cast iron
beryllium
steel
boron
fission of radioactive materials
fusion of radioactive materials
burning of the fuel rods in oxygen
none of these
to absorb the excess neutrons
to increases the energy of the neutrons
to slow down the neutrons
CANDU type reactors
boiling water reactors
pressurized water reactors
gas cooled reactors
highly enriched uranium as fuel and light water as moderator and coolant
natural uranium as fuel and heavy water as moderator and coolant
enriched uranium as fuel and ordinary water as moderator and cooler
only fertile material