Neutron capture is the process by which neutrons are taken up by the nucleus of an atom. This process is an important part of the synthesis of heavy elements, as it is the only way to produce elements heavier than barium. Neutron capture is also the primary mechanism used to cool fuel rods in nuclear reactors, as control rods made of boron readily absorb stray neutrons to control fluctuations.
Neutron capture works to produce new elements by bombarding atomic nuclei with extra neutrons faster than the atom can shed them through gamma decay. The extra neutrons accumulate in the nucleus and eventually trigger a beta decay into a heavier, more stable element.
Inside nuclear reactors, boron control rods act to absorb neutron flux and maintain a controlled chain reaction among the uranium or plutonium fuel rods. As the fuel rods shed neutrons, the heat they generate is used to drive a turbine. The waste neutrons that are a byproduct of the process flow easily through the negatively charged electron shells of the boron atoms. Unlike protons, neutrons lack an electromagnetic charge and are not affected by the polarity of other particles. Once trapped inside the boron nucleus, the neutrons are no longer available to drive the chain reaction, keeping the rate of atomic fission manageable.