The first atomic weapon used (Little Boy) was a fission bomb. It worked by creating a driving two pieces of nuclear material together creating a super critical mass and releasing a large amount of energy in a short time span. Nuclear reactors (at least PLWR) are a controlled fission process.
I understand that and "fatman" was a fusion bomb, which is the compacting of fissionable material to the point of nuclear fusion. Which is much more powerful and is less likely to accidents and is the process with which the USA's bombs have been made every since.