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

Explanation

**Step-by-Step Solution:**

1. **Understand the Question**: The question asks about the role of heavy water in nuclear reactors.

2. **Identify Heavy Water**: Heavy water is chemically known as deuterium oxide (D2O). It differs from regular water (H2O) because it contains deuterium, a heavier isotope of hydrogen.

3. **Role of Heavy Water**: In nuclear reactors, heavy water is primarily used as a **moderator**. A moderator is a substance that slows down the neutrons produced during nuclear fission, allowing for a sustained chain reaction.

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