Does anyone know exactly what constitutes weapons grade uranium as opposed to non weapons grade?
Weapons-grade is enriched in U-235, a very expensive and technically demanding process. This stuff came from one of the existing nuclear powers, probably the former Soviet Union.
At the same time, there's no way they could know it's weapons grade without a mass spectrum.
It is the percentage of the fisionable isotope U235. Fuel rods for Nuclear power use Uranium enriched to about 5-6% max. Weapons grade is 98% pure U235 or better. The balance is U238 a non fisionable isotope.