Controlling the power output of a boiling water reactor (BWR)—such as Exelon Nuclear’s Quad Cities Unit 1 or 2 (Figure 1)—requires changing the reactivity of the core by repositioning the control rods and controlling coolant flow through the core.
1. First in line. Exelon Nuclear’s Quad Cities Unit 1 will have its reactor recirculation flow control systems upgraded with electronic adjustable-speed drives in the spring of 2009. Courtesy: Exelon Nuclear
For a given control rod line, increasing flow causes steam bubbles (“voids”) to be reduced, which increases the amount of liquid water in the core. With more liquid available, more neutrons are slowed down (moderated) to a speed suitable for splitting fissile fuel. More fission means more thermal power. Decreasing flow through the core has the opposite effect on power output.
When a BWR is operating on the so-called “100% rod line,” its power output can be varied from roughly 70% to 100% of maximum rating by varying the speed of the recirculation pumps or by throttling a flow control valve.
The BWRs of interest in the U.S.—the BWR-3 through -6 designs—have two recirculation loops (Figure 2). Each loop has one pump whose flow is controlled by either of two systems: a motor-generator (M-G) set that uses large, medium-voltage induction motors to drive the recirc pumps or a flow-control valve design. Either system provides controllable recirculation flow through the reactor core.
2. Gone fission. A simplified diagram of a typical BWR plant and the recirculation loops that control the thermal output of the reactor. The pumps’ flow controls were the subject of an extensive upgrade option analysis by Exelon Nuclear. Source: Exelon Nuclear
The earlier (BWR-3 and BWR-4) designs use M-G sets. This control design allows variable adjustment of motor voltage and frequency and uses a voltage regulator to keep their ratio constant. The motor, operating at medium voltage, drives the generator through a fluid coupling that acts like a clutch. The speed and output of the generator rise and fall as the volume of fluid in the coupling is varied by changing the position of a scoop tube, or weir. As the generator’s output increases or decreases, the speed of the recirc pump follows suit. Figure 3 shows an M-G set from Quad Cities Nuclear Generating Station in Illinois.
3. Long in the tooth. A typical M-G set for controlling a BWR-3 recirculation pump. Courtesy: Exelon Nuclear
The flow-control valve design is standard at the later BWR-5 and BWR-6 plants, which also use an M-G set to power the pump at low frequency (15 Hz) and low speed (25% of maximum). A direct connection of the pump motor to the medium-voltage source provides for a faster speed at line frequency (60 Hz). A modulating flow-control valve in the loop adjusts the recirculation flow as required.
High time to upgrade
Both control designs have exhibited problems that have resulted in excessive maintenance costs, generating inefficiencies, and enough loss of control to warrant a unit de-rate. Making matters worse, spare parts have become scarce, which has resulted in extended outages for unplanned repairs.
Among the parts of both designs that have become troublesome are flywheels, scoop tubes, voltage regulators, and scoop tube positioners. Backups for those components (if available), as well as large inventories of M-G lubricating oils, fluid for the coupling, hydraulic fluid for control valve power units and shuttle valves, and generator brushes must be kept on the shelf. Spare parts for auxiliary lube oil, cooling water, and ventilating systems also must be kept on hand.
Within the reactor building, the primary piping and pump pressure boundaries are critical to safety, but the power and instrumentation portions of the water recirculation systems are not. However, their fault tolerance and obsolete components have caused reliability problems at several BWRs. Specifically, the old electromechanical analog technology used to control the speed of recirc pumps is vulnerable to hysteresis, sensitive to temperature, and prone to “hunting” due to mechanical dead bands and a limited range of output frequencies. Any of these conditions can reduce a unit’s capacity factor, a key measure of its performance.