A moderator material should have:
- Large atomic mass
- Small atomic mass
- Any of above
- None of these
Correct answer: 2. Small atomic mass
Explanation: In a nuclear reactor, a moderator is a material used to slow down neutrons produced in fission reactions. Slowing down neutrons increases the probability of their interaction with fissile isotopes (such as uranium-235), promoting sustained chain reactions.
The effectiveness of a moderator depends on its ability to slow down neutrons without significantly absorbing them. Neutrons lose more energy in collisions with lighter nuclei due to the conservation of momentum, so materials with smaller atomic masses are better moderators.
Commonly used moderator materials include light elements like hydrogen (in the form of water or heavy water) and graphite. These materials have small atomic masses and are effective in slowing down neutrons without excessive absorption.