堆芯熔化(meltdown)是指核反应堆温度上升过高，造成燃料棒熔化并发生破损事故。失去冷却水后，堆芯水位下降，燃料棒露出水面，燃料中的放射性物质产生的热量无法去除，随后温度持续上升会导致这种情况。这是核电站可能发生的事故中最为严重的事态。
接着堆芯会持续发热，进而发生熔化现象并且会与水蒸气反应。
It then gets even hotter even quicker, and bits of it will start to melt and react with the steam.
全部三个失事的反应堆都至少出现了堆芯部分熔化。
All three distressed reactors are thought to have at least partly melted down.
堆芯损伤状态包括堆芯无损伤、燃料元件包壳破损、燃料过热和燃料熔化等四种。
Core damage states include no core damage, fuel clad failure, fuel overheat and fuel melt.
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