What Happened In Chernobyl
Contrary to what action movies suggest, an explosion is not a fireball that spreads debris and fire. An explosion is primarily a closed container - be it a pressure cooker or a nuclear reactor - that suddenly gives way to too much pressure. This happens when a liquid is vaporised because the gas takes up much more space than the liquid. For example, when water is boiled, the steam forms and builds up pressure because it cannot escape. At a certain point, the container gives way and the explosion occurs.
Why was there water vapour at Chernobyl?
The principle of nuclear reactors is to heat water with a nuclear fuel and turn it into steam. This steam is fed into a turbine and turns it, producing electricity. The steam is then cooled down, becomes water again and is reused. The difficulty lies in controlling the production of steam. If you heat the water too much, you will produce too much steam and if you don't release it, the system will break down somewhere.
What caused the explosion?
As is often the case with accidents, there is no single explanation, but a combination of several factors which, when put together, led to the disaster. In this case, the accident occurred when the engineers wanted to conduct a test. The test was to prove that the installation was perfectly safe in the event of a power cut. The power supply is indeed a crucial element of a nuclear reactor because it allows the water to circulate continuously. This water not only turns the turbines but also cools the nuclear fuel. If the water stops circulating, the fuel will heat up and deteriorate while the water will be transformed into steam. To compensate for power cuts, all power plants have huge generators, the "diesels". The problem is that they take a few dozen seconds to start up. The test was to show that the electricity still produced by the turbines could be used to run the water pumps until the diesels were started. But the engineers were unable to control the reactor at that point.
Why did the test fail?
Firstly, it did not take place as planned. It should have happened a few hours earlier but was postponed at the request of the authorities to meet the demand for electricity following a problem at another plant. The first consequence was that it was conducted by an inexperienced night crew who failed to understand what was going on. Secondly, during this postponement, the reactor was operating at half power. This led to the invisible build-up of a by-product of the nuclear reaction, xenon. Operations had no way of detecting it, nor did they know the risk it posed. Yet this element would prevent the reactor from being properly controlled.
The second reason for the test's failure was human error. First, the reactor was slowed down too much by a handling error. When the operators wanted to restart it, the xenon that had formed prevented them from doing so. This element is indeed a 'poison' for the nuclear reaction. Despite this, the engineer in charge of operations decided to continue with the experiment when safety rules should have prompted him to stop. Worse, he decided to increase the power to the maximum beyond the safety measures, all at once and not gradually. Eventually the power went back up. The water was turned off for the test. A few seconds later, the reactor's power was 100 times higher than its nominal power. It exploded.
Why could the reactor not be controlled?
Firstly, the presence of xenon made it very difficult to control. Secondly, to understand this, one must imagine the means provided for the control of the nuclear reaction. The operators have an "accelerator" and a "brake". In concrete terms, these are control rods that are inserted or withdrawn in the reactor. But in this type of reactor, the accelerator (graphite) and the brake (boron) are placed one above the other on the same rod. So when the operators decided to remove the brake to restart the reactor, they also introduced the accelerator. But when they wanted to do the opposite procedure when they decided to launch the emergency shutdown procedure, the accelerator got stuck in the hottest part of the reactor which had started to deform. The accident was inevitable.
So it was a design flaw in the plant?
The operators did not know that when they pressed the brake, they also pressed the accelerator for a short time. But above all, the control rods were set up excessively slowly (about 18 seconds). Another flaw was that, for reasons of economy, conventional water was used in this reactor. However, when it is vaporised, it promotes the nuclear reaction. Finally, the reactor did not have what is known as a "containment vessel", designed to keep the core enclosed in the event of an explosion.
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