Patent Number: 041586050
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the drawings, there is shown in FIG. 1 a typical nuclear reactor vessel 10 including the vessel head 12. The vessel 10 encloses a reactor core 14 which includes a plurality of elongated fuel assemblies 16 oriented adjacent one another. The assemblies 16 are supported by a lower core plate 18 which is perforated to allow passage of coolant and which in turn is supported by a core barrel 20. The core barrel 20 is supported from a ledge 22 of the reactor vessel 10, and is restrained in lateral movement by a radial support system 24 affixed to the vessel 10. The main flow of reactor coolant fluid typically enters the vessel 10 through one or more inlet nozzles 26, passes downward about the outer periphery of the core barrel 20 and about the affixed neutron shields 28, is turned one hundred and eighty degrees in a lower plenum 30, passes upward through the lower core plate 18 and core 14, and exits through outlet nozzles 32. It is of prime importance that the flow of coolant is carefully controlled into and about the fuel assemblies 16 of the core 14. Baffling of coolant flow about the core 14 has typically been performed by a baffle plates 34 and formers 36 assembly, through which a small bypass flow of reactor coolant is also passed. This bypass flow must be minimized since it decreases the thermal efficiency of the reactor, but must be large enough to adequately cool the surrounding components. In order to also minimize bypass flow between the outermost fuel assemblies 16, which are typically operating at a lower power density than more central assemblies, and the baffle plates 34, the baffle plates 34 are oriented close to the assemblies 16. Because the baffle plates 34, shown best in FIGS. 2 and 3, are typically significantly thinner than the core barrel 20, the differential thermal expansion between them must be accommodated in the means fastening the formers 36 to the barrel 20 and to the baffle plates 34, typically bolts. The differential thermal expansion is compounded not only by the fact that the baffles 34 are closer to the core 14 and the hotter coolant fluid than the core barrel 20, but also because the heat generation in these components changes throughout the reactor operating cycle. Excessive fastener loads are alleviated by this invention, the pasic principle of which is to split the baffle plates 34 transversely at one or more elevations and allow expansion without significant interference at the junction region 38, thereby reducing loadings on the fasteners. To minimally disturb the coolant flow on either side of the baffle plates 34, the upper and lower plates 34 at each junction region 38 should be aligned longitudinally, to present a substantially continuous inner surface 40 and outer surface 42, as shown in FIG. 2a. Further, to minimize any cross flow leakage across the baffle plates 34 at the junction region 38, the baffle plates preferably overlap so as to provide a barrier of high resistance to flow. Both of these features can be accomplished by utilization of baffle plate extensions 44, as shown in FIG. 2a. The longitudinal clearances 46 accommodate the longitudinal expansion of the plates 34, and the transverse clearance 48 present a barrier to coolant flow. The transverse clearances 48 should therefore be sized as small as possible consistent with manufacturing techniques and maintenance of a generally smooth inner surface 40 and outer surface 42 throughout the entire height of the baffle assembly. For the embodiment shown, a transverse clearance 48 of 0.020 inches is consistent with these criteria. The longitudinal clearances 46 should be sized to accommodate, without significant interference, the expansions of consecutive plates 34. They will therefore vary dependent upon such parameters as the lengths of the baffle plates 34 and the temperatures the plates 34 are exposed to. In the embodiment shown in FIG. 2a, the upper longitudinal clearance is 0.120 inches, and the lower is 0.060 inches. The extensions 44 are approximately one inch long. FIG. 4 shows another embodiment which will maintain a zero transverse clearance 48. This is accomplished by providing a curved edge 50 on at least one of the extensions 44. As consecutive baffle plates 34 expand, the curved edge 50 will maintain contact with its mating extension 44, without presenting excessive resistance to the movement. FIGS. 5 through 7 present alternative embodiments which will also function to control thermal expansion and reduce fastener stresses without allowing excessive cross flow through the baffle plates at the junction regions 38. In FIGS. 5 and 5a consecutive baffle plates 34 are provided with male 52 and female 54 mating surfaces. The surfaces 52, 54 will allow longitudinal thermal expansions without interference, the expansions being taken within the longitudinal clearance 56. The male 52 and female 54 surfaces should therefore be sized to accept the expansion. The transverse clearances 58 should be sized to minimize the clearance area, without providing a significant resistance to the expansion movement. In this configuration, however, the clearances 56, 58 would allow cross flow through the junction region 38 unless otherwise prevented. Cross flow is therefore minimized by positioning the formers 36 to extend over the clearances. The formers 36 are preferably affixed to the baffles 34 by fasteners, such as bolts 60, through the male surface 52. FIG. 6 is an embodiment similar to that of FIG. 5a. Here, however, the transverse clearances 57 are enlarged to better facilitate attachment of the mating baffle plates 34 and increase some of the manufacturing tolerances. Here, the formers 36 are positioned to extend over the clearances. FIG. 7 shows another embodiment, similar to those of FIGS. 2a and 4, which also utilizes the formers 36 to minimize leakage across the baffle plates 34. The baffle plates 34 are provided with extensions 62 that can expand into the longitudinal clearances 64. The transverse clearance 66 is made as small as possible consistent with manufacturing techniques. Any leakage across the baffle plates 36 will therefore be minimized. To further alleviate leakage, the longitudinal clearances 64 on the baffle outer surface 42 are aligned with the formers 36. Aligning the longitudinal clearances 64 above or below the respective former 36 transverse centerline allows sufficient surface to affix the baffle 34 and former 36 by fastening means such as a bolt 68. Yet another embodiment is shown in FIG. 8. Here, a mating baffle plate is affixed to each former 36. Each baffle plate 34 is permitted to freely expand with temperature increases, without interfering with adjacent baffle plates 34. Thus, no significant thermally induced stresses will be imposed at the baffle-former attachment points 70. The baffle plates 34 have been shown with extensions 72, although flat edged plates can be utilized if flow is otherwise properly controlled to minimize core bypass flow and avoid vibration inducing impingement upon the fuel assemblies, such as those instances where any leakage flow would be from the core 14 outward. Affixing a separate baffle plate 34 to each former 36 provides the advantage of minimizing stresses, but would be more complex to manufacture and install than the other embodiments discussed. It is therefore seen that this invention provides a baffling arrangement which effectively baffles reactor coolant flow into and about the core of a nuclear reactor and which effectively reduces thermally induced stresses upon the baffling components. It will be apparent that many modifications and additions are possible in view of the above teachings. For example, mating baffle plates may be arranged at an angle, not necessarily presenting a horizontal upper or lower surface. Also, the number of plates may be varied, as may the geometric configuration and orientation of the mating extensions. It therefore is to be understood that within the scope of the appended claims, the invention may be practiced other than as specifically described.