Heavy water is used in nuclear reactors as:

Options

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

Explanation

To solve the question about the use of heavy water in nuclear reactors, let's break it down step by step.

### Step-by-Step Solution:

1. **Understanding Heavy Water**:
- Heavy water is chemically represented as D₂O, where D stands for deuterium, an isotope of hydrogen. In contrast, regular water is H₂O, which contains the most common hydrogen isotope (protium).
- Heavy water contains deuterium instead of protium, making it heavier than regular water.

2. **Role of Neutrons in Nuclear Reactions**:
- In nuclear reactors, neutrons play a crucial role in sustaining nuclear fission reactions. These reactions involve the splitting of heavy atomic nuclei, which releases energy.

3. **High-Energy Neutrons**:
- When nuclear fission occurs, it produces high-energy neutrons. These neutrons need to be slowed down (or moderated) to increase the likelihood of further fission reactions.

4. **Moderation Process**:
- The process of slowing down these high-energy neutrons is called moderation. A moderator is a substance that reduces the speed of fast neutrons, making them more effective in sustaining the fission chain reaction.

5. **Why Heavy Water is Used**:
- Heavy water (D₂O) is an effective moderator because it can slow down neutrons without capturing them. This property allows it to maintain a sustained nuclear reaction efficiently.
- Other common moderators include regular water (H₂O), graphite, and certain metals, but heavy water is particularly useful in certain types of reactors, such as CANDU reactors.

6. **Conclusion**:
- Based on the above points, the correct answer to the question is that heavy water is used in nuclear reactors as a means of slowing down the speed of high-energy neutrons.

### Final Answer:
Heavy water is used in nuclear reactors as a source for slowing down the speed of high-energy neutrons.

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