Adding boron to primary water
Boron 10 is an efficient absorber of slow neutrons, used in PWR reactors. A small amount of boric water is mixed with the primary water that serves as a coolant and moderator in the core. The hydrogen protons of the primary water slow the neutrons down by a series of collisions until they are absorbed by boron nuclei. The absorption probability for neutrons in boron 10 increases as the neutrons are slowed down. The capture of a neutron by a boron nucleus results in the formation of an alpha particle and a nucleus of lithium 7, This neutron absorption by boron reduces the number of neutrons available for the chain reaction. (For the sake of visibility the paths of the recoil protons have been enlarged in the picture).