I need to write a short research paper on Hiroshima. The subtopic I choose is wide open. Do you have any suggestions? I considered writing about how the Catholic Church became interested in Japan after WWII, but I'm not sure there is enough information out there. Can you suggest any topics?

There are endless possibilities for a research paper on Hiroshima, depending on your area of interest. There were several Jesuits in Hiroshima at the time the bomb exploded in 1945. In addition, Pope John Paul II visited Hiroshima in 1981 and was clearly affected by his visit (there's a New Yorker article about his visit in the links below). Obama was the first U.S. President to visit the Hiroshima War Memorial in 2016, and your paper might explore why US Presidents did not visit the site before that time.
Other possible research paper topics include a study of the memoirs of the survivors (see the link to the research library at the University of Washington below), including the commonalities and differences in their accounts. Another topic to explore is the research conducted on the medical effects of the bomb, both in the immediate aftermath of the explosion and in the months afterward. John Hersey's book Hiroshima, published in 1946, is an account of several survivors of the atomic bomb detonation from the moment the bomb exploded to one year after the event.  
https://guides.lib.uw.edu/research/qg4

https://www.newyorker.com/news/news-desk/what-the-pope-saw-at-hiroshima