Heavy water is used in nuclear reactors as

Options

A. Source of `alpha`- particles
B. Slowing down the speed of high energy neutrons
C. Transporting heat of the reactor
D. Heating purposes

Explanation

**Step-by-Step Solution:**

1. **Understanding Heavy Water**: Heavy water, chemically represented as D2O, is water in which the hydrogen atoms are replaced with deuterium, a heavier isotope of hydrogen.

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

3. **Importance of Neutron Moderation**: Slowing down the neutrons increases the probability of these neutrons interacting with fissile material (like Uranium-235 or Plutonium-239), which is crucial for sustaining the nuclear chain reaction.

4. **Conclusion**: Therefore, the correct answer to the question is that heavy water is used in nuclear reactors as a neutron moderator.

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

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