New totally stupid theory (that doesn't work):
The bomb is radioactive guy going ballistic, not some intentional act.
Hiro goes back to save claire, to somehow get her to radioactive guy's wife before she dies, hmmm, but then they wouldn't need to save her, as she would have healed her before homecoming - damn.
I do still think the "bomb" is nuclear guy losing control somehow.
Oh well, I don't really need to guess, I just enjoy the trip on this one.
The bomb is radioactive guy going ballistic, not some intentional act.
Hiro goes back to save claire, to somehow get her to radioactive guy's wife before she dies, hmmm, but then they wouldn't need to save her, as she would have healed her before homecoming - damn.
I do still think the "bomb" is nuclear guy losing control somehow.
Oh well, I don't really need to guess, I just enjoy the trip on this one.