Personally, I really want to keep the Manhattan Project name, but I think I've suggested this before:
- Manhattan Project is a World Project.
- First Nuclear Test is a National Wonder
- Nuclear Silo is a regular building.
One civilization has to build the Manhattan Project before anyone can build nuclear weapons. Once the Manhattan Project is built, any civilization can build a First Nuclear Test. The civilization that actually builds the Project gets their First Nuclear Test for free. Once you build a First Nuclear Test, then you can build Nuclear Silo, and a Nuclear Silo is required to build any nuclear unit. This would all be doable in XML, with one Python routine to place the free First Nuclear Test in the city that completes the Manhattan project.