M5 alloy was a new zirconium alloy used in high burn up assembly such as fuel cladding, end plug, guide tube and grid spring and so on.

M 5合金是一种适用于高燃耗组件的新型锆合金，主要用作燃料棒包壳、端塞、导向管和定位格架材料。

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This meant likely degradation of their zirconium alloy cladding, the possible release of hydrogen, and - by Tepco's admission - the risk that a nuclear chain reaction could begin.

BBC: Fukushima: As bad as Chernobyl?