I see your point; A HEU core

I see your point;A HEU core may require as little as 25 kg of weapon grade (~93.5% U235) HEU.25 * 144 = 3.6 tonnes of uranium required to get the necessary HEU (lest call it 4.0 tonnes).If the ore grade is o.2% the uranium is still only 1/500th the material mined;So to get the 4.0 tonnes of uranium for one bomb, you would need,3.6 * 500 = 2,000 tonnes of oreSo 10 weapons would require the mining of 20,000 tonnes at 0.2% grade.So several train loads of ore must be refined from a significant deposit, to obtain an initial small arsenal.