In 1986, the Chernobyl Nuclear Power Plant in Soviet Ukraine was the third most powerful on Earth. To accommodate its army of workers, authorities constructed the city of Prypiat two kilometers from the plant. Bustling with a population of 45,000, life in this “nuclear town” was luxurious by Soviet standards: meat and dairy were available in shops, and it boasted two swimming pools and an ice rink. This idyllic scene was shattered after April 26, when a series of explosions tore through Chernobyl’s Unit 4 – home to its fourth nuclear reactor. To really grasp the scope of this disaster, it’s helpful to understand how nuclear reactors work. First of all, they exist to create heat. This heat vaporizes water into steam, powering turbines to produce electricity. They create heat through a process called fission. Fission is when the nucleus of an atom splits into smaller components. When fission occurs, energy and tiny subatomic particles called neutrons are released. We can induce fission by forcing a neutron to collide with another atom’s nucleus – but we can’t do this without a supply of neutrons that have already been freed from their original atom. The nuclei of some atoms – like uranium-235 – are extremely unstable. They naturally want to undergo fission to allow them to split into smaller, more stable parts. This natural fission can start a chain reaction, with freed neutrons colliding with other atoms, splitting them and releasing more neutrons. Packing uranium-235 atoms close together in fuel rods creates just this kind of chain reaction. But there’s a problem: neutrons travel so fast that they’re unlikely to hit other uranium atoms. To slow them down and thus increase reactivity, nuclear plants use substances like water and graphite. To control the power of the reaction, nuclear plants also have control rods, made of materials like boron, which absorb neutrons. These are inserted into the reactor’s core and adjusting the depth of the control rods regulates the power of the reaction. Meanwhile, cooling fluid circulates through the reactor itself, moderating its overall temperature. So, what happened inside Unit 4 on April 26? Well, unbelievably, operators were conducting a safety test on this very system. If Chernobyl ever experienced a power loss, electricity was still required to pump the cooling fluid into the reactor to stop it from overheating. Diesel generators were on hand for this, but they took 45 seconds to kick in – a dangerous delay. But Chernobyl’s steam turbines didn’t immediately stop after a power loss, so it was possible that their dying rotations would produce enough electricity to bridge the 45-second gap before power was restored. The test aimed to confirm this.