  Between the time of the discovery of nuclear fission in early 1939 and the
end of 1946, approximately 90 nuclear piles were constructed in six countries.
These devices ranged from simple graphite columns containing neutron sources
but no uranium to others as complex as the water-cooled 250-megawatt plutonium
production reactors built at Hanford, Washington. This paper summarizes and
compares the properties of these piles.
