That's like comparing the rate at which wood rots to the rate at which it burns. Radioactive decay is the spontaneous decomposition of unstable atoms. Nuclear fission inside a reactor is a chain-reaction which causes the atoms to split, harnessing the exothermic products of the reaction to heat water and drive aturbine.

The U235 decay chain goes like this:

>Uranium-235 →Thorium-231 → Protactinium-231 →Actinium-227 →Thorium-227 →Radium-223 →Radon-219 →Polonium-215 →Lead-211 →Bismuth-211 →Thallium-207→ Lead-207 (stable)

The fission products of a nuclear reactor are far less predictable, but include isotopes of Iodine, Caesium, Strontium, Xenon, and Barium. That's because the neutrons which collide with the U235 nuclei crack them apart.