https://disarmament.unoda.org/en/our-work/weapons-mass-destr...
https://scipython.com/blog/uranium-enrichment-and-the-separa...
The critical mass required for a weapon shrinks as enrichment increases; implosion designs would require an infinite mass at or below 5.4% enrichment (see https://en.wikipedia.org/wiki/Enriched_uranium).
Weapons-grade uranium is more like 85%+ U-235. Enrichment above around 20% is what really raises red flags.
Which, as I understand it, is because at 20% enrichment you've already done about 70% of the work needed to get to 85%.
How would this help? Nuclear power plant and enrichment facility are separate entities.
All it takes is the enrichment to produce the fissile material for a weapon.
As far as I know countries have agreed to not build weapons, with the exception of those that already have them, there is an international body that monitors enrichment sites, but checks are voluntary a country can choose to not accept inspections and/or build additional secret enrichment sites.
The fissile material is not sufficient for a weapon though, as I understand there is quite a bit of science that goes into making a bomb.
Additionally, first generation weapons are large and unwieldy, i.e it takes a bomber to deploy a single weapon with a very small yield.
Miniaturization, building a weapon small and light enough to put on a missile is a significant problem that took the current powers years to get over.
But that's about it, if you can figure out how to make a small bomb of variable yield, you can make bombs small enough to fit a large backpack, and thermonuclear weapons that fit in a ballistic missile as well.
The solution to these issues is just to manage the enrichment supply chain. If a country wants nuclear power but can't be trusted, supply then with at cost uranium.