Carbon or Graphite rods are used in atomic reactors as moderators for sustained nuclear chain reaction through nuclear fission process. In this process

In a nuclear fission reactor, the process of moderation is essential for sustaining a chain reaction. When heavy isotopes like Uranium-235 undergo fission, they release high-energy 'fast' neutrons with kinetic energies typically around 2 MeV [3]. However, these fast neutrons are not easily absorbed by other fuel nuclei to trigger further fission. Carbon or graphite rods act as moderators by facilitating elastic collisions with these fast neutrons [1]. Through these collisions, the neutrons transfer their kinetic energy to the moderator atoms, effectively slowing them down until they reach 'thermal' energy levels [3]. These 'slow' or thermal neutrons have a much higher probability of being captured by the fuel atoms, thereby ensuring a continuous and controlled nuclear chain reaction [1]. Protons are not the target of this moderation process in atomic reactors.

Sources

1. [1] https://www.sciencedirect.com/topics/immunology-and-microbiology/fission-neutron
2. [3] https://www.osti.gov/servlets/purl/5603483
3. [2] https://www.sciencedirect.com/topics/materials-science/moderator-material