Heavy water is used as a ………… in nuclear reactors.

Answer

**Step-by-Step Solution:**

1. **Understanding Heavy Water**: Heavy water is chemically represented as D2O, which means it contains two deuterium atoms (isotopes of hydrogen) and one oxygen atom.

2. **Role of Heavy Water in Nuclear Reactors**: In nuclear reactors, heavy water is primarily used as a moderator. A moderator is a material that slows down fast neutrons produced during nuclear fission.

3. **Function of a Moderator**: The main function of a moderator is to reduce the speed of neutrons so that they can effectively sustain a chain reaction. Slower neutrons are more likely to be captured by fissile material (like Uranium-235 or Plutonium-239), which is crucial for maintaining the reaction.

4. **Conclusion**: Therefore, heavy water (D2O) is used as a moderator in nuclear reactors.

**Final Answer**: Heavy water is used as a moderator in nuclear reactors.

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