Nuclear Fission

Nuclear fission, as covered in NCERT, is the splitting of a heavy nucleus such as uranium-235 into two medium-mass fragments after it absorbs a neutron, releasing a large amount of energy because the fragments lie closer to the peak of the binding-energy curve. Crucially, each fission event also releases two or three fresh neutrons, and these can be absorbed by other uranium nuclei to cause further fissions, producing a self-sustaining chain reaction. The option about fragments fusing back is wrong because fusion of heavy fragments does not occur and would not release neutrons. The option relying on gamma rays alone is wrong because photons do not induce fission; neutrons do. The option about capturing electrons is wrong because fission is neutron-induced, not electron-induced, and electrons play no role in splitting the nucleus. As stated in NCERT Class 12, Chapter 13 (Nuclei), control rods made of neutron-absorbing material such as cadmium soak up surplus neutrons to keep the chain reaction steady in a reactor, while moderators slow the neutrons to make capture more likely. A plausibility check: only because more neutrons emerge than are consumed can the reaction grow or be sustained, which is exactly the neutron multiplication that defines a chain reaction.