Cruciform control rods are employed for BWRs as shown in Fig. 3.14, and boron carbide (B4C) or hafnium is usually used as a neutron absorber. The control rods are inserted from the bottom of the reactor pressure vessel into the cruciform region formed between four fuel assemblies. There are two types of control rod drive mechanisms: hydraulic pressure piston drive and electric motor drive. Most of BWRs use the hydraulic pressure-driven system to move the control rods in 15.2 cm increments (one notch). The control rods in the advanced BWR (ABWR) design are driven in 1.8 cm increments (one step) by electric fine motion motors which make it possible to simultaneously move the maximum 26 control rod groups. For B4C control rods, stainless steel tubes, which are arranged in a blade sheath, are filled with B4C powder. For hafnium control rods, metal hafnium plates or rods are inserted into the sheath.

All control rods are inserted at reactor shutdown and control rods of about 5-10 % are inserted to control the excess reactivity during reactor operation at the rated power.

An inherent feature of power and reactivity control of BWRs is to control reactivity by changing the coolant flow rate in the core, which can be controlled by changing the pumping speed of the coolant recirculation pumps. BWR cores
have a coolant void and its value decreases as the coolant flow rate increases from the normal condition. Since the coolant also serves as a moderator in the core, the reduction in void fraction leads to a large effect on neutron moderation and results in progression of neutron spectrum softening. Therefore, the reactivity and reactor power increase. This leads to an increase in the void fraction again and results in an equilibrium state of the reactor power level corresponding to the void fraction at reactivity balance. On the other hand, a decrease in the coolant flow rate leads to a reactivity decrease and then the reactor power equilibrates at a lower level. Another feature of BWR cores is that the core power distribution hardly changes before and after a variation in core coolant flow rate.

This capability for reactivity control by coolant flow rate can be applied to compensate for the reactivity variation during reactor operation; fissile mate­rials are consumed and reactor power gradually drops with reactor operation. Control rods can be withdrawn little by little to maintain the reactor power and to control the reactivity, but it gives rise to a distortion of the core power distribution. The change in coolant flow rate makes it possible to control the reactivity without distorting the core power distribution and moreover change in coolant flow rate is relatively easy to implement.