Among most suggestions are projectiles spamming thousands of emitters into the air and cauing a massive lag attack on the server. (Mini-nuke, anyone?)
However, my Idea for a Nuclear based item is one that only works for Minigames. Instead of launching a projectile. It causes all players to die, while orang-ish dustlike emitters flow past the player. And casting a orange light that lights up quite a bit of area around the player, but is not solid. Is a faint orange-ish colour.
With a server preference (for RTB) to end the round or not.
The Modern Warfare 2 nuke explosion being a slight example. Just without the HQ textures and blinding lights.
The item would be a 2 button remote.