Cost / density. You quite literally want the cheapest dead weight. Cost of lead/tonne : USD 1900 (approx) Cost of Uranium/tonne: At least USD 60000 (approx reg

Cost / density. You quite literally want the cheapest dead weight. Cost of lead/tonne : USD 1900 (approx) Cost of Uranium/tonne: At least USD 60000 (approx regular uranium) even if you are able to source it.. and then you still have to build containment for this, get permits etc.. For that price, you could get much higher efficiencies if you went for a cheaper metal - even something like Tungsten which is extremely dense will be much cheaper.