The pebbles will most likely be made of Lithium because it is a solid/liquid at the operating temperatures of the cooling system and therefore has a high density. It is also an element with a very low atomic mass, which makes the crosssections for interactions with fast neutrons comparably high. So the Lithium will have collisions with the fast neutrons from the D-T-fusion that leave the plasma confinement with most of the energy from the reaction. These will heat up the pebbles due to the energy transfer and from time to time a Lithium-6 atom will catch a neutron. This will make the Lithium core fission into a Helium code amd Tritium core. Tritium is one of the two fuels required for fusion and has a very low natural abundance due to its rather short half life. So breeding it seems to be the best option to obtain it in sufficient quantities.