There isn't just *one* radioactive element used in nuclear reactions. Many different radioactive elements can be involved, depending on the specific reaction.

Here's a breakdown:

* Fission: In nuclear fission, heavy elements like uranium (U) and plutonium (Pu) are split into lighter elements, releasing energy. These elements are radioactive.

* Fusion: Nuclear fusion involves fusing lighter elements like deuterium (D) and tritium (T) into heavier elements, releasing even more energy. Both deuterium and tritium are isotopes of hydrogen and are radioactive.

* Other reactions: Many other radioactive elements can be used in nuclear reactions, including thorium (Th), neptunium (Np), and americium (Am). These elements are involved in various processes like reactor fuel production or nuclear waste management.

So, the key point is that numerous radioactive elements are used in nuclear reactions, and the specific element depends on the type of reaction and its purpose.