Patent Number: 051006090
Section: description

DETAILED DESCRIPTION OF THE INVENTION Since a natural-circulation BWR contains no recirculating devices nor external loops, regulating the speed of the prime drive motor so as to produce a controlled recirculation flow rate or throttling flow control valves located in external recirculation loops to obtain flow rate regulation are unavailable. Accordingly, the problem to be solved is to devise an improved means of obtaining effective load-following capability and/or enhancing spectral shift capability in sparger-type BWRs and for which the drawbacks in the form of added components are minimal and are acceptable considering the benefits obtained. With reference to the drawing, it will be appreciated that much of the reactor internals are conventional and have been omitted from the drawings as such components are unnecessary to the modifications which need to be made to the BWRs in accordance with the precepts of the present invention. The reactor internals, their construction, and operation are well known in the art, such as illustrated by reference to the following publications: Glasstone and Sesonke, Nuclear Reactor Engineering, pp 748-753, 3d Edition, VanNostrand. Reinholt (New York, NY, 1981); Wolfe and Wilkens, "Improvements in Boiling Water Reactor Designs and Safety", presented at American Nuclear Society Topical Meeting, Seattle, Wash., May 1-5, 1988; Duncan and McCandless, "An Advanced Simplified Boiling Water Reactor", presented at the American Nuclear Society Topical Meeting, Seattle, Wash., May 1-5, 1988; and Lahey and Moody, The Thermal Hydraulics of a Boiling Water Reactor, especially Chapter 2, pp 15-44, American Nuclear Society (LeGrange Park, Ill., 1977). Conventional BWRs, the ABWR, and the SBWR, all are described and discussed in the foregoing references, all of which are expressly incorporated herein by reference. While the description refers primarily to natural circulation BWRs, forced circulation reactors can be modified in accordance with the present invention also. Referring to FIG. 1 more particularly, reactor pressure vessel (RPV) 10 is seen to admit feedwater via inlet 12 and exhaust steam via outlet 14. Connected to inlet 12 is sparger 16. Sparger 16 is a ringshaped pipe having suitable apertures through which the feedwater is passed to within RPV 10. The design of spargers and their apertures is conventional and well-known to those skilled in the art. As described generally above and in particular in the references cited, with respect to the flow path of water within RPV 10, sub-cooled water located in the downcomer region identified at 24 flows downwardly between RPV 10 and shroud 26 in the annulus region identified at 28. The water flowing through annulus 28 then flows to the core lower plenum region identified at 30. Again, for simplicity many of the reactor internal components have not been illustrated in the drawing as these items are conventional and will be readily apparent to those skilled in the art. The water then flows through a guide tube region located within shroud 26 and below core 32, thence through the fuel orifices and past the fuel support casting and nosepiece lower tie plates. The water then enters the fuel assemblies disposed within core 32 wherein a boiling boundary layer is established, thus causing a lower non-boiling region and an upper boiling region within the fuel assemblies. Flow by-passing is to be provided as is necessary, desirable, or convenient in conventional fashion. Next, a mixture of water and steam enters core upper plenum 34 which is formed within shroud head 36 and disposed atop core 32. Core upper plenum 34 provides stand-off between the mixture exiting core 32 and entering standpipes 38 that are disposed atop shroud head 36 and in fluid communication with core upper plenum 34. It will be observed that downcomer region 24 is formed between the walls of RPF 10 and standpipes 38 which form the chimney. It will be appreciated that a variety of additional confining or direction means/members could be used as the chimney in place of standpipes 38. The liquid water elevation or level established within RPV 10 during normal operation of the BWR is identified at 44. Normal operation is defined to be a reactor output at expected or normal grid electrical demand with all components of the reactor operating nominally. Note that sparger 16 is located at about water elevation or level 44. Sparger 16 is located above the skirt bottom of the separator. The mixture flowing through standpipes 38 then enters steam separator/dryer assembly 40 that is to be provided in conventional or unconventinoal fashion. Separator 40 provides outlet communication for separated water to enter downcomer 24 and for steam to enter steam dome 42 and thence to be withdrawn from RPV 10 via outlet 14. The separated water in downcomer 24 and recycled feedwater from the turbine island portion of the power generating station entering inlet 12, then combine and the flow circulation commences again. From the foregoing description, it will be readily apparent that steam will condense if water level 44 is lower than the elevation of sparger 16 and there will be little or no sub-cooling. If water level 44 is raised to an elevation above sparger 16, there is less time for condensation and the sub-cooling will increase. The upward flow at the indicated sparger elevational level reduces with higher water levels and becomes is virtually zero at a suitably high water level so there will be no entrainment of feedwater towards steam separator dryer 40. Load following, then, is performed simply by reducing or increasing the feedwater flow and, thus, the elevation of water level 44. Consequently, the amount of steam that is condensed will be increased or decreased and so will the amount of sub-cooling and, consequently, the reactor power. Thus, it will be observed that the power level generated by the reactor can be increased or decreased by suitable operation of the sparger BWR disclosed herein. For spectral shift operation, it will be appreciated that the sparger BWR will be operated so as to reduce the sub-cooling and/or recirculation rate so as to increase the void fraction in the core and, thus, reduce the power level of the reactor. Withdrawal of the conventional control rods, not shown in the drawings, re-establishes the power level and enhances spectral shift operation of the reactor. As noted above, when operating the sparger BWR of the present invention in a spectral shift mode, load following still is possible responsive to grid electrical demand and/or other demands made of the reactor. Spectral shift may be compromised and/or control rod movement may be required, yet load following is believed to be achievable primarily through judicious operation of feedwater admitted into the reactor via the sparger disposed therewithin. As to the materials of construction, preferably all components are manufactured from materials appropriate for their use within a nuclear BWR. Further, it will be appreciated that various of the components shown described herein may be altered or varied in accordance with the conventional wisdom in the field and certainly are included within the present invention, provided that such variations do not materially vary within the spirit and precepts of the present invention as described herein.