You can think of meltdown as the moment something gets too hot to keep its own shape. Heat builds up, nothing carries it away, and the thing that was holding everything in place and sinks. The two halves say it : melt, and down. Every use of the word points at that same moment — the point where what was containing the trouble stops being able to contain it.
From the word’s history
Wiktionary gives meltdown as melt (verb) + down (adverb), a deverbal from the verb melt down. The noun therefore keeps exactly the action of the verb: heating until shape is lost and the mass sinks.
Wiktionary 記載 meltdown 由 melt（動詞）加 down（副詞）構成，是從動詞片語 melt down 直接變來的名詞。所以這個名詞完整保留了那個動作：熱到失去形狀、整團往下塌。也因為它出身於一個動作，它描述的永遠是一個過程的結果，不是一種長期狀態。
Wiktionary entry for meltdown (etymology: melt + down, deverbal from melt down), supplied in this evidence packet
登入就可以看每一支怎麼連回核心畫面。