plates

pallets

pins

any one of the above

does not absorb neutrons

absorbs neutrons

accelerates neutrons

eats up neutrons

slowing down fast neutrons so that Uranium fission continues by slow motion neutrons

accelerating fast neutrons

absorbing all neutrons

using moderator

fast moving

slow moving

critical neutrons

neutrons at rest

light weight atoms

heavy weight atoms

critical atoms

zero weight atoms