Patent Number: 047939664
Section: description

DETAILED DESCRIPTION OF THE EMBODIMENTS The apparatus shown in the drawings is a nuclear reactor 11. The reactor 11 is of the light-water pressurized type (PWR). A PWR is presented here only for the purpose of describing this invention with reference to a concrete embodiment. To the extent that this invention is applicable to reactors of other types, such as boiling-water, liquid-metal, or heavy-water reactors, such application is within the scope of this invention. The reactor 11 includes a pressure-tight vessel 13 of conventional construction. The vessel 13 includes a plurality of inlet nozzles 15 for conducting the coolant into the vessel 13 and a plurality of outlet nozzles 17 for conducting coolant out of the vessel. Typically, as shown, there are two pairs of two inlet nozzles 15 each, and two pairs of two outlet nozzles 17 each, interposed between the inlet nozzles. A shell section 19 is sealed (welded) to the inner wall of the vessel opposite each pair of inlet nozzles 15. The shell sections 19 prevent the coolant from being injected into the upper regions of the vessel, essentially guiding the coolant to the lower regions of the vessel. Within the vessel there are the lower internals 21 (FIG. 3), which include the nuclear core 22, and the upper internals 23. The core 22 includes the fuel assemblies 25 which include thimbles 27 for receiving the neutronabsorbing rod control clusters 29, the grey rod control clusters 31 and the water displacement control-rod clusters 35 (FIG. 2). The fuel assemblies 25 are mounted between upper and lower supports 39 and 37 which are sometimes referred to as upper and lower core plates. The structure of the lower internals 21 is conventional. The upper internals (FIGS. 1, 2) include the cruciform guides 41 for the RCC's and for the grey rods. These guides 41 include a plurality of plates 43 which extend vertically from the walls of the arms of the cruciforms 41 and which are formed to guide the RCC's 45 or grey rods 47 (FIG. 1). The guides 49 for the WDRC's 35 are contained within generally square or rectangular shells 51. The sides of each shell 51 are parallel to and extend along inwardly extending arms 53 of four adjacent cruciform guides 45 or 47. The guides 49 are formed in plates 55 which extend vertically along the shells 51. The cruciform guides 41 and the shells 51 have slots 57 (FIG. 3) through which coolant flows. The plates 55 also have holes 59 (FIG. 2) which are penetrated by coolant. In this application and in its claims, the cooperative parts or assemblies of the reactor, including the guides 41 and 49, the shell 51, the WDRC's 35, the RCC's 45, and the grey rods 47, are referred to as controlrod assemblies. In the normal operation of a nuclear reactor, during the earlier part of the fuel cycle, the WDRC's 35 are in the core 22, the RCC's 45 are in the upper internals and the positions of the grey rods 47 are dependent on the loading. The coolant flows inwardly through the inlet nozzles 15 as depicted by the arrows 60 (FIGS. 1,3) and through the annular sections 61 between the sections 19 and the wall of vessel 13. In this annular section, the coolant is guided downwardly into plenum 63 (FIG. 2) whence the coolant flows upwardly through lower core plate 37, the core 22 and the upper core plate 39, as depicted by the arrows 65. The coolant then flows into the upper internals, out of the guides, and out through the outlet nozzles 17. In the region of the upper internals 23 the flow is deflected at right angles and the coolant flows through the upper internals generally transversely, as depicted by the arrows 67. Coolant flowing at a high speed transversely to the guides 41 and 49 and shells 51 of the upper internals and the WDRC's 35, as occurs in apparatus in accordance with prior-art teaching, would cause these components to vibrate or be subject to high stress and to fail. The upper internals 23 are encircled by a shroud 71 (FIGS. 1,3,4,5) which is interposed between the upper internals and the nozzles 15 and 17. The shroud 71 has holes 73 except in the regions 75 opposite the outlet nozzles 17 as shown in FIG. 4. Flow of coolant directly through the outlet nozzles 71 is thus prevented. In the shroud 71, shown in FIG. 4, the holes 73 are generally of the same area and are generally equally spaced except in the regions 75. In the shroud 81, shown in FIG. 6, the holes 83 progressively increase in area vertically from the bottom of the shroud to the top. In the shroud 91, shown in FIG. 7, the holes 93 are of the same area but increase in number progressively vertically from the bottom to the top. The concepts of FIG. 6 and FIG. 7 may be combined. The holes 93 may increase progressively in number and area from the bottom to the top. In the interest of clarity, hole-free regions which are opposite the outlet nozzles 17 are not shown. Shrouds 81 and 91 include such regions. At the periphery of the internals, the groups 101 of residual WDRC's extend only between two or three outer arms 103 (FIG. 1) of the cruciform grinder. These peripheral WDRC groups are enclosed in irregularly shaped shells 105 (FIGS. 1,5). The shells 105 have correspondingly shaped plates 107. Annular plates or barriers 109 extend vertically between the shroud 71 and the outer walls 111 of the peripheral shells. There is a small gap 113 between each plate and the wall 111. The purpose of these barriers is illustrated in FIG. 5. The coolant confined by the plates 107 decreases in pressure progressively in pressure from the lower region 115 defined by the plates 107 to the upper region 117. Different pressures are depicted in FIG. 5 for illustrative purposes. In the absence of the barriers 109, the high pressure at the bottom would drive the coolant to the upper region of the space between the shroud 71 and the peripheral shells, limiting the volume flowing transversely to the control-rod assemblies and increasing the velocity of the coolant. The barriers 109 preclude such flow. In the practice of this invention the outflowing coolant is distributed over a large volume and its velocity is reduced to a low magnitude. While preferred embodiments of this invention have been disclosed herein, many modifications thereof are feasible. This invention is not to be restricted except insofar as is necessitated by the spirit of the prior art.