That's what the lithium blanket is for.
The liquid lithium absorbs the neutrons, converts their kinetic energy into heat, which in theory can be used to run a steam-electric generator. If carbon absorbed the neutrons first, they wouldn't reach the lithium. A different coolant would have to be used. I think that's sortof how the early fission reactors worked: carbon (graphite) acted as a moderator, absorbed excess neutrons, and prevented a meltdown.