In one conventional boiling water reactor (BWR) a cylindrical pressure vessel includes in serial flow communication from the bottom thereof to the top thereof a reactor core for heating a reactor coolant, such as water, for generating steam, an upper plenum in which a water and steam mixture rises, a moisture separator for removing a majority of the liquid from the steam, and a steam dryer for removing substantially all the remaining moisture from the steam. The dried steam is discharged from the vessel through an outlet nozzle for flow to a conventional steam turbine. Surrounding the reactor core is an annular core shroud which is spaced radially inwardly from the reactor pressure vessel to define an annular downcomer. Disposed near the top of the upper plenum is a conventional feedwater sparger which receives relatively cool feedwater which is discharged into the reactor coolant for flow down the downcomer.
Since the reactor coolant is heated in the core its density decreases and it rises through the core and the upper plenum. The coolant is then mixed with the feedwater which lowers its temperature, and therefore increases its density, and then the reactor coolant flows downwardly by gravity through the downcomer to a lower plenum of the reactor pressure vessel disposed below the core. The reactor coolant turns from a downward direction to an upward direction in the lower plenum and enters an inlet of the reactor core to repeat this natural recirculation process.
In order to increase reactor coolant recirculation, BWRs typically include some form of pump in the downcomer, such as an impeller-driven reactor internal pump (RIP) or a fluid-driven jet pump. Such pumps are mounted to an annular pump deck disposed in the downcomer which separates the downcomer from the lower plenum. The pumps are spaced circumferentially around the pump deck and force, or pump, the reactor coolant from the downcomer into the lower plenum, and in turn into the core inlet. The pump deck seals the downcomer from the lower plenum for allowing effective pumping action of the pumps. Without the pump deck to seal the downcomer, a significant portion of the pressurized reactor coolant discharged from the pumps would recirculate upwardly in the downcomer into the pump inlet which would decrease the total flow into the core.
Impeller-driven RIPs are typically preferred in modern BWRs since they provide effective pumping capacity in a relatively short length as compared to conventional jet pumps. However, following a pump trip in which the pumps are rendered inoperable, natural recirculation of the reactor coolant is substantially inhibited by the RIPs, and primarily by the impellers thereof. Although the pump deck prevents local recirculation in the downcomer while the RIPs are operable in the pumping mode, it also prevents natural recirculation of the reactor coolant following the pump trip when the RIPs are rendered inoperable.