By contrast, when the abundant isotope uranium-238 captures a neutron it still has a binding energy deficit of 1 MeV after internal rearrangement. If it captures a neutron with a kinetic energy exceeding 1 MeV, then this energy plus the energy released by rearrangement can over come the binding energy and cause fission. Heavier elements require relatively more neutrons to stabilize the nucleus. These neutrons are shed very rapidly by the excited fragments. More neutrons are produced on average than are consumed in fission.
Fission is a statistical process.
The nucleus rarely splits into pieces with nearly the same mass and atomic number. All of the kinetic energy is released to the environment instantly, as are most of the instantaneous gamma rays. Criticality A neutron entering a pure chunk of one of the slow-fissionable isotopes would have a high probability of causing fission compared with the chance of unproductive absorption. Non-fissionable materials mixed with these isotopes tend to absorb some of the neutrons uselessly, and increase the required size of the critical mass or may even make it impossible to achieve altogether.