In a thermal nuclear fission reactor, the choice of materials for the moderator and the control rods is critical to maintaining a controlled, steady-state chain reaction. Which of the following statements correctly explains the physical principles behind the choice of these materials?
The moderator contains light nuclei (such as 1H^1\text{H}1H or 12C^{12}\text{C}12C) because elastic collisions with nuclei of similar mass transfer the maximum kinetic energy from the neutrons. Control rods contain materials with a high neutron capture cross-section (such as 10B^{10}\text{B}10B) to absorb neutrons without undergoing fission.
The moderator contains heavy nuclei (such as 208Pb^{208}\text{Pb}208Pb) because their large mass allows neutrons to rebound with minimal energy loss, maintaining high velocities. Control rods contain light nuclei to slow down surplus neutrons so they do not cause fission.
The moderator contains materials with a high neutron capture cross-section to absorb fast neutrons before they escape the core. Control rods contain light nuclei to elastically scatter neutrons, keeping them within the fuel rods.
The moderator contains light nuclei that undergo nuclear fusion with thermal neutrons to assist heat generation. Control rods contain heavy nuclei that undergo sub-critical fission to regulate the neutron population.
