Final answer:
The Chernobyl disaster was caused by an unauthorized experiment conducted with some safety features disabled, leading to a catastrophic increase in reactor pressure and a subsequent explosion.
Step-by-step explanation:
The major cause of the power plant explosion at Chernobyl was an unstable reaction during an unauthorized experiment while operating at low power with some safety devices shut off. The chain reaction in one of the reactors became uncontrollable and exceeded the design capacity, leading to extreme steam pressure and a reactor rupture. The absence of a containment building meant that radioactive material was released into the environment as the graphite moderator caught fire, resulting in acute radiation sickness among first responders and long-term environmental impact.