The Boogie-man of Rare Metals
We already know that uranium is the basis of most of nuclear reactions. It consists of two major isotopes: the 235 and 238. Uranium 238 can be found abundantly in nature, while 235 isotope is much more rare. Ironically, the rare Uranium 235 is the one needed for most nuclear reactions.  Fortunately, the separation of uranium isotope is very complicated, that is why only few Countries can afford it. Meanwhile, chemists and physicists prefer to use depleted form of uranium for laboratory use since it only contains about 0,2% of real thing. 
References: University of Nottingham