The current magnitude limit of the Hubble telescope is 31.5. The James Webb telescope magnitude limit is going to be 34. Any magnitude number greater than this will remain optically unobservable at current technology levels. Uhoh mentions neutrinos, but that is a different issue.
This site states that a 35% of the energy from a nuclear explosion is released as thermal radiation, including infra-red, optical, ultraviolet and x-ray. A megaton of TNT releases 4.2 peta-joules of energy. That is 420 peta-joules for the given example, 35% of which would be roughly 147 peta-joules of thermal radiation. it's visibility would decline according to the inverse-square rule.
My back hurts too much to do that calculation right now. But, when, due to dissipation over distance, the magnitude from that equation exceeds 34 the explosion would be too dim for us to perceive it based on current technology. So, Yes, there is a limit to how far away we can observe a nuclear explosion of 100 megatons in space.