 Sufficient enriched uranium had arrived by May to start it up, and it went critical on 9 May 1944.It was only the third reactor in the world to do so, the first two being the Chicago Pile-1 reactor at the Metallurgical Laboratory and the X-10 Graphite Reactor at the Clinton Engineer Works.Improved cross-section measurements allowed Christy to refine his criticality estimate to 575 grams.