#1285 What caused the Chernobyl disaster?

What caused the Chernobyl disaster? Reactor number 4 at the Chernobyl nuclear power plant exploded after a test to simulate cooling the reactor went wrong. The disaster was blamed on the workers, but they were not trained properly, and the design of the reactor was flawed.

Let’s look at how a nuclear power plant works. Nuclear fission is initiated inside the reactor core. Enriched uranium is used. It has too many neutrons and it tries to lose neutrons. These neutrons fly off and when they hit another uranium atom they split it open, releasing energy and neutrons. The energy is released because the new atoms that are formed from the uranium have a lower atomic mass and the extra mass is released as energy because energy = mass. The neutrons hit other atoms and the process continues. This is called nuclear fission. Water is pumped into the reactor core under very high pressure. The higher the pressure is, the higher the boiling point is. The lower the pressure is the lower the boiling point. Water boils at the top of Everest at 70℃. Because the pressure is so high, the water can be heated by the energy coming from the core without boiling. The extremely hot water is passed through a heat exchanger where it heats other, not pressurized water, which produces steam and turns a turbine, making electricity. The nuclear reactor has control rods which are made of boron. Boron is an element that absorbs neutrons. If the control rods are lowered into the reactor, they absorb neutrons and slow down the reaction, reducing the heat. If the control rods are pulled out, the reaction increases, producing more heat. The water going through the reactor cools it as well, and that, along with the control rods, keep the reactor safe. That is generally how a reactor works.

The Chernobyl Nuclear Power Plant was started in stages between 1978 and 1984. It was a RBMK-1000 reactor. RBMK stands for the Russian words Reaktor Bolshoy Moshchnosti Kanalnyy (Реактор большой мощности канальный), which means “high-power channel-type reactor” in English. It uses 2% enriched uranium 235 as fuel and has graphite moderator, which is there to slow down the neutrons so there is more chance they hit other atoms and start a fission reaction. An RBMK reactor doesn’t have a heat exchanger, which means the water going into the reactor is the water being turned into steam. More water is pumped in to cool the reactor and control rods are used as well.

So, what are the design flaws with an RBMK reactor? The first is the fact that there is no heat exchanger, which means there is steam in the reactor. All nuclear power cores have something called a void coefficient. A liquid is always used to cool the nuclear reactor core, by removing neutrons, and the efficiency of that liquid is connected to how dense it is. Regular water is dense and cools the reactor effectively, but steam has air bubbles, or “voids” and it not only becomes less able to cool the reactor, it increases the amount of fission. Water absorbs neutrons much more effectively than steam does. Having voids gives it a positive void coefficient, which is not good with a reactor. The voids make the reaction faster, which produces more heat, which creates more steam, which produces more voids, which makes the reaction faster, and so on.

The second flaw is that the control rods have graphite on the ends of them. When control rods in regular nuclear plants are inserted into the core, they reduce the number of neutrons and the amount of fission immediately. The graphite on the control rods in RBMK reactors is designed to displace water from the core, increasing the fission reaction until the part of the control rod behind the graphite has entered the core.

The third flaw was not a design, but an operational problem. The people running the plant were too young, too inexperienced, and not trained well enough. Also, because of the culture of fear in the USSR at the time, nobody could speak out about problems for fear of the consequences.

On the day before the disaster, April 25, 1986, a test of the turbines in reactor number 4 was started. They wanted to see how long the turbines would continue spinning if electrical power was lost. The power was lowered and owing to problems, the night shift, who were supposed to come in after the test had finished, ended up running it. The power dropped far more than they expected, so they removed all of the control rods to bring the power back up. The core began to get hotter, producing more steam, which in turn made the core hotter because of the positive void coefficient. Once the test was finished, one of the operators pressed the button to reinsert the control rods. All of the control rods entered the core at the same time, displacing the water with their graphite tips and the fission reaction increased dramatically. The water heated to steam instantly and there was a steam explosion which blew the core apart and jammed the control rods. The first explosion broke the coolant lines and all the remaining coolant turned to steam instantly, creating the second and larger steam explosion. This blew reactor 4 apart and shot pieces of the graphite from the core out of the building. The core was completely exposed to the sky. There was a very beautiful light shooting up into the sky from the core, which was caused by the air being ionized in a column above the core.

The Chernobyl power plant is currently covered by a concrete sarcophagus. The surrounding area will probably be safe to live in again in 3,000 years and the whole are will be safe in 20,000 years. So, that’s great. And this is what I learned today.

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Sources

https://www.iaea.org/newscenter/focus/chernobyl/faqs

https://en.wikipedia.org/wiki/Chernobyl_disaster

https://en.wikipedia.org/wiki/Chernobyl_Nuclear_Power_Plant

https://en.wikipedia.org/wiki/RBMK

https://world-nuclear.org/information-library/safety-and-security/safety-of-plants/chernobyl-accident

https://www.oecd-nea.org/jcms/pl_28271/chernobyl-chapter-i-the-site-and-accident-sequence