This fallout material decays over a long period of time, and is the main source of residual nuclear radiation.

尘降物需要很长的时间才会衰减，这是核辐射残留的主要来源。

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For human beings, natural radiation is the main source of radiation. It is much more than the radiation exposure from all nuclear facilities for peaceful purposes.

天然辐射是人类所受辐射照射的主要来源，远大于用于和平目的所有核设施带给全世界人口的辐射照射量。

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