Mark 18 nuclear bomb

The test was a complete success at full yield.

Its normal mixed uranium/plutonium fissile core ("pit") was replaced with over 60 kg of pure highly enriched uranium or HEU.

With a natural uranium tamper layer, the bomb had over four critical masses of fissile material in the core, and was unsafe: the accidental detonation of even one of the detonator triggers would likely cause a significant (many kilotons of energy yield) explosion.

An aluminum/boron chain designed to absorb neutrons was placed in the fissile pit to reduce the risk of accidental high yield detonation, and removed during the last steps of the arming sequence.

[1][2] Beginning in March 1953, the United States deployed a number of Mark 18 bombs.