They were actually designed to be very safe. It was thought that they literally couldn't fail dangerously. Chernobyl was a huge fluke (that had preventions put in place to ensure it never happened again) that was just a lot of weird things combining at once. The other reactors at Chernobyl continued operating for decades safely, similarly to three mile island which only stopped on 2019 because it wasn't profitable, but now it appears it is again. Both of these nations (and child nations/successors) continued to operate many more nuclear plants without issues. Nuclear is, by far, the safest energy source, including green energy like solar.