I'm not 100% sure of what you mean by "subtopics." My guess is that you could be referring to themes and symbols that present in and throughout the book.
One major theme that runs through each of the survivors is the idea of survival itself. The will to live is a powerful thing, and Hersey illustrates it throughout the short novel. What I appreciate, though, about Hersey's portrayal of the survival is how community-orientated it is. It's a tribute to the Japanese people. While each survival story in the book is focused on a single person, their account always points out how survivors really came together to help each other out. It wasn't because they were family members or even friends (most of those were killed). It was a community of survivors who existed to work toward the survival of the group. Going along with that is another subtopic -- the topic of Japanese stoicism. It's amazing to read that a frequent attitude of the Japanese people was to sit and deal with the pain and assume someone else was worse off.
If you are going for a symbolic subtopic, I would pick water. Normally, movies and books portray water as pure and cleansing. It's a gift. It's good. Not so in this book. People in Asano Park end up drowning in rising tides. People drink the water and become sick from it because it has been contaminated for a variety of reasons. Water really comes to symbolize just how pervasive and devastating the bomb was.