Control rods

Nuclear power plants are designed to provide heat, and thus power, at a steady rate, called "baseload" power in the industry. But the system needs to be slowed down sometimes or shut down for refueling. The control rods are the metal tubes that fit over fuel rods below them. They are made of an alloy that absorbs the neutrons being thrown off by the nuclear reaction in the reactor to slow the rate of fission. Plant operators can also release boron into the reactor water, which also slows the pace of fission.