 Bradbury pushed continued development of nuclear weapons to take them from laboratory devices to production models . There were numerous improvements that could make them more safe , reliable and easy to store and handle , and make more efficient use of scarce fissionable materiel . While Bradbury gave priority to improved fission weapons , research still continued on " Alarm Clock " , a boosted nuclear weapon , and the " Super " , a thermonuclear weapons design . The new fission designs were tested during Operation Sandstone in 1948 . The Mark 4 nuclear bomb became the first nuclear weapon to be mass @-@ produced on an assembly line . 
