Correct Option
A self-sustained chain reaction in a nuclear reactor relies on specific conditions:
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1. More neutrons are released in each of the fission reactions: Nuclear fission of heavy nuclei, such as Uranium-235, typically releases an average of 2 to 3 neutrons per fission event. This surplus of neutrons is essential for the propagation and self-sustainment of a chain reaction.
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2. The neutrons immediately take part in the fission process: Neutrons released during fission are capable of initiating further fission events. While fast neutrons often require moderation to increase their probability of causing fission in thermal reactors, their availability and ability to participate in the fission process are fundamental for a chain reaction to occur and be sustained. The term "immediately" refers to their prompt availability for subsequent reactions.
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3. The fast neutrons are slowed down by graphite: Graphite serves as a moderator in many nuclear reactors. Its function is to reduce the kinetic energy of fast neutrons, converting them into thermal neutrons. Thermal neutrons are significantly more effective at inducing fission in fissile materials like Uranium-235, thereby increasing the likelihood of sustaining the chain reaction.
Incorrect Options
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4. Every neutron released in the fission reaction initiates further fission: This statement is incorrect. Not every neutron released in a fission reaction causes further fission. A significant fraction of neutrons may escape the reactor core, be absorbed by non-fissile materials (e.g., control rods, structural components), or undergo non-fission capture. For a chain reaction to be self-sustained, it is only necessary that, on average, at least one neutron from each fission event causes another fission.
Options (B), (C), and (D) are incorrect because they either omit a correct statement (Statement 2 in option B) or include an incorrect statement (Statement 4 in options C and D).