Plutonium, with a yield of 19 kilotons per kilogram, and the bomb on board having 5.2 kilograms of plutonium, the constructed bomb has a theoretical yield of 98.8 kilotons. The Fat Man bomb dropped on Nagasaki contained 6.2 kilograms of plutonium and yielded only 20 kilotons, but with better technology and design refinements, the terrorists probably got substantially more than 20 kilotons (but nowhere near the "ideal" 98.8 kilotons).