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Why did the Chernobyl accident spread radioactive material over a large area?

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Labeled diagram showing the process of a nuclear power plant with a pressurized-water reactor. Water circulates through the reactor core, heats to produce steam in a steam generator, which drives a turbine connected to an electric generator. Steam is then cooled in a condenser using water from a cooling tower, which releases nonradioactive water vapor. Control rods, pumps, and transmission lines are also indicated. — nuclear power Schematic diagram of a nuclear power plant using a pressurized-water reactor.

The Chernobyl disaster resulted in the widespread dispersal of radioactive material due to a catastrophic combination of design flaws and human error during a poorly managed experiment at Unit 4.12

When the reactor's chain reaction spiraled out of control, a series of explosions blew off the reactor's heavy steel and concrete lid, exposing the core directly to the atmosphere. This breach, coupled with an ensuing fire in the graphite reactor core, propelled massive amounts of radionuclides into the air. Because the facility lacked a robust, Western-style containment structure—relying instead on a design that functioned more like a simple confinement—it was unable to prevent the release of these materials.1234

Once released, the radioactive particles were carried great distances by air currents, spreading contamination across Belarus, Russia, and Ukraine, and eventually reaching as far west as France and Italy. This event remains the most significant release of radioactive material in the history of nuclear power generation.23