the "non-proliferation treaty" ?

Yes, nuclear isn’t hard, they’re so strictly regulated because there aren’t other dual use technology that are more controllable, destructive and easily repurposed by sufficiently advanced nations.

My personal opinion is that if we discover a more devastating weapon of some sort, such as kinetic bombardment using bunches of O'Neill space colonies as projectiles, nuclear becomes comparatively benign and that kinds of event can lead to more de-regulation.