I'm working on something like this myself for my servers. For the most part the heavy lifting is this line to kill the entire other team

for(i = 0; i < level.players.size; i++)
{
    level.players[i] thread [[level.callbackPlayerDamage]](self, self, 2147483600, 8, "MOD_SUICIDE", self getCurrentWeapon(), (0,0,0), (0,0,0), "j_head", 0, 0 );
}

Then you could mess with visions sets, timescales, sounds, etc to make it more "nuke like". For example:

player playsound("uin_start_count_down");
setDvar("timescale", "0.33");
self SetVisionSetforPlayer("taser_mine_shock", 0);