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How did the containment structure limit radioactive releases at Three Mile Island?

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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.

During the 1979 Three Mile Island accident, the reactor's containment structure played a critical role in limiting the release of hazardous materials into the environment.1

Following a stoppage of core cooling that resulted in a partial meltdown, a significant portion of the core's radioactivity—along with hydrogen gas generated by the reaction of fuel cladding with superheated steam—was released into the containment vessel. Despite the occurrence of a hydrogen deflagration within the structure, the containment successfully acted as a robust physical barrier.12

By effectively enclosing these materials, the structure prevented all but a very small amount of radioactivity from escaping into the atmosphere. This performance is widely credited with preventing a major radioactive release and ensuring that the accident did not pose a significant threat to the health of the surrounding population.12