Here's why:

* Actinides are a series of elements on the periodic table with atomic numbers 89 to 103.

* Naturally occurring means that these elements are found in nature, not created in labs.

* While elements heavier than Uranium exist (like Neptunium, Plutonium, etc.), they are all synthetic meaning they are created in nuclear reactions and not found naturally on Earth.

So, while Uranium is radioactive and has a long half-life, it is the heaviest actinide found in nature.