In response to the public alarm over fallout , an effort was made to design a clean multi-megaton weapon , relying almost entirely on fusion . The energy produced by the fissioning of unenriched natural uranium , when used as the tamper material in the secondary and subsequent stages in the Teller-Ulam design , can far exceed the energy released by fusion , as was the case in the Castle Bravo test ; realizing that a non-fissionable tamper material is an essential requirement in a ' clean ' bomb , it is clear that in such a bomb there will be a relatively massive amount of material that does not contribute energy by either fission or fusion . So for a given weight , ' dirty ' weapons with fissionable tampers are much more powerful than a ' clean ' weapon ( or , for an equal yield , they are much lighter ) . The earliest known incidence of a three-stage device being tested , with the third stage , called the tertiary , being ignited by the secondary , was May 27 , 1956 in the Bassoon device . This device was tested in the Zuni shot of Operation Redwing . This shot used non-fissionable tampers ; an inert substitute material such as tungsten or lead was used . Its yield was 3.5 megatons , 85 % fusion and only 15 % fission
Castle Bravo
Castle Bravo
Nuclear weapon design