Better use it as small wonder for Crusades and later make nukes just available with tech.. it doesn't matter that making this SW obsolete, you still can build units flaged as nuclear weapons (so that nukes are actually already allowed in medieval with crusades.. only then you don't have nukes.. and then make crusaders get obsolete by upgrade.. and in modern times, because crusades SW is already built, it allows nuclear weapons, so you can just make actual nukes available with rocketry or fission or special nuclear weapons tech).. i really have never understood why there's this Manhattan Project wonder.. just takes more time until you can build nukes.
But then again.. has anyone tested, if you give SW the flag to allow building of nuclear weapons, does it give it for all the nations (even if they have not yet built it themselves) or only to the one it has already built it