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Which constraint on reaction rate arises when enriched uranium-235 causes control rod failure in a nuclear reactor?

A)Moderator neutron absorption cross-section decreases
B)Delayed neutron fraction becomes negligible
C)Xenon-135 concentration reaches equilibrium faster
D)Prompt neutron lifetime drastically shortens

💡 Explanation

A larger fraction of fission neutrons become 'prompt,' making control much more difficult because the reactor response is then mostly immediate, and dominated by prompt criticality rather than delayed. Therefore, reactivity ramps up very quickly because delayed neutrons are vital for stable control, rather than merely shifting xenon equilibrium.

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