Power plants could\can exist with out the possibility of critical mass detonations.

But mass detonations are far easier, so it makes senses they developed those first.

Radiation can be measured in Fukoshima w/o a critical mass detonation.

Yes but it still requires nuclear fission, which is the same processed used to detonate nuclear bombs.

It's like trying to say grenades exist, but TNT doesn't. It just doesn't make sense.