AIIMS 2007 Physics Moderators in Nuclear Reactors Assertion Reason Question

To analyze the question regarding heavy water and normal water as moderators in nuclear reactors, let’s break down the assertion and the reason provided:

Assertion:

Heavy water is a better moderator than normal water.

Reason:

Heavy water absorbs neutrons more efficiently than normal water.

Evaluation of Assertion and Reason

1. Understanding Moderators:

   • Moderators are materials used in nuclear reactors to slow down fast neutrons produced during nuclear fission. Slower (thermal) neutrons are more likely to induce further fission in fissile materials like Uranium-235 or Plutonium-239.
   • Common moderators include normal water (H₂O) and heavy water (D₂O).

2. Heavy Water vs. Normal Water:

   • Heavy water (D₂O) consists of deuterium, which is an isotope of hydrogen containing one neutron along with one proton. Normal water (H₂O) consists of hydrogen atoms with no neutrons.
   • The presence of the additional neutron in deuterium means that heavy water has a lower neutron absorption cross-section compared to normal water. This allows heavy water to slow down neutrons more effectively without capturing them.

Analysis of the Statements

• Assertion: Heavy water is indeed a better moderator than normal water. This is true because heavy water effectively slows down neutrons while allowing a higher proportion of them to remain available for further fission reactions.

• Reason: The statement that heavy water absorbs neutrons more efficiently than normal water is false. In fact, heavy water has a lower absorption rate for thermal neutrons compared to normal water, which means it allows more neutrons to be available for sustaining the chain reaction.

Conclusion

Thus, while the assertion is true, the reason is false. Therefore, the correct answer is:

C) Assertion is true but Reason is false.

Clarification of Other Options

• A) Both Assertion and Reason are true and Reason is the correct explanation of Assertion.

  • Incorrect: The reason is false; hence this option is not valid.

• B) Both Assertion and Reason are true but Reason is not the correct explanation of Assertion.

  • Incorrect: The reason is false, so this cannot hold true.

• D) Both Assertion and Reason are false.

  • Incorrect: The assertion is actually true.

Summary

In conclusion, heavy water serves as a superior moderator due to its ability to slow neutrons effectively without absorbing them as much as normal water does, making the assertion true while the reason fails to describe the situation accurately. This understanding is crucial in nuclear physics, particularly in the design and operation of reactors.