Chernobyl Explained

December 1st, 2023 (11 months ago) • 5 minutes

Chernobyl is a 2019 historical drama television miniseries produced by HBO and Sky UK, revolving around the Chernobyl nuclear disaster of April 1986 and the unprecedented cleanup efforts that followed.

It's an amazing series and one of the best miniseries I have watched. I enjoyed the minor details they displayed in the show, although they are not the most accurate in terms of historical facts.

The show's explanation of how an RBMK reactor works is very clear and easy to understand for a layperson.

In short, the Chernobyl explosion happened because of a combination of human error and design flaw.

Some of the phrases that stuck with me from the series:

Nuclear Reaction 101

A nuclear reactor operates based on Uranium-235 isotopes undergoing fission, which is the splitting of the atom into two smaller atoms, releasing a large amount of energy in the process. This energy heats water, creating steam that spins a turbine to generate electricity.

When enough uranium-235 is packed together, the reaction can go out of control, as each fission releases more neutrons that split more uranium-235 atoms. This is known as a chain reaction. To control this and maintain balance, several measures are in place:

Absorbers - materials that absorb neutrons and slow down the reaction:

1. Boron
2. Xenon - a product of uranium decay

Moderators - materials that slow down neutrons to speed up the reaction.

Neutrons travel at high speeds, too fast to effectively collide with other atoms to split them. Therefore, a layer of moderator is used to slow down the neutrons

1. Water (or heavy water) - also used as a coolant to cool down the reactor
2. Graphite

In RBMK reactors, the control rods are made of both boron and graphite. This design is used because if control rods made only of absorber material (boron) are removed, the reaction will still speed up, albeit at a slower rate (since water is not as effective a moderator as graphite). Therefore, a combination of boron and graphite in control rods allows for better control of the reaction.

A control rod made of boron and graphite together

What happened

On the night of the explosion, the reaction was already running at a very low power level, and the operators were trying to run a test to see if the turbine could provide enough power to the reactor's cooling system in case of a power outage. When a reactor is run at a low power level, one of the products of uranium decay, called xenon, will build up in the reactor. It doesn't show up immediately due to its half-life but will build up over time, hence decreasing the power further over time.

Under normal conditions, you would increase the power level slowly over time to counter this effect, but that did not happen. The operators pulled out all of the control rods to speed up the reaction due to pressure from the top management.

When the brakes were pulled out, the accelerators were pulled in. Water, which is usually there to slow down the reaction, started to boil as steam, reducing the number of absorbers. Neutrons that were released from the fission started to jump out and fuse with the xenon atoms that had been building up, reducing the number of absorbers even further. Now, all sorts of counterbalance to the reaction were practically gone.

As the power spiked exponentially, the operators did what they could, which was to press the AZ-5 button, an emergency button used to lower the control rods instantaneously to stop the reaction. However, when the control rods were lowered, they produced a flux at the bottom of the reactor, which caused a spike of energy above the limit of the reactor, leading to an explosion. The reason behind this is because he graphite rods are usually shorter than the uranium rods to balance the neutron flux (how atoms are displaced) in order to keep the reaction balanced and maximize output. Due to this, pushing down the rods displaced the water and forced the energy output to spike in 1 area.

The AZ-5 button was a design flaw that was not known to the operators.

The aftermath

The explosion caused the roof of the reactor to be blown off, exposing the reactor core to the air. The graphite rods caught fire, and the fire was fueled by the air coming in from the roof. The fire was put out by the firefighters, but the graphite rods continued to burn for days, releasing radioactive particles into the air.

My thoughts

It's such a tragedy that the explosion could have been prevented if the operators were properly informed and trained. Transparency and communication are so important in any organization, and it's sad to see that the lifes were lost due to many cover ups.