In the popular HBO mini-series “Chernobyl,” early episodes show scientists Valery Legasov, and Ulana Khomyuk, a composite character, expressing their alarm at a nuclear reactor core explosion. We try to explain the same thing in a simpler manner.
How does a nuclear reactor actually work?
Atomic energy is harnessed in two ways:
1. Fusion - is when two atoms are fused together. Though we have managed to make an uncontrolled fusion reaction called a thermonuclear bomb, no one has so far been able to control fusion safely in a way that creates energy. This happens in the Sun all the time. If anyone ever successfully pulls it off on Earth, that person will become very popular and very rich.
2. Fission - is a process that can make an atomic bomb, but scientists have found a way to control it. This is the process in which a stable atom is made unstable, forcing it to split up and send particles called neutrons flying out, searching for another place to land.
When the atom decays, it gives off a little bit of energy and it shoots out some particles called neutrons. Those neutrons are absorbed by other nuclei that are stable, but when they absorb the neutron, they become unstable. So now they break apart, sending more neutrons. Thus, you get a chain reaction.
Some naturally occurring elements are awesome for this kind of activity. Uranium, for instance, has radioactive properties, and gives up neutrons like they’re Christmas candy.
This is why Uranium-235 is commonly used in nuclear power plants. The energy this process creates is given off in the form of heat, which turns water into steam that powers a turbine to create electricity, just like any other power plant.
If the fission reaction is uncontrolled, you get a bomb. If it’s controlled, you get a nice and smooth generation of energy.
To make this controlled, nuclear power plants conduct fission inside the reactor core. The reactors also have things like cold water to prevent overheating, elements that can slow down or stop the chain reaction, and moderators to ensure the process happens in a controlled space.
How does a nuclear reactor explode?
Chernobyl used the Soviet designed RBMK reactor. On the night of April 26, 1986, a system test was being run on reactor No. 4 to simulate a power outage.
The power was set too low, and the RBMK reactor became unstable. There were not enough fail-safes, and there was a lot of human error.
Somehow the reactor got supercritical, and the staff weren’t able to control it. Couple that with a reactor that has inherent design flaws, and suddenly you have high chances of a complete core meltdown.
The Elephant's Foot, Chernobyl
Control rods are used in reactors to increase or decrease the energy output of a nuclear reaction. In this case, they were made of the element Boron, which is the opposite of Uranium in that it is good at absorbing neutrons and not going crazy. The control rods absorb those neutrons that, if they’re not absorbed, would otherwise cause the reaction to become uncontrolled.
Graphite was used to moderate the uranium neutrons, effectively keeping everything under control.
Water was used to cool down the core and also absorb neutrons. But a sudden power spike turned the water into steam and created gaps known as “voids.”
RBMK reactors were designed to have a very high positive void coefficient, meaning when voids are created, power output increases. The reasons for making it so high are notably Soviet: it cost less money to run the reactor and it allowed for the plant to both create electricity and enrich uranium to be used in nuclear bombs.
Chernobyl personnel tried to emergency stop the test by inserting the control rods, but this only caused more voids - which led to more power, and a subsequent steam explosion that ruptured the core and fractured one of the rods. With the reaction continuing unhindered, a much larger explosion then blew open the core, opened a giant hole in the roof and scattered graphite everywhere.
The presence of graphite outside of the building and on part of the roof led scientists to surmise that the core was gone. And that meant all the highly radioactive material was exposed to the air and lifted by the ongoing graphite fire to be spread over hundreds of miles.
Chernobyl and the nearby town of Pripyat, now in independent Ukraine, are still no-go zones, and will be for the foreseeable future.
Why didn’t it explode like a nuclear bomb?
To get a fission chain reaction that creates a nuclear bomb, everything needs to be compact. Nuclear reactors, on the other hand, are built with enough space, so that an atomic bomb-type explosion can’t occur.
Chernobyl suffered a meltdown, meaning the core got so hot that it turned into a "nuclear glob of radioactive material", but that’s not in danger of exploding.
What did happen with Chernobyl is that meltdown almost fell into pools of coolant water below. If not for the actions of the three brave volunteers who went down and drained the water - shortening their lifespans in the process - that glob would have reacted with the water, causing another steam explosion that would have decimated the immediate area and made about half of Europe uninhabitable for a few hundred thousand years.
Thankfully, that didn’t happen.
Why would anyone still use nuclear energy?
The Soviets immediately took steps to change the RBMK reactor design, lowering its positive void coefficient to make the reactors more stable. And not every reactor type comes with a positive void coefficient; in fact, some are negative.
American nuclear reactors have always been designed in a way that can contain accidents. For example, the U.S. came close to its own Chernobyl when the Three Mile Island plant near Harrisburg, Pennsylvania experienced a partial meltdown in 1979. Though alarming, it was determined that no radioactive material escaped the facility.
Americans build their power plants with the idea that, if something was to happen and the core was to get overheated and released radioactivity, it would stay inside a containment vessel. Chernobyl had no containment vessel, and therefore it went all over the place.
The technology we use to control the process that creates nuclear power is, of course, much better now than it was in 1986.
There are ways now to build nuclear power plants that are inherently safe, that human error can’t cause any problems. The default state is ‘off,’ so if anything happens, it goes off and the control rods go in.
Another Chernobyl is not likely to happen in the distant future. Besides, nuclear power plants bring with them plenty of advantages that outweigh the potential to cause disasters. Then again, it's important to note that we never dreamed an accident like Chernobyl was possible in the first place.
"There are things we know that we know. There are known unknowns. That is to say there are things that we now know we don't know. But there are also unknown unknowns. There are things we do not know we don't know."
- Donald Rumsfeld