The Gadget

A man is pictured next to a large metal orb with wires surrounding the orb.
Norris Bradbury, head of bomb design at Los Alamos, stands next to the Gadget, 1945.


The purpose of the Manhattan Project was, according to a primer given to new arrivals at Los Alamos, “to produce a practical military weapon in the form of a bomb in which the energy is released by a fast neutron chain reaction in one or more of the materials known to show nuclear fission.” Plutonium is one such material. It exists only in trace amounts in nature but can be made in a nuclear reactor.

At Oak Ridge, Tennessee, the Manhattan Project built a pilot-scale reactor for making plutonium called X-10. In November of 1943, workers removed 5 tons of irradiated uranium from X-10 then chemically separated plutonium from the irradiated uranium. Gram quantities of plutonium were sent from Oak Ridge to Los Alamos in the summer of 1944. Thousands of experiments were performed at Los Alamos to study the properties of plutonium. At Pond Cabin in a canyon isolated from the main mesa-top laboratory at Los Alamos, Physicist Emilio Segré studied the spontaneous fission rate of the plutonium created at X-10. Away from the radiation interference from the main laboratory, Segré discovered that plutonium would not work in a gun-type atomic bomb because a plutonium projectile and target would melt before joining to make a nuclear explosion.

In September of 1944, a scaled-up version of the X-10 reactor, the plutonium producing B Reactor at Hanford, Washington reached criticality for the first time. By the end of 1944, three DuPont Corporation built reactors and chemical separation plants at Hanford were making plutonium nitrate paste to ship to Los Alamos for final processing.

The uranium enrichment plants at Oak Ridge were only expected to produce enough uranium for one gun-type bomb by 1945. Emilio Segré’s calculations showed that the implosion method was the only known way to make a working bomb with the plutonium coming from Hanford. Early implosion experiments at Los Alamos had failed to produce shock waves timed with the correct force and symmetry to compress plutonium to the point of nuclear explosion. Laboratory Director J. Robert Oppenheimer focused Los Alamos’s resources on perfecting plutonium implosion. Three divisions and hundreds of workers raced to make the implosion design work.

Oppenheimer told General Leslie Groves, who oversaw the entire Manhattan Project, that a test of the complex implosion design bomb would be necessary. Groves believed a test was a waste of precious plutonium but agreed to a test to avoid any last-minute disasters with the weapon. Oppenheimer named the proposed test of the implosion bomb “Trinity” while the test bomb itself was named “Gadget.”

The Gadget was a sphere of 32 explosive “lenses” hexagonal or pentagonal in shape made of two different types of high explosives. These lenses surrounded a plutonium sphere at the center of the Gadget. Special detonators were designed to set off the explosive lens with the perfect simultaneous timing needed to squeeze the plutonium at the Gadget’s center and start an atomic explosion. Converging shockwaves created pressures 500,000 times greater than earth’s surface air pressure at the center of the Gadget.

On July 16, 1945, the Manhattan Project successfully tested the Gadget 35 miles from Soccoro, New Mexico. The first atomic bomb test ushered in the nuclear age and changed the world. The plutonium implosion bomb design tested with the Gadget was used in the “Fat Man” atomic bomb detonated over the Japanese city of Nagasaki on August 9, 1945, killing over 40,000 people.

Manhattan Project National Historical Park

Last updated: July 27, 2023