Munkchester will use its nuclear arsenal in the following circumstances:
If an invasion force lands in Munkchester proper (as in the city-state and surrounding area). this won't happen though because you won't be getting anywhere near Munkchester because you do realise how hard that would be, right?
If nuclear weapons are used against Munkchester and/or the Munkcestrian military. This includes colonies. It would be a proportional response, of course. If you nuke Munkchester proper that would result in the deaths of 99% of the Munkcestrian population and Munkchester would respond in kind by nuking you until 99% of your population is dead.
idk right now