My grandmother was born in the United States (Utah) in the 1920s to Japanese immigrants. When she was very young, her family moved back to Japan, where her father died soon after. During WWII, she was living and working in Hiroshima in a factory. The factory was converted to be part of the war effort, and she was transferred to the office. When the bomb fell, she was protected by the office and desk and miraculously avoided radiation poisoning. After the war, she tried unsuccessfully for six years to return to the US (her native country) with her mother, due in part to anti-Japanese sentiment/policies.
She was approached to tell her story about 30 years ago as part of a book, but she refused. When he was alive, my grandfather (himself a very vocal Heart Mountain internee) would press her to tell her story, which only caused her to retreat further. My family has always wanted to know and document her story, but we were always hesitant to dredge up painful memories. She has only ever discussed memories sparingly and indirectly, but with more frequency in the last few years.
I feel a sense of obligation to tell her story, and I'd love to know if anyone has any experience with this sort of thing. I'm not sure if she'd be willing to share with us, much less with a journalist/historian.
My great grandfather was in World War I and would never talk about it. The family was always afraid to ask him questions. One day when my parents went to visit him toward the end of his life, he suddenly started talking about the war. My dad said he talked for hours and it was totally unprovoked and unexpected. My parents are the only ones he ever shared anything with.
Your grandmother is a trauma victim who most likely suffers from post-traumatic stress disorder. You might want to consult a mental health professional. I am guessing they will tell you not to push too hard and let her share if/when she feels comfortable. It sounds like you are handling things in a very thoughtful and sensitive way. I hope she decides to share more of her experiences with you! Keep a recording device handy when you talk to her just in case!
It's great that she is starting to share more now. There are getting to be fewer and fewer of her generation left and it's so important to learn from them when we can!
I'd like to recommend that you read Death in Life: Survivors of Hiroshima by Robert Jay Lifton. It provides an extensive but easy to understand examination of the psychology of Hiroshima survivors, based on interviews that Lifton conducted with hibakusha from all walks of life.
This won't help you with your problem directly, but it's a very good book and I think it might help you understand your grandmother more (and anything she does share with you).