What is the fundamental difference between a nuclear reactor and an atom bomb?

The fundamental difference between a nuclear reactor and an atom bomb lies in whether the chain reaction is controlled or uncontrolled. Each fission triggers exactly one more fission producing steady continuous energy used for electricity generation. The multiplication factor k is much greater than 1. 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.