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The correct answer is Option B. The fundamental difference between a nuclear reactor and an atom bomb lies in whether the chain reaction is controlled or uncontrolled. In a nuclear reactor the chain reaction is carefully controlled using control rods (cadmium or boron) to maintain multiplication factor k equal to 1. Each fission triggers exactly one more fission producing steady continuous energy used for electricity generation. In an atom bomb there are no control rods or moderators. The multiplication factor k is much greater than 1. All the fissions happen within microseconds releasing all the energy in a devastating explosion. Option A is incorrect: Both atom bombs and nuclear reactors primarily use fission not fusion. The hydrogen bomb uses fusion triggered by a fission bomb. Option C is incorrect: Both reactors and bombs can use U-235. What differs is the enrichment level and whether the reaction is controlled. Option D is incorrect: Physical size is irrelevant to the fundamental difference. Critical mass determines bomb feasibility not reactor vs bomb distinction.
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