About nuclear fission

Because there are 2-3 neutrons in the product after neutron bombardment of uranium -235, it will produce a chain reaction, which is convenient for continued fission, but it must be controlled to avoid nuclear explosion.

Moreover, neutrons are light, easy to be accelerated and easy to obtain.

Iron is at the critical point of surface binding energy of elements. Fission of elements heavier than iron will release energy, and fusion of elements lighter than iron will release energy, so iron cannot be used.

All elements with atomic number above 84 are radioactive, and some elements with atomic number below 83, such as K and Rb, are also radioactive, so uranium 235 is radioactive.