ABSTRACT
Control rods have four primary functions they are required to perform. They are used to raise or lower the power level of the core by changing the value of the effective multiplication factor and the reactivity ρ. Control rods can be used to change the shape of the neutron flux, and changing the flux shape can be used to eliminate hot spots, or flux peaks, and to promote better burnup of the fuel. Aside from the control rods themselves, many nuclear power plants have employed new fuel assemblies where materials known as burnable poisons are dissolved directly in the fuel. Burnable poisons can also be used to flatten the radial power distribution within an individual fuel assembly. The most common form of chemical shim in pressurized water reactors (PWRs) is a form of boric acid, which can be directly dissolved in the coolant to absorb excess neutrons and to make many subtle adjustments to the global power level.
