I have had a question that I cannot get an answer for. I recently had an exam for energy in the nucleus which included a question about why lead nuclei would be inadequate for use as a moderator in a reactor. When I got the answer for the question, it stated that the lead nuclei reflect neutrons on account of the nuclei being "too heavy." This didn't make sense to me because the absorption of neutrons by uranium nuclei, being a much heavier substance, absorbs neutrons, not reflects. Can anyone explain this to me? I was told to post it here.