The fission process is controlled by the reactor operators through the use of neutron-absorbing materials. The primary control is an array of control rods or blades that can be withdrawn from the core to the degree needed. In the PWRs, the control rods are moved within selected empty tubes within the assembly. In the BWRs, cruciform (cross-shaped) control blades are moved across the faces of the fuel assembly, typically narrower than those in a PWR fuel assembly. Reactor fuel designers also use burnable poisons within the fuel assembly to control the fission process. These poisons are placed in appropriate amounts within the fuel assembly so that they burn away, making the fuel assembly more reactive, as the continued fission process is making it less reactive. PWRs also use neutron control by dissolving neutron-absorbing sodium borate in the reactor coolant, gradually lowering the concentration from the peak after refueling to the minimum before the next refueling.
American Nuclear Society. 1988. Design Criteria for an (independent Spent Fuel Storage Installation (Water Pool Type): An American National Standard. ANSI/ANS-57.7–1988. American Nuclear Society. LaGrange Park, Illinois.
Duderstadt, J.J. and L.J.Hamilton. 1976. Nuclear Reactor Analysis. John Wiley& Sons. New York.
Lahey, R.T. and F.J.Moody. 1997. The Thermal Hydraulics of a Boiling Water Nuclear Reactor. Second Edition. American Nuclear Society. La Grange Park, Illinois.