Where did you hear that? It was an industrial area
Probably here at FR; however, Amazon books gets a great many references from Google, and here are two others:
"...Survivors from the Catholic community--the bomb had fallen on Nagasaki's Catholic district and cathedral--..."
In fact, Christianity was sealed in Nagasaki in the blood of martyrs. On a hill not far from the epicenter of the blast of Nagasaki, is a hill where about 20 Japanese were crucified on orders of Tokugawa (the Shogun at the time) for not renouncing Christianity. In that group, were little Japanese children, too, each nailed to their own smaller crosses overlooking the city.