Nuclear explosions are impossible due to the design of a nuclear reactor. The MIT reactor, for example, is equipped with a large amount of water and core structural materials that slow down neutrons before they reach other fissile atoms. Even if an uncontrolled reaction were to occur, it would be too slow to cause an explosion. Additionally, the reactor is designed to shut down on its own as the temperature increases, making a thermal explosion impossible. The Chernobyl disaster was a stark reminder of the power of nuclear energy and the consequences of human error.
Valery Legasov, Boris Shcherbina, and Ulana Khomyuk were tasked with investigating the disaster and determining the cause of the explosion. They concluded that the key reasons behind the disaster were due to the failures of those in charge, including chief engineer Anatoly Dyatlov. The RBMK reactor that exploded in Chernobyl was composed of a graphite core sandwiched between two biological shields. This core contained thousands of channels with fuel rods made up of uranium atoms that are easy to split.
The core also had channels for control rods, boron compounds, and graphite tips designed to neutralize the reaction. Water flowed through these channels and the entire structure was lined with steel and sand. When neutrons ping inside the uranium atoms, they pass through the solid graphite that surrounds them. Graphite and water slow down these neutrons, making them more likely to be captured by the network of uranium atoms. If this process occurs over and over again in a chain reaction, it generates a lot of heat which causes water in the channel to boil and turn into steam.
This steam is then used to create energy. Control rods are used to equilibrate the reaction and prevent it from getting out of control. If too much energy is generated, these control rods are placed in the core to prevent neutrons from colliding with each other as frequently. However, if the plant itself loses energy, it can quickly lead to disaster. On April 26th 1986, a safety test was conducted at Chernobyl with the aim of solving this problem. The control room team attempted to bring back energy levels by removing control rods from the core in order to reactivate the reaction.
This delay caused an accumulation of xenon atoms which blocked nuclear fission and caused the core temperature to drop so much that it stopped boiling water and producing steam. The ratio of water to steam is known as the vacuum coefficient. In other nuclear reactors, this coefficient is negative: more steam means less reactivity. However, in Chernobyl's RBMK reactor, this coefficient was positive: more steam meant more reactivity which accelerated nuclear fission and caused an explosion. The Chernobyl disaster was a tragic reminder of how human error can have devastating consequences. It also highlighted how important it is for those in charge of nuclear reactors to understand their limitations and adhere to safety protocols.