The term 'Atom Pile' is a reference to the first nuclear REACTOR, which was built in 1942 by Enrico Fermi and his team at the University of Chicago. The REACTOR was called the Chicago Pile-1, and it was the first successful attempt to create a sustained nuclear chain reaction. The crossword solution for this term is 'REACTOR', which is an appropriate description of the device. A nuclear REACTOR is a device that uses nuclear fission to generate energy, and it is the most common type of nuclear power plant.