I’ll look for it. I just read it last week.

This is a Bing fact-sheet source. But it is available on most sites that discuss this topic.

Weapons Development: Enriching uranium to 60 percent is a critical threshold that brings it closer to weapons-grade material. While uranium enriched to 20 percent is considered "highly enriched," moving from 60 percent to 90 percent is technically easier than the initial enrichment to 60 percent.