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Radiation leaked from the nuclear power plant.
Literal
Radioactivity [subject-が] nuclear power-plant [from-から] leaked
放射能 = radioactivity (放射 radiation + 能 capability). 原子力発電所 = nuclear power plant — a compound chain: 原子 (atom) + 力 (power) + 発電 (electricity generation) + 所 (place). The kind of word that looks intimidating but parses transparently once you know the pieces.