The code name "Trinity" was assigned by J. Robert Oppenheimer, the director of the Los Alamos Laboratory, after a poem by John Donne. The test used an implosion-design plutonium device, informally nicknamed "The Gadget", of the same design as the Fat Man bomb later detonated over Nagasaki, Japan, on August 9, 1945. The complexity of the design required a major effort from the Los Alamos Laboratory, and concerns about whether it would work led to a decision to conduct the first nuclear test.
Fears of a fizzle led to the construction of a steel containment vessel called Jumbo that could contain the plutonium, allowing it to be recovered, but Jumbo was not used. A rehearsal was held on May 7, 1945, in which 108 long tons (110 t) of high explosive spiked with radioactive isotopes were detonated. The Gadget's detonation released the explosive energy of about 20