The heavy atom often used in nuclear fission reactions to produce thermal energy is uranium.

Here's why:

* Fissionable: Uranium-235 is a fissile isotope, meaning it can be easily split by neutrons, releasing a large amount of energy in the process.

* Abundant: Uranium is relatively abundant in the Earth's crust.

* Practicality: Uranium is readily processed and can be used in nuclear reactors.

While other heavy atoms can undergo fission, uranium is the most commonly used due to these factors.