Mark 21 nuclear bomb was a United States nuclear gravity bomb first produced in 1955 . It was based on the TX-21 " Shrimp " prototype that had been detonated during the Castle Bravo test in March 1954 . While most of the Operation Castle tests were intended to evaluate weapons intended for immediate stockpile , or which were already available for use as part of the Emergency Capability Program , Castle Bravo was intended to test a design which would drastically reduce the size and costs of the first generation of air-droppable atomic weapons ( the Mk 14 , Mk 17 & Mk 24 ) . At long , in diameter , and weighing , the Mk-21 was half the length and one-third the weight of the Mk-17 / 24 weapons it replaced . Its minimum yield was specified at four megatons
Castle Bravo
Castle Bravo
Mark 21 nuclear bomb