Anordnung von kühlwasserumw alzpumpen innerhalb des druckgefässes von siedewasserreaktoren

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AEG proposes a new version of the Boiling Water Reactor, namely with coolant recirculation pumps installed within the reactor pressure vessel, thus abolishing external recirculation loops and allowing a still more compact design of reactor containment. Section 1 reviews the development of coolant recirculation systems for boiling water reactors. Internal jet pumps fed by external recirculation loops for only about a third of the total coolant flow is the present state of the art. Speed variation of the external recirculation pumps controls reactor power. Section 2 describes the design of the proposed internal pumps. The number of such pumps varies proportional with reactor power, each pump handling the flow roughly corresponding to 85 MWe. Guide vanes and axial impeller are situated in the downcomer area between vessel wall and core shrouding, the shaft penetrating the vessel bottom head vertically. Mechanical seals, axial bearing and speed controlled motor are accessibly situated underneath the reactor, and the individual components as well as their arrangement virtually are the same as usual in the normal BWR recirculation pumps. Due to the relatively small power of about 600 kW of each pump, static transformers can be used to generate the variable frequency for pump speed control; they have advantages in comparison with the normally used motor generator sets. Section 3 gives an outline of the general criteria for the chosen design. Reliability was the most important consideration, and the large saving associated with the internal pump systems allowed use of the most reliable though costly design. For example, the water-lubricated radial bearings are of the hydrostatic type to eliminate wear, and redundancy in pumping power allows to operate the reactor at full power with one of the pumps out of service. Section 4 deals with the effect of the pump system on other parts of the nuclear steam supply system. In particular, the internal pumps do not affect size or cost of the pressure vessel and allow a very compact containment design. Section 5 analyses in some detail the influence of the pump system on reactor availability, including transient analysis in the case of pump failures, safety against cavitation and repair possibilities. The general conclusion is that the internal pump system should improve reactor availability. Section 6 explains the developmental program. Two slightly different full size prototype pumps are being built by two renowned pump manufacturers and will be tested in a rig built by AEG which not only simulates reactor conditions, but also incorporates auxiliary and supply systems as planned for a BWR system, in particular the static transformers for frequency variation. The purpose of this testing is not that of demonstrating the feasibility of the pumps, but rather that of debugging. Section 7 finally compares the described pump system with the jet pump recirculation system. Apart from an important saving of plant capital cost and of reduced pumping power, the system also offers advantages in the areas of reactor control and safety.