Chernobyl: The Physics Behind the Explosion

The furor over the Chernobyl miniseries continues. Many of the people who watched it have started exploring a long chain of books and documentaries focused on that terrible nuclear accident, each with different levels of precision and fantasy. However, if what you’re looking for is a more technical point of view, with advanced details about the reactor design, then you need to set aside 20 minutes for the video published by the great Scott Manley on YouTube.

One of the hardest things the Chernobyl series teaches us is the price of lies, negligence, and political irresponsibility, but there is a whole physical and technical facet that we need to analyze more deeply. The final episode delivers a good amount of information about it, although for artistic reasons and “temporal compression”, some pieces were left off-screen.

This is where Scott Manley steps in with his recent video. Who is Scott Manley? Probably one of the best communicators of science and space news on YouTube. Thanks to him I learned to play Kerbal Space Program, but if you want to know about a mission, a rocket, or a specific technology, you should visit his channel.

How a Nuclear Reactor Works

Scott’s video begins with an essential description of how a reactor operates. An atom of uranium or plutonium is “split” into lighter atoms, releasing energy and neutrons — in a word, fission. Some of those neutrons strike and split other atoms, initiating a chain reaction. One of the most critical tasks of a reactor is to regulate, moderate, and balance the relationship between neutrons and reactions. To do this, it uses control rods that absorb excess neutrons, and a nuclear moderator that reduces their speed. Two of the most adopted materials in moderation are deuterium (heavy water) and carbon (as graphite).

The Chernobyl reactor used the RBMK design (Reaktor Bolshoy Moshchnosti Kanalnyy), roughly “High Power Channel-type Reactor”, based on uranium with graphite moderation. This configuration allows (present tense because there are RBMK units online) the use of natural, unenriched uranium as fuel, which translates into extremely powerful reactors, and also economical ones (both in construction and maintenance). The “channels” are, loosely, pipes that run through the reactor core, carrying ordinary water (not heavy). A series of electric pumps move the water through the core. The liquid absorbs heat from the reactor, leaves the core, and its steam is separated to spin the turbines.

The Famous Tests

So we come to the famous tests. In an emergency, the reactor must shut down, but not the cooling process, because heat production continues after interrupting the reaction. Without external sources, the water pumps depend on diesel generators, which need about a minute to reach adequate performance. During that crucial minute, the pumps require an additional source of power, and in this case, it is the residual energy of the turbines that are starting to stop. At least, that was the hypothesis.

The problem was that this combination (the remaining turbine energy holding up the pumps until the generators activate) had never been successfully achieved. The tests at Chernobyl would demonstrate the definitive operation of the emergency system, and since the reactor was going to be shut down for maintenance, they scheduled the procedure. However, energy consumption that day had been higher than normal... preventing the shutdown of Reactor 4.

Scott continues for ten more minutes once he reaches this point (a pity there is no Spanish translation, although his English is very clear), but I think you already have a solid foundation. The buildup of xenon-135 that steals neutrons, the sharp drop in thermal generation to 30 megawatts, the pressure to complete the test, the withdrawal of the control rods, the start of the test, the reactor surge and acceleration of the reaction, the slowness of the rods to return to position, the absorption “void”... boom. The last reading of the control system was 33 gigawatts, ten times more than the station could tolerate.