I've been having the exact same issue, but it's only on some cities, some that aren't even the capital. Some cities get nuked and they're destroyed, the animation plays, the turn ends, all is well. But when hitting some other cities (typically those with a high population, I've noticed and/or the capital city) the start of the nuke animation begins (the map with the alignment lines, the missile falling onto the city), but the mushroom cloud doesn't appear and the city seems fine and is still clickable (but when I hover over buildings - that all look perfectly fine - the tile info tells me they're 'ruins'). When I click to end the turn, it just loads forever. Reloading the game doesn't fix it at all. It makes nukes pretty much impossible to work with.