In a nuclear reactor, moderators slow down the neutrons which come out in a fission process. The moderator used have light nuclei. The heavy nuclei will not serve the purpose, because:
| 1. | they will break up |
| 2. | elastic collision of neutrons with heavy nuclei will not slow them down |
| 3. | the net weight of the reactor would be unbearably high |
| 4. | substances with heavy nuclei do not occur in the liquid or gaseous state at room temperature |
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