What is meant by critical mass in a nuclear chain reaction?

Explanation

**Step-by-Step Text Solution:**

1. **Understanding Nuclear Chain Reaction**:
A nuclear chain reaction is a series of reactions where a single nuclear reaction causes an average of one or more subsequent nuclear reactions, leading to a self-sustaining series of reactions. This typically involves the fission of heavy nuclei, such as uranium-235 or plutonium-239.

2. **Defining Critical Mass**:
Critical mass is defined as the minimum amount of fissile material needed to maintain a nuclear chain reaction at a steady rate. If the mass of the fissile material is below this critical mass, the chain reaction will die out; if it is at or above this mass, the reaction can sustain itself.

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