Union $235 billion. Can you imagine how much money it is? Emergency response, cleaning up, relocating people, paying the expenses of the survivors, carrying out evacuations and decontamination of the environment they had to bear all expenses. Politically, it is said to be a major reason for the breakdown of the Soviet Union. Scientists all around the world started conducting research on preventing such future nuclear disasters. As a direct result of this disaster, the World Association of Nuclear Operators was founded in 1989. The aim of this international organisation was to conduct safety checks of all the nuclear power plants in the world. And to conduct research into how their safety can be further increased. I already mentioned one of its examples, how the present-day nuclear reactors use water as the Moderator as well as the Coolant. In order to prevent any positive feedback loop. Even though there were high levels of radiation in this area, the remaining parts of the Chernobyl nuclear power plant, continued to be in operation until the year 2000. To meet the power needs of Ukraine. After that, this power station was slated to be dismantled. And the process of doing so continues even now. It is expected that by 2028, this process would end. About 2,400 people work in this plant now. Either they work as the guards to defend the exclusion zone, or they are the firefighters, scientists, technicians or service staff. Because the radiation levels are so high there, they work only 2 shifts a week. And they are regularly monitored to check the levels of radiation absorbed by them. The area around the plant, in the 30 km exclusion zone, has been completely abandoned by humans. Before the war began, tourist groups did go to that zone, but more or less, the area has been taken over by nature. Big animals like wolves, deers, lynx, beavers, eagles, boars, and bears are found in this area. Some of these animals are even endangered. But they have a thriving population here, because of the lack of humans. Radiation has indeed affected some animals, and deformities have been spotted, but for the most part, there wasn't a major impact of the radiation on the animals. Friends, this was the story of the World's Worst Nuclear Disaster, if you liked the video, you can watch the video on Bhopal Gas Tragedy to understand that in detail. I have made numerous videos on such historical events, you can click on this playlist to watch them all. Let's meet in the next video. Thank you very much!