The larger portion of the total explosive yield in this design comes from the final fission stage fueled by 238U , producing enormous amounts of radioactive fission products . For example , an estimated 77 % of the 10.4-megaton yield of the Ivy Mike thermonuclear test in 1952 came from fast fission of the depleted uranium tamper . Because depleted uranium has no critical mass , it can be added to thermonuclear bombs in almost unlimited quantity . The Soviet Union 's test of the " Tsar Bomba " in 1961 produced " only " 50 megatons of explosive power , over 90 % of which came from fusion , because the 238U final stage had been replaced with lead . Had 238U been used instead , the yield of the " Tsar Bomba " could have been well-above 100 megatons , and it would have produced nuclear fallout equivalent to one third of the global total that had been produced up to that time
Ivy Mike
Ivy Mike
Uranium-238