Introduction:
Deaerators are devices used to remove dissolved gases, primarily oxygen, from feedwater in power plants. They are commonly used in fossil fuel power plants to prevent corrosion and damage to the boiler and other equipment. However, deaerators are not typically used in water-cooled and moderated nuclear power plants. The correct answer for why deaerators are not used in these plants is option 'D' - radioactivity.

Explanation:
1. Purpose of a Deaerator:
- Deaerators are designed to remove dissolved gases, mainly oxygen, from the feedwater.
- Oxygen can cause corrosion and pitting in the boiler and other equipment, leading to reduced efficiency and potential damage.

2. Water-Cooled and Moderated Nuclear Power Plants:
- Water-cooled and moderated nuclear power plants use water as both a coolant and moderator for the nuclear reactions.
- The primary coolant, usually water, circulates through the reactor core to remove heat generated from the nuclear fission process.
- This water is highly radioactive due to the presence of activated isotopes and fission products.

3. Radioactive Environment:
- Nuclear power plants operate in a highly radioactive environment.
- The presence of radioactive materials in the coolant makes it necessary to maintain a closed-loop system and prevent any leakage or release of radioactive substances.

4. Challenges with Deaerator:
- Deaerators typically have venting systems to remove the non-condensable gases, including oxygen, from the feedwater.
- The vented gases are released into the atmosphere in fossil fuel power plants.
- In a nuclear power plant, the vented gases would contain radioactive isotopes, posing a risk to the environment and personnel.

5. Alternative Methods:
- Water treatment processes are employed in nuclear power plants to control and minimize corrosion and fouling.
- These processes include ion exchange, pH control, and chemical additives to reduce the effects of dissolved gases.
- The primary focus is on maintaining the purity and quality of the coolant while minimizing the release of radioactive substances.

Conclusion:
Deaerators are not used in water-cooled and moderated nuclear power plants primarily due to the radioactive nature of the coolant. Maintaining a closed-loop system and preventing the release of radioactive substances into the environment is of utmost importance in nuclear power plants. Alternative methods are employed to control corrosion and minimize the effects of dissolved gases in the coolant.