| 1 | It is found in trace amounts in groundwater throughout the world. | |
| 2 | The other radioactive material often in the cooling water of a nuclear reactor is tritium. | |
| 3 | Two types, or isotopes, of hydrogen are used - deuterium and tritium. | |
| 4 | The best fuels for fusion are two types, or isotopes, of hydrogen - deuterium and tritium | |
| 5 | The other radioactive material often in the cooling water of a nuclear reactor is tritium. |