Were there scientists or witnesses brought to him to explain what had happened? How did they describe it? Was he ever brought to the site to witness the destruction? Are there documents or records of how and what were his the initial reactions were after the bombs were dropped?
It seems that the senior members of the Japanese government and military already understood the potential of atomic bombs. In fact, Japan did have its own nuclear program, and according to historian Sadao Asada, its scientists had informed the government that "no country would be able to develop an atomic bomb during the course of the war." Even after the destruction of Hiroshima, the Army insisted that "although the United States claims it to be an atomic bomb, it actually appears to be a conventional bomb with extraordinary destructive power."
As for the Emperor's reaction, Asada cites the diary of Kido KÅichi (Lord Keeper of the Privy Seal) as well as the memoirs of Nagaakira Okabe (Imperial Chamberlain) and writes:
In an audience with Emperor Hirohito at 1:30 [on August 7], Kido noted how worried the Emperor was. Hirohito, a scientist specializing in marine biology, was quick to grasp the destructive power of the atomic bomb. The emperor peppered Kido with questions about the bomb. He had been apprised by court attendants of the Hiroshima bomb on the afternoon of August 6 and was informed the following morning that it was an atomic bomb. He demanded more details from the government and the army about the devastation of Hiroshima and was "strongly displeased" that he was not getting enough information.
Sorry I can't be more descriptive. Hopefully someone else will be able to add information!
Source: Sadao Asada, "The Shock of the Atomic Bomb and Japan's Decision to Surrender: A Reconsideration," Pacific Historical Review 67, no. 4 (1998): 477-512.