I was looking it up and of all the accounts, apparently Stalin didn't seem to be that interested as he never asked any questions or seemed intrigued. He just said it was good and he hoped it would be used on Japan. Why did he act this way and how did he react when he realized the actual power of it?
He already knew about the bomb by the time Truman told him about it. As for his reaction to us bombing Japan, he was upset and actually locked himself away out of being depressed that the Soviet Union didn't join the war against Japan. Not out of honor, but because FDR promised him lands lost in the Russo-Japanese War if they would help us defeat Japan after V-E Day. He ended up invading Japan anyway, which shocked the Japanese and forced them to end the war more than the impact of the atom bomb.
Source: Racing the Enemy by T. Hasegawa
Really good resource on this here: http://www.dannen.com/decision/potsdam.html
Truman didn't have a very high opinion of Stalin ("the little squirt") or a very good relationship with him like FDR had. He and his appointee Secretary of State James Byrnes seemed to think Stalin was a bit of a dope who didn't understand what he was being told. We know better now, he was just using a poker face, and messaged Georgii Zhukov that evening to accelerate the Soviets' own nuclear program.
Stalin had agreed at Yalta to enter the war against Japan three months after the defeat of Germany. U.S. desire to have him do so was in flux after successfully testing the bomb at Trinity. Suddenly, the postwar order looked better with de facto unilateral U.S. occupation of Japan and keeping the Soviet Union out of China. Stalin wanted access to those same Manchurian railways at stake in the Russo-Japanese war of 1905. He also wanted the disputed Kuriles and Sakhalin. The Potsdam Declaration was notably issued with references to the Soviet Union removed, which Vyacheslav Molotov tried to insert, but it had been released to the press already. The Soviet Union still used the pretext of enforcing the Potsdam Declaration with its declaration of war on August 8, 1945.
In short, Stalin was using a poker face. Unlike Truman, who was informed by Leslie Groves and Henry Stimson of the bomb in April, and was excited to tell Stalin, Stalin had known for a long time due to his spies in the Manhattan Project. He didn't want to tip his hand on this, or show fear as he wanted to keep his claims in East Asia despite the bomb changing the situation.
Roberts, Geoffrey. "Stalin at the Tehran, Yalta, and Potsdam Conferences." Journal of Cold War Studies, Fall 2007, Vol. 9, No. 4, Pages 6-40
In a word: Soviet spies in US during Roosevelt & Truman administrations.