Scarlet of the After War

One year after the end of the Pacific War, Kadode, a surviving member of a kamikaze squad, works at a building site in Tokyo, which he calls the grave of his former comrades. Then, Kadode hears that if he can find "Scarlet," he can earn a large reward for it. It turns out Scarlet is a third atomic bomb that the U.S. military was about to drop on Tokyo at the end of the war! The battle over the "third bomb" is about to begin, in a story of postwar nuclear survival!

(Source: Kodansha, translated)