Found on a sign near this:
The Urakami Branch Prison was located on a hill straddling the three neighborhoods of Matsuyama-machi, Oka-machi, and Hashiguchi-machi and had an area of 20,000 square meters (13,000 occupied by buildings). It was the closest public facility to the hypocenter.
The explosion of the atomic bomb at 11:02 a.m., August 9, 1945 killed all 134 employees and residents, convicts, and accused prisoners. The four meter-high, 0.25 meter-thick reinforced-concrete surrounding wall collapsed near the base, and the wooden buildings burned to ashes. Only the kitchen chimney remained standing.