Ordinary water is not used as a moderator in nuclear reactors because

Options

A. Cannot slow down neutron
B. Absorbs neutrons
C. is expensive
D. Accelerates neutron

Answer

### Step-by-Step Solution:

1. **Understanding the Role of a Moderator**:
- In a nuclear reactor, a moderator is used to slow down fast-moving neutrons produced during fission reactions. Slower neutrons are more likely to cause further fission when they collide with fissile material (like uranium-235).

2. **Identifying the Problem with Ordinary Water**:
- Ordinary water (H₂O) contains hydrogen atoms, which are primarily in the form of protium (¹H). When fast neutrons collide with these hydrogen atoms, they can be absorbed.

3. **Neutron Absorption by Ordinary Water**:
- When a fast neutron interacts with a hydrogen atom in ordinary water, it can be absorbed, resulting in the formation of deuterium (²H). This absorption reduces the number of free neutrons available to sustain the fission chain reaction.

4. **Conclusion**:
- Because ordinary water absorbs neutrons instead of allowing them to slow down and facilitate further fission, it is not effective as a moderator in nuclear reactors. Therefore, other materials that do not absorb neutrons, such as heavy water (D₂O) or graphite, are preferred.

### Final Answer:
Ordinary water is not used as a moderator in nuclear reactors because it absorbs neutrons, which reduces the number of neutrons available to sustain the fission chain reaction.

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