The only way to ensure that a civil uranium enrichment program remains strictly civil is via transparency and monitoring. A country that has mastered uranium enrichment technology for fueling civil power reactors could use the same technology to produce bomb-grade uranium. It actually takes more work to enrich natural uranium into fuel for power reactors than it takes to further enrich power reactor fuel into bomb material:

https://scipython.com/blog/uranium-enrichment-and-the-separa...

You should read the Treaty on the Non-Proliferation of Nuclear Weapons (NPT) as it addresses several of those issues. Possession of highly enriched uranium isn't necessarily an act of war by itself.

https://disarmament.unoda.org/en/our-work/weapons-mass-destr...

Natural uranium on earth is currently about 0.7% U-235; civilian power reactors typically need low-enriched uranium which is 3% to 5% U-235.

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.

Modern weapons use plutonium not uranium, uranium weapons can be constructed.

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.

IAEA inspections verify your claimed inventory and enrichment facilities. They are trying to detect if any nuclear materials are being skimmed/diverted. As for weapons, nuclear fuel is very low enrichment (usually under 5%). Iran surpassed 60%, which has no peaceful use, so that is why it was said they were perusing weapons.

Imo that's a pretty complicated topic. On one side if you just build LWRs you just don't need very highly enriched uranium or plutonium so posession of those is a red flag. On the other side fast breeder reactors are the ones which are able to produce the least harmful waste. But fast breeders and closed fuel cycles produce and handle plutonium which in turn can be used for bad things.

Energy needs like 5% enrichment while weaponizing needs much higher and much more difficult to obtain 85% enrichment