Heavy water is used as moderator in a nuclear reactor. The function of the moderator is

Options

A. To control the energy released in the reactor
B. To slow down the neutrons to thermal energies
C. To cool the reactor faster
D. To absorb neutrons and stop chain reaction

Answer

**Step-by-Step Solution:**

1. **Understanding the Role of Neutrons in Nuclear Reactions:**
- In a nuclear reactor, neutrons are essential for initiating and sustaining the fission process. Fission occurs when a heavy nucleus, such as uranium-235, absorbs a neutron and becomes unstable, leading to its splitting into smaller nuclei and the release of energy.

2. **Identifying Fast Neutrons:**
- When fission occurs, the neutrons released are typically fast-moving (high-energy). These fast neutrons are less likely to be absorbed by the uranium nuclei for further fission reactions.

3. **Purpose of the Moderator:**
- A moderator is a substance used in nuclear reactors to slow down fast-moving neutrons. By reducing the energy of these neutrons, they can be converted into thermal (slow) neutrons.

4. **Effect of Slowing Down Neutrons:**
- Thermal neutrons have a higher probability of being absorbed by uranium-235 nuclei, which increases the likelihood of further fission reactions. This is crucial for maintaining a sustained chain reaction in the reactor.

5. **Conclusion:**
- Therefore, the function of the moderator, such as heavy water, is to slow down fast-moving neutrons to thermal energies, allowing uranium to effectively use them to carry out fission reactions.

**Final Answer:**
The function of the moderator is to slow down fast-moving neutrons to thermal energies so that uranium can use them to carry out fission reactions.

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