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The world’s first nuclear explosion was an implosion device that used plutonium for fissile material. The Manhattan Project scientists who designed the device dubbed it the "gadget". It was detonated successfully on 16 July 1945 near Alamagordo, New Mexico, in what was called the Trinity test. A weapon of the same design was used a few weeks later in the attack on Nagasaki, Japan, on 9 August 1945.

Unlike uranium, plutonium does not occur naturally on Earth; it must be created in a nuclear reactor. It is a byproduct of all power reactors, but in order to be used as fissile material, it must be chemically separated from the rest of the highly radioactive waste. This is a costly and hazardous process requiring specialized knowledge, facilities, and equipment. It is thus highly improbable that terrorist organizations would learn to master this technique and more likely that they would steal the plutonium.

Therefore, it is crucial that countries around the world secure all existing stockpiles to reduce the risk of theft.It takes a smaller amount of plutonium than HEU to achieve a self-sustaining chain reaction of nuclear fission. However, plutonium’s physical properties are such that a gun-type device cannot combine two separate masses fast enough to achieve this critical mass.

For this reason, a nuclear explosion using plutonium actually begins as an implosion that relies on a sophisticated arrangement of high explosive lenses that must fire inwards simultaneously from all directions towards a plutonium pit.

In order to test an implosion device, the compression of the pit must be uniform and needs to be tested quickly enough to avoid a premature nuclear explosion, a so-called fizzle. Before or after this particular moment, the conditions are not right for sustaining the chain reaction until most of the fissile material has been consumed. This is a far more challenging engineering problem than building a gun-type device, but experts at the Nuclear Control Institute warn that it could still be accomplished by a small group of people with the right training and experience if they have access to plutonium.

For an instant, the plutonium is compressed to a high density, making the mass critical. However, simply compressing the plutonium to critical mass does not ensure that a nuclear chain reaction will begin. For this to happen, high energy neutrons need to be available at the moment of compression. Since relying solely on natural decay of plutonium is too risky, the certainty provided by a neutron initiator is needed.

The implosion design can also be used with HEU, allowing a smaller device to achieve the same yield as a gun-type device. China’s first nuclear test in 1964 was a 22 kiloton HEU implosion device named “596.” After the first Gulf War, UN inspectors discovered that Iraq was attempting to build an implosion bomb using HEU.