30 Containment systems Control rods reactivity

A cut-away schematic of a BWR equipped with external coolant pumps is shown in Fig. 17. The recirculation system comprises the external piping, pumps, and valves located at the lower region of the vessel. The penetrations through the bottom of the vessel that contain the control rod drive mechanisms that are vital for controlling the reactivity of the core are not shown in Fig. 17. Of particular importance, as far as the integrity of the reactor is concerned, is the control rod drive tubes and related mechanisms, because they are in contact with the coolant and... 

The fuel subassemblies of the core and of the blanket are placed in the discharge header mounted on the pressure chamber of the reactor vessel (fig.5). The core loading of 220 subassemblies contains fuel elements with UO2 fuel and a blanket material. More recently some MOX experimental fuel elements were also loaded into the core. There are 12 positions in the core for the rods of the control and safety system. Of the 12 control rods 2 are automatic control rods, 6 are reactivity compensators, 3 are scram rods and 1 is a temperature effect compensator. The core is surrounded by the blanket, containing depleted uranium dioxide. The axial blanket is mounted in the subassemblies of the core (Fig. 6) and the radial blanket is formed by fuel subassemblies containing blanket material. Above the core is a central column containing the control rod drives and thermocouples. The column is only as wide as necessary for the absorber rod systems it does not cover the full width of the active core zone. 

BWRs do not operate with dissolved boron like a PWR but use pure, demineralized water with a continuous water quality control system. The reactivity is controlled by the large number of control rods (>100) containing burnable neutron poisons, and by varying the flow rate through the reactor for normal, fine control. Two recirculation loops using variable speed recirculation pumps inject water into the jet pumps inside of the reactor vessel to increase the flow rate by several times over that in the recirculation loops. The steam bubble formation reduces the moderator density and... 

Control of core reactivity is maintained by control rods, which drop into the core upon receipt of a signal from one of several diverse monitoring and actuation systems, or loss of power. Any water subsequently added to the core to control heat removal contains boron, which maintains the low levels of reactivity. 

Reactivity control is achieved through the use of control rods with in vessel drives no soluble boron system is foreseen. To reduce the reactivity at the beginning of the cycle, the loaded portion of fuel contains burnable poison. As in standard PWRs, the rod clusters are moved in guide thimbles but since the steam generator acts as a vessel head, there is no possibility of... 

The reactivity and power control rod system consists of two identical clusters. Each control rod unit consists of an array of tubes containing B4C to provide a gravity-driven rod drop and a powered drive-in. Gas expansion modules (OEMs) located at the periphery of the active core are passive reactivity feedback assemblies that insert negative reactivity into the core during the loss of flow events. 

Each well contains three neutron detectors. Two neutron detectors provide neutron flux signals to the PPIS for use in the reactor trip circuitry. The third neutron detector provides a neutron flux signal for use by the NCS and Rod Control Systems for reactivity control during plant operation. The detectors used are fission chambers. The ranges covered are shown in Figure 4.3-11. 

VSR - Vertical Safety Rod - Boron-containing rods suspended over open channels in the pile during operation, so arranged that when the safety circuit is tripped the rods will fall into the pile by gravity. The structure and number of rods varies considerably from pile to pile. The VSR system is the fastest reactivity control system and absorbs almost as much reactivity as the Ball 3X system. 

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