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.

> 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%.