Graphite is used as a ......... in atomic reactors.

Explanation

**Step-by-Step Solution:**

1. **Identify the Role of Graphite in Atomic Reactors:**
- Graphite is used in atomic reactors primarily as a moderator. The moderator is a material that slows down fast-moving neutrons produced during the fission process.

2. **Understand the Function of a Moderator:**
- The purpose of slowing down neutrons is to increase the probability of further fission reactions. Slower neutrons are more likely to be captured by fissile materials like Uranium-235, thus sustaining the chain reaction.

...