As humanino indicated the absorption of a neutron is the nuclear reaction, and most often it is accompanied by gamma emission. Only a few isotopes are fissile, e.g. isotopes of Th, U, Pu, and heavier elements.
Cadmium, silver, indium, 10B, hafnium (Hf), gadolinium (Gd), dysprosium (Dy), erbium Er) and some other rare Earth's are good neutron absorbers.
In Boiling Water Reactors (BWRs), 10B in the form of B4C and Hf are the preferred absorbers for Control Rod which reside incore for reactivity control (and power distribution) during reactor operation. Gd in the form of gadolinia (Gd2O3 is a burnable (meaning it depletes during irradiation) absorber used in the fuel, and is mixed in with the UO2.
In Pressurzed Water Reactors (PWRs), control rods contain Ag-In-Cd, Hf, Dy, or B4[/sup]C, although Hf has a problem with hydrogen absorption, so it's use is problematic and for the most part, isn't used these day. Usually control rods are withdrawn above the core in PWRs. Some PWRs use grey rods using Ni-alloys to tailor the neutron flux (power distribution) during operation.
PWRs use boric acid in the reactor coolant water to control reactivity during operation, and this solution is buffered with KOH or LiOH.
PWR fuel may incorporate burnable absorbers such as Gd (gadolinia), Er (erbia), or 10B in the form of ZrB2. The objective is to select a burnable absorber with low residual. There are also clusters (removable) that can be inserted into the guide tubes of a PWR fuel assembly, similar to control clusters, that contain boron-containing pyrex, and are sometimes called discrete burnable absorbers.
