The correct answer is Option C — Cadmium. Cadmium is NOT used as nuclear reactor fuel. Instead it is used to make control rods in nuclear reactors. Cadmium has an extremely high neutron absorption cross-section meaning it captures neutrons very efficiently. Control rods made of cadmium or boron are inserted into the reactor to absorb excess neutrons and control the rate of chain reaction. Option A is incorrect as a distractor: Deuterium (heavy hydrogen) is used in fusion reactors as fuel and heavy water (D2O) containing deuterium is used as moderator in fission reactors. Option B is incorrect as a distractor: Uranium (specifically U-235) is the primary fuel in most nuclear fission reactors worldwide. Natural uranium has 0.7% U-235. Option D is incorrect as a distractor: Plutonium-239 is produced in reactors from U-238 and is used as fuel in fast breeder reactors like India's PFBR at Kalpakkam. Boron is also used for control rods along with cadmium.
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