Q:

energy production in the nuclear fission

according to this image of nuclear fission, before fisson reaction, the total mass of uranium and the slow neutron is 236 amu. and after adding the masses of Barium, krypton and the 3 neutrons after the fission reaction, you will again get 236 amu.
Question: if we get back all the mass, where did energy come from? is it the binding energy that is released?

A:

Look up the masses to high precision (i.e. don't just count nucleons). You don't get back all the mass.
The mass of $ ^{235}\mathrm{U}$ is 235.0439299 u not 235 u and so on.

