 The Chalk River Laboratories opened in 1944, and the Montreal Laboratory was closed in July 1946. ZEEP went critical on 5 September 1945, becoming the first operating nuclear reactor outside the United States. Using 5 short tons (4.5 t) of heavy water and 3.5 short tons (3.2 t) of uranium metal, it could operate continuously at 3.5 W, or for brief periods at 30 to 50 W. The larger NRX followed on 21 July 1947. With five times the neutron flux of any other reactor, it was the most powerful research reactor in the world. Originally designed in July 1944 with an output of 8 MW, the power was raised to 10 MW through design changes such as replacing uranium rods clad in stainless steel and cooled by heavy water with aluminium-clad rods cooled by light water.
