I agree, my estimate was way low, so ten tones would reduce green house gas sufficiently to be worth the project, if the problems are only moderately difficult. Potassium is more abundent and lower density than the others you mentioned. K40
(12 parts per million in natural potasium) has a very long half life and produces both beta- and beta+ Do we get gamma when anialation of the antimater occurs? If so, more energy, but gamma penetrates the confinment. Neil