Though sandwiched between two very practical elements on the periodic table -- thorium and uranium -- protactinium is pretty much useless. The dearth of applications can primarily be attributed to the element's rarity -- it's present at only a few parts per trillion in Earth's crust. But its high toxicity and incredible radioactivity don't help much either. Back in the 1960's, the Great Britain Atomic Energy Authority isolated 125 grams of the stuff from 60 tons of nuclear waste. That "bounty" served as the world's only stock material for many years.
(Image: Institute for Transuranium Elements)