Thirty-eight years ago today, on April 26th, 1986, a nuclear catastrophe unfolded in the quiet early hours near Pripyat, Ukraine. The Chernobyl disaster would become the worst nuclear accident in history, reaching the maximum severity level and ranking as the costliest disaster ever recorded. A reactor meant to be the source of power for thousands became a source of unimaginable destruction and long-term suffering. It was a tragedy born from isolationism, secrecy, design flaws, and poor operational choices. In this blog post, we’ll focus on critical design flaws, particularly the positive void coefficient and the flawed control rod design, that combined to cause the uncontrolled power surge and resulting explosions.
Fission
Fission is the backbone of nuclear power. It is the forceful splitting of an atom's nucleus into smaller nuclei. We split the nucleus because doing so releases potential energy by moving an element from an unstable state to a more stable one. We can split nuclei on demand by shooting them with neutrons. The nucleus absorbs this neutron, which breaks it apart. Just like when you're breaking in pool, imagine the white ball as a neutron, and the rest as the nuclei.
Typically, Uranium-235 is used because it's fissile, meaning it can be readily split by a neutron. There are more neutrons than protons (92 protons, 143 neutrons) within the nucleus. Since protons are positively charged, they repel each other. To prevent them from shooting off, the strong nuclear force keeps them together, making the nucleus “unstable” in a stable sense (with no interference it won’t split), yet fissile (easily split through interference.) However, since the strong nuclear force acts over very short distances, whereas the repulsion between protons acts over the entire, large nucleus, the overall binding effect is not as strong as it would be in smaller nuclei. Because this nucleus isn't as tightly bound per particle, it is susceptible to fission when disturbed by a neutron.
Okay, so we want this U-235 nucleus to become stable. Not because we're nice, but because the process of it becoming stable releases the energy we need to power things. To begin, we actually make it even *more* unstable for a second. We shoot a neutron into the nucleus, and when it absorbs that neutron, it becomes an even more unstable isotope, U-236.
Since U-236 is very unstable, it immediately splits apart (fissions) into smaller, more stable nuclei called fission products. The total mass of these resulting fission products (and any other neutrons that are released) is slightly less than the mass of the U-236 it started as. Where did the mass go? Remember the E=mc^2 equation? The mass was converted directly into the energy released during the split. Because c (the speed of light) is ridiculously large, even a tiny amount of mass turns into a monumental burst of energy, mostly released as heat. This is what we're after. U-235 is not abundant. In natural uranium, it makes up only 0.7%. This is not enough to sustain reactions. That's where enrichment comes in, which increases the proportion of U-235 to its other isotope (that being U-238). The RBMK reactor used 2% enriched U-235.
We can create continuous power through the chain reaction. During fission, alongside the fission products released, fast-moving neutrons are also produced. If these go on to strike other U-235 nuclei, they will fission, which releases more energy and fission products (including more neutrons), which go onto hit more U-235 nuclei and this continues. This is self propagating, and if this continues uncontrollably, with more than one neutron hitting nuclei, it is called supercriticality. However, if less than one neutron hits nuclei, this is called subcriticality. A sustained, controlled reaction is called criticality, where exactly one neutron from each fission reaction causes another fission event. This keeps power output steady.
Main Reactor Components
Moderator
U-235 fissions best with slow moving neutrons. Neutrons released from fission of U-235 nuclei are fast-moving. How do we slow this down? We introduce a moderator, which is placed inside the core, around the fuel (our U-235). When fast-moving neutrons hit this moderator, it transfers some of its kinetic energy into the nucleus of the moderator, slowing the neutron down, just like when you hit another ball while playing pool. The ball that hit the other one slows down as some of its kinetic energy is transferred into the stationary one. After multiple collisions with the moderator, the neutron slows down to what’s known as its thermal energy level, where it's highly efficient for fission with U-235. In this RBMK reactor, graphite was used as the moderator, as it slows down neutrons efficiently without absorbing too many of them.
Coolant
The primary form of energy produced by fission is thermal (heat). We need a way to take this heat and use it. This is where a coolant comes into play. It can either be in the form of a liquid or gas, which will circulate through the core, into a system where steam is collected from that heat, which is used to turn turbines, which generates electricity! In the RBMK, the coolant used was water. Water is pumped to the bottom of where the fuel is stored (fuel channels), where fission is happening. As it moves up pressure tubes, it absorbs the thermal energy (fancy for heat) and boils. In the RBMK reactor, the tubes are connected directly to where the steam can be used to power the turbines, so there's no separate area for steam to be collected and then sent to the turbines. Therefore, the water serves two purposes: as coolant, and to provide the turbines directly with steam to power them, without needing an exchanger.
Control rods
We need to be able to maintain criticality, or deliberately move into different levels of criticality in case we want to increase or decrease reactivity. Control rods are used for this. Control rods are made of materials extremely good at absorbing neutrons. They are essentially neutron sponges. An RBMK reactor has 211 control rods, composed of boron. When the control rods are lowered into the moderator (our graphite), they absorb the neutrons instead of them bouncing off the graphite, which slows down the reaction. The further they're lowered into the core, the more neutrons are absorbed. Withdrawing control rods means fewer neutrons are absorbed, which allows for more neutrons to find U-235 nuclei, increasing the fission rate. Fully inserting control rods absorbs enough neutrons to make the reactor subcritical, and shut down the reaction.
RBMK reactors produced immense amounts of power, and as such they did have safety measures and procedures. But as we'll see later, fundamental design and operational choices made them futile.
Positive Void Coefficient - the main problem
Let's think about reactivity. It's the measure of how quickly the chain reaction increases or decreases. Positive reactivity means the reaction rate and power are increasing, and the opposite for negative. Zero reactivity means it's stable (at criticality). This is what our control rods manage. A void coefficient describes the reactor's reactivity when voids (steam bubbles) form inside the coolant (water). It tells us that if more water turns to steam, does the reactor's power increase (positive coefficient) or decrease (negative coefficient).
In the West, water cooled reactors have water serve as the coolant and the moderator. So if water turns into steam, cooling becomes less effective, as steam doesn’t absorb heat as well as water (since it's less dense), which, in turn, also means that moderation becomes less effective. Since neutrons need to be slowed down (moderated) to efficiently fission U-235, reducing moderation makes the chain reaction less efficient, and so in these reactors more steam means less reactivity, which is a negative void coefficient. This is a safety feature: if the reactor gets too hot, the chain reaction slows down.
In RBMK reactors, when steam increased, a positive void coefficient would be achieved. Graphite was the primary moderator, while water flowed through the channels in the graphite to help with cooling. This water also absorbs some neutrons. But the problem was when this water boiled. As steam production increased, the neutrons that would have been absorbed by the dense water would now actually increase fission. Since water was used as a coolant, when it boiled, it would produce steam (increasing the voids), making it less dense. Since it's less dense now (mostly steam) it absorbs fewer neutrons than its liquid form would. But remember, we still have graphite present slowing neutrons down. As mentioned earlier, in a normal water cooled reactor, the liquid is used as both the coolant and moderator. So when the voids (steam bubbles) increase, the reactivity actually decreases, as the neutrons are not slowed down (moderated) as effectively. Here in the RBMK, the graphite is still moderating, slowing neutrons down. Because fewer neutrons are absorbed by water, more neutrons survive to be slowed down by the graphite, making them available for fission. More fission causes more heat, creating more voids (steam), absorbing fewer neutrons, making more neutrons available for fission. This was a positive feedback loop, and a very positive void coefficient. As the amount of steam increases more heat is generated, leading to more steam, leading to more heat.
The 1986 accident
Unit 4 was being prepared for a routine shutdown on the 25th April, 1986. Operators wanted to use this opportunity to conduct a test that was to see whether, after a loss of main power, the momentum from the turbines spinning down would be enough to power the main pumps that provide cooling water until the backup generator kicked in. This wasn’t new - a test the previous year showed that the power dropped off too quickly, but that was before they upgraded their machines, so they wanted to try again. Prep for this test carried on into the early hours of the 26th. However, deliberate actions from the operators compromised the reactor: they disabled the automatic shutdown mechanisms, most likely to stop the reactor's safety systems from interfering with the low power tests they were trying to do. This, along with other actions like withdrawing too many rods (remember, this increases reactivity) meant that when they moved to shut down the reactor, it was already in an extremely unstable condition.
What actually triggered the disaster was when they tried to shut down the reactor. The RBMK control rods were meant to absorb neutrons through boron, but their tips contained graphite which was meant to displace water before the boron entered the core. As we have said numerous times, graphite is a moderator, and slows down neutrons, increasing reactivity initially before the boron decreases it and induces subcriticality. This caused a huge surge in power the second the shutdown button was pressed, and the rods began inserting into the already unstable core. The incredibly hot fuel + water caused the fuel assemblies to break apart, which increased the surface area for heat transfer, leading to rapid explosive production of steam, which increased pressure in the fuel channels.
On April 26th, 1986, at 1:23am, Alexander Akimov pushed the AZ-5 (shutdown) button. The rods were only partially inserted into the already unstable core. The intense pressure generated by the steam blew the plate cover (weighing 1000 tonnes) off the reactor vault, which destroyed pressure tubes and jammed the rods. Huge amounts of steam spread through the core, leading to the first steam explosion. This destroyed the reactor building and released radioactive fission products into the atmosphere. Not even three seconds later, a more powerful explosion happened. This is the blast that ejected tonnes of graphite out of the vault. Since graphite was only found inside the core, the fact that it was in plain sight on the roof of the turbine building was iron-clad proof that the core had exploded.
Two workers were killed immediately, one being Valery Khodemchuk, an engineer on that shift working on the circulating pumps. The ejected graphite started many fires. These fires provided a medium for radioactive material to disperse, and this was the reason for the famous helicopter drops, where tonnes of elements like sand and boron were dropped onto the radioactive material in order to smother it.
Impact and Aftermath
The Soviet Union tried to conceal the accident, but radiation monitoring stations in Sweden detected high levels of radiation on the 28th of April, which forced them to admit a major incident had happened. This was the largest uncontrolled radioactive release into the environment ever recorded, and large quantities of radioactive substances like iodine-131 (which accumulates in the thyroid gland) were released into the air for around 10 days. Most of the material was deposited close by as debris, but lighter material was spread by wind over Ukraine, Belarus, Russia, Scandinavia and Europe.
There were many casualties. Of immediate responders (firefighters) who attended the fires on the roof of the turbine building, six died by the end of July due to doses received on the first day alone. They responded with no specialised protective gear, as it was believed these were regular fires. 30 people died within the first few weeks as a direct result of the radiation. Two workers were killed instantly by the explosion, and 28 more suffered from a fate worse than death: Acute radiation syndrome (ARS). Doses received were high enough to result in ARS, which happens when a person is exposed to more than 700mGy within a short time frame. Symptoms of ARS include nausea, vomiting, headaches, burns, and fevers. If the entire human body is exposed to doses between 4000 - 5000 mGy, there's a fatality rate of 50%. 8000 - 10,000 is considered fatal. Doses for the most severely affected first responders reached as high as 20,000 mGy. Pripyat, the town for nuclear workers, was evacuated the next day, and 116,000 people within a 30km radius were evacuated by the 14th of May.
An exclusion zone was established shortly after the accident, initially set to a 30km radius. To date, access is heavily restricted, and you need a permit to enter. Radiation today has significantly decreased due to the decay of short-lived isotopes like iodine-131, but levels still remain above background radiation in many areas. However, thousands of people still work in the zone. They decommission projects and monitor the environment. These people live in the city of Slavutych, specifically built for evacuated personnel. It's located outside the zone.
The city of Pripyat is now a ghost town. Buildings crumble due to weather and time, and nature is beginning to reclaim the city, with trees growing on roofs and inside buildings. One of the most iconic landmarks is the never-used Ferris wheel in the amusement park, which was scheduled to open just days after the disaster. Personal belongings are still inside many apartments. Funnily enough, there's a thriving ecosystem there, with populations of wolves, deer, and horses.
The destroyed unit 4 reactor is now covered by a massive arch-shaped structure that was completed in 2017. It was built over the original, crumbling sarcophagus made to contain radioactive material. This is meant to do the same for the next 100 years, to allow for the eventual dismantling of the reactor debris. Units 1, 2, and 3 are currently undergoing decommissioning, and the site won’t be cleared by 2064. Despite the grim nature of the area, Chernobyl had become a large tourist destination, with guided tours operating with strict safety rules, although this has stopped due to the Russia Ukraine war.
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