In a nuclear reactor, what is the function of:
The moderator,

Explanation

### Step-by-Step Solution:

1. **Understanding the Role of the Moderator**:
- In a nuclear reactor, the moderator is a crucial component that plays a significant role in the fission process.

2. **Function of the Moderator**:
- The primary function of the moderator is to slow down the fast-moving neutrons produced during the fission reaction. This is essential because slow (thermal) neutrons are more likely to induce further fission reactions in the fuel.

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