A nuke will make it to where anybody lacking "The Hero" achievement cannot attack motherbase. After completing Episode 31 you can build a Nuclear Weapon for 750k GMP, 75k Fuel, 50k Metal and also 50k heroism. The process will, depending on your motherbase level, take roughly 24-28 hours to complete. The time this takes is in
REAL time, not game time. It should also be noted, that using your offline resources to build your online nuke has been patched. (
https://www.reddit.com/r/metalgearsolid/comments/3nrfcm/mgsv... ) In order to build your nuke using offline resources you must go into offline mode. Materials may be obtained (online mode) via dispatch missions and/or FOB missions. In FOB missions you may gain some for the win, but for the most part you must obtain them via "stealing" the freight cars. Here are a few videos to help you if needed:
Video 1: (credit to AFGuidesHD)
Video 2: (credit to wiiare.in)
Hope this helps! Let me know how I can improve my guide if needed.