| 1 | Radium is a radioactive element. | |
| 2 | Radium and uranium are radioactive elements. | |
| 3 | Because it is heavy you expect it is radioactive. | |
| 4 | These radioactive elements are by-products of the fission reaction that generates power in the nuclear plants. | |
| 5 | Rare earth mines often also contain radioactive elements, such as uranium and thorium. |
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