Why did the Chernobyl nuclear power plant explode?

Chernobyl disaster


In the early hours of 26 April 1986, the operators of the Chernobyl nuclear power plant were preparing to carry out an experiment to check whether, after a power outage, the turbine was capable of generating enough electricity to operate the safety systems. Paradoxically, nuclear reactors require external electrical power to operate the control rooms and hydraulic pumps that circulate the water inside the reactor core and extract the heat generated within.

To do this experiment, the reactor had to be operating at very low powers. A delay in the start of the experiment caused contamination by xenon-135, an element that absorbs neutrons and therefore slows the fission rate in the reactor. This caused the power to drop below the limits required for the experiment. To correct the situation, the operators decided to disconnect most of the control rods from the automatic regulation system, which they removed manually, and reduced the injection of cooling water. In a nuclear reactor, control rods are used as a safety mechanism because they absorb a large amount of neutrons, so adjusting their position is the easiest way for operators to control the amount of fission reactions occurring within the core. However, these actions by the Chernobyl operators left the reactor in a highly unstable state they were not quite aware of.

When the experiment itself began, the reactor suffered a sudden increase in power, at least up to ten times more than was anticipated in the reactor design, and all the cooling water evaporated in a few moments. First, there was a steam explosion that created cracks in the reactor and allowed oxygen to enter the core. A few seconds later, there was a second, stronger explosion, probably caused by the combustion of the hydrogen, which had accumulated inside the reactor. The hydrogen was formed by the reaction of graphite – used by this type of reactor as a neutron moderator – and water vapour at very high temperatures. Other studies explain that the second explosion was produced by the rapid heating of the structural materials of the nucleus, caused by the loss of water after the first explosion. This second explosion left reactor 4 of the Chernobyl plant wide open and released a large quantity of radioactive elements into the environment.

Although the responsibility for the accident was first attributed almost exclusively to the plant operators, subsequent studies have shown that design problems in the reactor also made it unstable when operating at low power as required by the experiment. Therefore, even though the plant engineers made serious technical errors during the performance of the test and the subsequent management of the accident, it is also true that they were unaware of the design defects in the reactor if it operated at low power.

Antoni Vidal-Ferràndiz graduated in Aeronautical Engineering and holds a PhD in Nuclear Energy from the Polytechnic University of Valencia.

Question sent by Daniel Butti Julià.

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