Nine reactors beat ZEEP into service

David Kramer’s article on nuclear developments in my native Canada (Physics Today, January 2021, page 23 ) was an enjoyable read. However, his assertion that the Zero Energy Experimental Pile (ZEEP) was the world’s second operating nuclear reactor after Enrico Fermi’s Chicago Pile-1 (CP-1) is erroneous; at least nine other stateside piles achieved criticality before ZEEP did so in September 1945.

Those nine US piles were CP-2 and CP-3 at Argonne National Laboratory (March 1943 and May 1944; CP-3 was the first heavy-water pile); the X-10 pilot-scale pile at Oak Ridge National Laboratory (November 1943); the 305 fuel-testing pile and the B, D, and F plutonium production piles at the Hanford Site (1944 to early 1945); and two small aqueous enriched-uranium devices, LOPO and HYPO, at Los Alamos National Laboratory (1944). ZEEP was the first pile outside the US to achieve criticality.