Why is heavy water used as moderator in a nuclear reactor ?

Explanation

### Step-by-Step Solution:

1. **Understanding the Role of a Moderator**:
- In a nuclear reactor, a moderator is a substance that slows down fast-moving neutrons produced during the fission reaction. Slowing down these neutrons increases the likelihood of them being captured by fissile material, such as Uranium-235.

2. **Properties of Heavy Water**:
- Heavy water (D₂O) is water in which the hydrogen atoms are replaced with deuterium, a heavier isotope of hydrogen. This gives heavy water unique properties that make it an effective moderator.

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