sentences:  Orange Herald remains the largest fission device ever tested.It is thought that the fusion boosting failed to increase the yield.A higher compression but smaller fission pit American weapon, the Mark 18 Super Oralloy Bomb, had a yield of 500 kilotons from a pit with slightly over 60 kilograms of highly enriched uranium, around 8 kilotons per kilogram of uranium, about the practical maximum 50% fission yield efficiency for very large or very highly boosted fission weapons.