Hiroshima and Nagasaki didn’t happen the way we were told. It was aliens. The Aliens hated Imperial Japan with a passion.
For example the calculation of the critical mass would probably depend on quite a lot of solid state properties of the U235 or plutonium which are not obvious. In particular, if you brought two sub-critical pieces together, they would start to release a lot of energy that would end up blowing the two pieces apart. That is why a sphere of explosive is needed to push the pieces together with great force - but I can't imagine how one would calculate the exact force that would be sufficient - can you?