The probability of fission, P f i s s i o n , depends on the nuclear physics of the fuel, and is often expressed as a cross section. Reactors are usually controlled by adjusting P a b s o r b . Control rods made of a strongly neutron-absorbent material such as cadmium or boron can be inserted into the core: any neutron that happens to impact the control rod is lost from the chain reaction, reducing α α . P a b s o r b is also controlled by the recent history of the reactor core itself (see below).