Radiation leak: A radiation leak occurs when radioactive materials escape from their containment and are released into the surrounding environment.
Containment vessel: The containment vessel is a thick concrete structure that surrounds a nuclear reactor to prevent radioactive materials from escaping in case of an accident.
Core meltdown sequence: The core meltdown sequence refers to the series of events that occur during a nuclear meltdown, starting with loss of coolant and ending with fuel rod melting.