Here’s a 1983 history of the Atomic Energy Commission.

A sample:

The first test of new weapons was conducted at Enewetak Atoll in April and May 1948. Operation Sandstone explored weapon designs and tested a new fission weapon to replace the clumsy tailor-made models used during World War II. By 1948 the Commission had both gun- type and implosion-type non-nuclear and nuclear components in stockpile and was well on the way toward producing an arsenal of nuclear weapons.