I have been searching but I don't know if it is the gamma rays released during the nuclear fission or if it comes from another heat source during fission if not what?
Its not just gamma rays that produces the heat. In a nuclear fission reaction a neutron collides with an atom and the atom splits forming two atoms of different elements and also releases a few neutrons. The mass of the two new atoms and the released neutrons is less than the mass of the original atom.
The nuclear fission reaction, splitting of the nucleus, is what causes the heat to be generated, not gamma rays. The nucleus splits into 2 smaller atoms when struck by a neutron, releasing neutrons to carry on a chain reaction and much heat energy that is absorbed by water and a heat exchanger.
SEE THE FOLLOWING.adsabs.harvard.edu/abs/1978VDIZ.....
