Uranium-238 isotopes are capable of undergoing fission when exposed to_________________?
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Answer: A. high energy (fast) neutrons alone
Explanation
Uranium-238 has a very low fission cross-section for thermal neutrons and primarily undergoes fission only when struck by high-energy fast neutrons. Thermal neutrons lack sufficient kinetic energy to overcome the fission activation energy barrier of the U-238 nucleus. Consequently, U-238 acts primarily as a fertile material rather than a primary fissile fuel in thermal nuclear reactors. Uranium-238 constitutes the vast majority of naturally occurring uranium reserves found within the Earth crust. Neutron cross-section measures the probability that a specific atomic nucleus will interact with an incident neutron. When U-238 absorbs a neutron without fissioning, it undergoes radioactive decay to eventually become fissile plutonium-239.
Option B is incorrect because low-energy thermal neutrons are incapable of causing significant fission in U-238 nuclei. Option C is wrong because thermal and fast neutrons produce vastly different nuclear reaction probabilities in U-238. Options D and E are incorrect because U-238 specifically requires high-energy fast neutrons to undergo direct nuclear fission.
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- Engineering
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- Mechanical Engineering
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