The 90% level is not a strict number. You can have nuclear explosion with lower levels of enrichment, but the total mass of the uranium is very large, therefor

The 90% level is not a strict number. You can have nuclear explosion with lower levels of enrichment, but the total mass of the uranium is very large, therefor not a practical weapon. For example you need 100kg of 60% enriched Uranium-235 https://www.researchgate.net/figure/Critical-mass-of-a-bare-metal-uranium-sphere-depending-on-the-type-of-fissile-isotope-and_fig1_366266734 https://www.researchgate.net/figure/Critical-mass-of-a-bare-...