In a nuclear reactor, moderators slow down the neutrons which come out in a fission process. The moderator used have light nuclei. Heavy nuclei will not serve the purpose because

Options

A. they will break up
B. elastic collision of neutrons with heavy nuclei will not slow them down
C. the net weight of the reactor would be unbearably high
D. substances with heavy nuclei do not occur in liquid or gaseous state at room teperature

Explanation

To understand why heavy nuclei cannot serve as moderators in a nuclear reactor, we can break down the explanation into a series of steps:

### Step-by-Step Solution:

1. **Understanding the Role of Moderators:**
- Moderators are materials used in nuclear reactors to slow down fast neutrons produced during fission. Slower neutrons are more likely to induce further fission in the fuel.

2. **Characteristics of Effective Moderators:**
- Effective moderators typically consist of light nuclei, such as hydrogen (protons), deuterium, or graphite (carbon). These materials are chosen because they can effectively slow down neutrons through elastic collisions.

3. **Elastic Collisions:**
- When a neutron collides with a nucleus, the collision can be elastic, meaning that both kinetic energy and momentum are conserved. In elastic collisions, the neutron can transfer some of its kinetic energy to the lighter nucleus, slowing itself down.

4. **Why Light Nuclei are Preferred:**
- Light nuclei, having fewer protons and neutrons, are more effective in transferring energy during collisions. For example, when a fast neutron collides with a hydrogen nucleus (a proton), the neutron can lose a significant amount of its kinetic energy.

5. **Limitations of Heavy Nuclei:**
- Heavy nuclei, such as lead or uranium, contain a larger number of protons and neutrons. When a neutron collides with a heavy nucleus, the mass difference is significant, and the energy transfer is less efficient.
- In fact, heavy nuclei tend to absorb neutrons rather than slow them down effectively. This absorption can lead to fewer neutrons available for sustaining the fission chain reaction.

6. **Conclusion:**
- Therefore, heavy nuclei are not suitable as moderators because they do not facilitate the effective slowing down of neutrons through elastic collisions, which is essential for maintaining a controlled nuclear reaction.