Return to the fact that each U235 atom that fissions results in the two “halves” being nucleuses of different elements depending on the number of protons they end up with. Like dinner plates rarely breaking in the same way, these fission products include a wide variety of elements, mostly unstable hence radioactive isotopes, of those elements.
When a majority of the U235 in a reactor has fissioned (a matter of several years) it looses power and the reactor must be refueled, i.e., old fuel rods replaced with new ones. The old rods, now with their plethora of radioactive materials (some with half-lives into the thousands of years) become nuclear wastes. As such they pose significant hazard to human health. How do we dispose of them? How do isolate them from human contact for 1000s of years.
As described in the following video, with few exceptions, this in a continuing and worsening problem in the US and in other countries around the world. Currently nuclear wastes are being stored at the facilities where they were produced, waiting for a more permanent solution. (The video is long. Watch as much as you wish to ascertain that the problem is more political than practical.) Note that there are countries that have reached what they consider acceptable solutions.