The Mark 18 nuclear bomb was a follow-on to the Mark 6 and Mark 13 , utilizing a fissile pit assembly with around 60 kilograms of HEU and delivering a yield of 500 kilotons , the largest pure-fission ( non-thermonuclear ) bomb design ever developed by the US . Mark 18 bombs were eventually recycled into '' ' Mark 6 Mod 6 ' '' bombs after thermonuclear weapons were deployed in quantity . The Mark 18 was tested once in Operation Ivy King
Ivy King
Ivy King
Mark 6 nuclear bomb