The story begins, in a sense, in classrooms around the world, where students are taught that the rate of decay of a specific radioactive material is a constant.
这个事情要从世界各地的教室里说起，在那里学生学习到特定核素的衰变速率是恒定的。
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The concept of heat threshold is brought forward for the melting process of radioactive source with constant heat rate.