A typical boiling water reactor (BWR) includes a pressure vessel containing a nuclear reactor core submerged in coolant water therein. The reactor core is effective for boiling the coolant water for generating steam which is discharged from the pressure vessel and used for producing power, such as powering a steam turbine-generator for producing electricity.
In order to cool the reactor core, a recirculation system is provided and includes an annular downcomer disposed between the reactor core and the inside of the pressure vessel, and suitable pumps for channeling the coolant water downwardly through the downcomer to a lower plenum of the pressure vessel wherein it is turned upwardly and through the reactor core for its cooling. As the water flows upwardly through the reactor core it is heated thereby for generating steam which is channeled upwardly from the reactor core.
Various types of pumps are known for providing recirculation in a reactor pressure vessel having various advantages and disadvantages. For example, conventional jet pumps may be disposed vertically inside the downcomer and include a suction inlet at a top end thereof and an outlet at the bottom end thereof. An injector nozzle is disposed at the suction inlet for injecting pressurized water which provides energy for operating the jet pump to create suction at the inlet for drawing in a portion of the coolant water from the downcomer. The water drawn in is then channeled through the conventionally configured jet pump housing wherein it is mixed with the injected water and diffused for providing pressurized water at its outlet having a suitable flow rate and pressure for providing recirculation to the reactor core. The injector nozzle is typically joined to external piping for receiving the pressurized water from a conventional driving pump located remotely from the pressure vessel. The remotely located driving pump and external pipes increase the complexity of the overall power plant and correspondingly require increased space in the reactor building and increased maintenance for ensuring the proper operation thereof.
Another type of pump used for recirculation flow in a reactor pressure vessel is conventionally known as a reactor internal pump (RIP) which is sealingly joined to the pressure vessel and forms a part thereof. In this way, no external pipes are required as in the above example having the remotely located pump. RIPs include a conventional impeller which is suitable sized and configured for providing a suitable flow rate and pressure for recirculating the coolant water through the reactor core. The impellers are typically configured for providing relatively low-head and high-flow to effectively meet the recirculation flow requirements of a typical reactor. However, one disadvantage of an impeller driven RIP is that during a power failure, the impeller will stop rotating and present resistance to natural recirculation flow of the coolant water inside the pressure vessel. Accordingly, suitable design provision must be provided to ensure effective recirculation flow of the coolant water in the event of a power failure which stops rotation of the impeller and results in a degraded natural circulation flow.
Impeller driven RIPs may be mounted below the lower head of the pressure vessel or may be side-mounted on the pressure vessel. The bottom mounted RIPs require suitable access space under the pressure vessel which requires a taller containment building and increases the difficulty of maintaining the RIPs in the limited and congested space under the pressure vessel. Side mounted RIPs, on the other hand, free up space below the pressure vessel and are readily accessible for improved ease of maintenance.
However, a suitable number of impeller-driven RIPS for obtaining the required overall recirculation flow rate through the reactor core typically requires low-head and high-flow configurations to be practical. High flow through the RIPs in turn requires a correspondingly large RIP and, unless the impeller is installed inside the reactor pressure vessel, a correspondingly large nozzle through the pressure vessel for channeling the coolant water therebetween. Larger nozzles increase the complexity and cost of the pressure vessel since the pressure vessel must be suitably designed for accommodating the relatively high pressures caused from the generation of steam therein.