Obviously, once the atomic bombs dropped in 1945, the war was almost over. There wasn't much time to rebuild, or even offer aid, to the cities of Hiroshima and Nagasaki before the Japanese surrendered and the Americans started arriving, which no doubt changed how things like war damage and reconstruction were handled.
But within that period before the end of the war, what did the Japanese government do in response to the bombings? The Wikipedia entry contains a lot of information about the bombings, but barely any about the subsequent relief efforts or response. Was there much of a relief effort for the survivors, or were there no resources to spare? Did the try to rebuild anything, or did they basically just abandon the sites and focus on other areas? Did they respond quickly, or did it take them some time to decide what to do?
And once the Americans did start arriving in Japan, did these efforts change, or did they continue largely unaffected by the surrender?