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.