I understand that there is difference between thermal and fast neutrons according to their energy. Now, if one wishes to add neutrons into atomic nuclei, it should be easy as neutrons are neutral, so they can pass by the electron clouds and they don't get repulsed by nuclei itself, so no high energies are needed. Moreover, energetic neutrons may split atomic nuclei of some elements easily (Uranium). But some elements have very high neutron absorption cross sections;
So for neutron addition one has to use fast or thermal neutrons?