What is a nuclear fission reaction ?

Explanation

### Step-by-Step Solution:

1. **Definition of Nuclear Fission**:
- Nuclear fission is a nuclear reaction in which a heavy nucleus splits into two smaller nuclei of comparable mass. This process releases a significant amount of energy.

2. **Example of Nuclear Fission**:
- A common example of nuclear fission is the reaction involving Uranium-235 (U-235), which has an atomic number of 92.

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