Sentence:  The report judged the risks of catastrophic nuclear power plant accidents (known as core meltdowns or <WSD>China syndromes</WSD>) to be socially acceptable. 
Meaning of the word marked with <WSD> tags:  A hypothetical kind of catastrophic failure in which a nuclear reactor melts through the floor of its containment system and penetrates the Earth's surface, continuing downward as if (from a Western Hemispheric point of view) traveling through the planet toward China.