NAGASAKI, Japan — When Miyako Jodai was 6 years old, the United States dropped an atomic bomb on her hometown, the port city of Nagasaki.

She was knocked unconscious, and her home was destroyed. She spent the next several days huddling with dozens of others in a cave on the side of a mountain.

“I was so scared,” she said. “I was crying and I stepped on some of the bodies of the injured people, because there was no room to walk.” When she finally ventured out, the city was still ablaze with towering flames.

Ms. Jodai was one of the fortunate ones. The bomb dropped on Nagasaki the morning of Aug. 9, 1945, killed about 74,000 people, about half as many as those who died in the bombing of Hiroshima three days earlier.