The Chernobyl disaster was caused by a combination of design flaws, human error, and technical malfunctions that occurred during a safety test at the number 4 reactor of the Chernobyl Nuclear Power Plant.

The reactor was a Soviet-designed RBMK (high power channel-type) reactor, which had several design flaws that made it more prone to accidents than other types of nuclear reactors. One of these flaws was a positive void coefficient, which meant that as the coolant in the reactor heated up and turned into steam, the reactivity of the reactor would increase. This positive feedback loop could cause the reactor to become uncontrollable if not properly managed.

During the safety test, the operators of the reactor made a series of mistakes, including disabling safety systems and removing control rods that were designed to absorb excess neutrons and regulate the reactor’s power. These actions caused the reactor to overheat and led to a sudden power surge that resulted in a steam explosion and a subsequent graphite fire that burned for several days.

The explosion and fire released a massive amount of radioactive material into the atmosphere, contaminating the surrounding area and causing serious health and environmental consequences. The disaster also exposed major flaws in the Soviet nuclear industry’s safety culture and management practices, which contributed to the accident and its aftermath.