This isn't entirely true. Although it may be true in the fact that it may last a long time, that still isn't considered renewable. In the event that we did run out millions of years from now, our only source would be outside of Earth. For it to be truly renewable, there would have to be a way to produce it. Which is why we have plutonium, a also highly radioactive element that is man made, and is uses more often in nuclear plants than uranium