The X-10 Graphite Reactor was the world's second artificial nuclear reactor after Chicago Pile-1, and was the first reactor designed and built for continuous operation. It consisted of a huge block, long on each side, of nuclear graphite cubes, weighing around , that acted as a moderator. They were surrounded by of high-density concrete as a radiation shield. In all, the reactor was wide, deep and high. There were 36 horizontal rows of 35 holes. Behind each was a metal channel into which uranium fuel slugs could be inserted. An elevator provided access to those higher up. Only 800 (~64%) of the channels were ever used.