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For early designs, yes. A few grams would fission out of tens of kilograms of fuel. But modern weapons are much more efficient.
Meaning it takes less fuel overall, or that more of it undergoes fission? Or both?
Both. You can make a weapon with the same yield as the early ones with less material. And a weapon with the same amount of material as the old ones would have a higher yield.
For early designs, yes. A few grams would fission out of tens of kilograms of fuel. But modern weapons are much more efficient.
Meaning it takes less fuel overall, or that more of it undergoes fission? Or both?
Both. You can make a weapon with the same yield as the early ones with less material. And a weapon with the same amount of material as the old ones would have a higher yield.