In a nuclear reactor, chain reaction is controlled by introducing

Options

A. Iron rod
B. Cadmium rod
C. Graphite rod
D. Gold rod

Answer

To solve the question regarding how a chain reaction in a nuclear reactor is controlled, we can break it down into the following steps:

### Step-by-Step Solution:

1. **Understanding Nuclear Fission**:
- In a nuclear reactor, uranium isotopes (commonly Uranium-235) undergo fission when bombarded with neutrons. This process releases a significant amount of energy and additional neutrons.

2. **Chain Reaction**:
- Each fission event can produce multiple neutrons (typically 2-3). These neutrons can then initiate further fission events in nearby uranium nuclei, leading to a chain reaction.

3. **Need for Control**:
- If left uncontrolled, the chain reaction can escalate rapidly, leading to an explosion. Therefore, it is crucial to manage the number of neutrons that continue to cause fission.

4. **Role of Control Rods**:
- To control the chain reaction, materials that can absorb neutrons are introduced into the reactor. These materials are known as control rods.

5. **Materials Used for Control Rods**:
- Control rods are typically made from elements that are effective at absorbing neutrons. Common materials include boron and cadmium.

6. **Function of Control Rods**:
- By adjusting the position of the control rods within the reactor, operators can control the number of free neutrons available to sustain the chain reaction. Inserting the rods deeper into the reactor absorbs more neutrons, while withdrawing them allows more neutrons to participate in fission.

7. **Conclusion**:
- Therefore, the introduction of control rods made of materials like cadmium or boron is essential for controlling the chain reaction in a nuclear reactor.

### Final Answer:
In a nuclear reactor, chain reaction is controlled by introducing control rods made of cadmium or boron.

---