They're not the same reactions

Normal decay is random. A U-235 decays and then nothing else happens, there's no chain reaction

In nuclear reactors the fuel is refined and set up in the reactor so it'll be critical so each atom that splits causes one other atom to split. Now you're not waiting for each one to randomly breakdown by instead trigging a chain reaction that works its way through the fuel fairly rapidly splitting the available atoms into smaller ones. If they need more power from the reactor they briefly adjust the control rods so each split triggers more than one other split to get up to a higher power level then reduce it back to 1-1 to hold at that level. The more power you need to pull from the reactor the faster you need to burn through the fuel

They also don't burn through all the fuel of the fuel rod, but it starts building up byproducts that muck with the power you can get out of it so they have to swap it out. Unfortunately the byproducts are wayyy less stable than the starting uranium/plutonium so now the random decay is occurring at a much faster rate creating a lot more radiation but less useful kinetic energy(heat) from the process. The really angry byproducts are mostly cleared out after a year because they decay that rapidly