Patent Number: 055531075
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the drawings in detail and in particular to FIG. 1 there is shown a nuclear reactor vessel 10 having a generally cylindrical vessel body 12 and a removable head 14 for heating coolant recirculating between the reactor vessel 10 and one or more steam generators (not shown). Commercial nuclear reactors normally are coupled with two to four steam generators. The coolant, which generally is deionized water containing parts per million quantities of boron and lithium, flows from a steam generator (not shown) through a large cold leg pipe 20 and into a cold leg nozzle 22 of the reactor vessel 10. The coolant flows downwardly through the annular space defined by the vessel body 12 and a generally concentric core barrel 24. The coolant then flows upwardly past the fuel assemblies, including interior assemblies (shown as one assembly 26) and peripheral assemblies (shown as one assembly 28) in the core 30, into the upper plenum 32, out of the reactor vessel 10 through a hot leg nozzle 34 into a hot leg pipe 36 and back to the steam generator. As employed herein, "peripheral" assemblies refer to those assemblies near the core barrel 24 and "interior" assemblies refer to the other assemblies in the interior of the array. The upper plenum 32 is generally defined by an upper core support plate 40 and an upper support plate 42 within the core barrel 24. Both support plates 40 and 42 are supported on the core barrel 24, which in turn is supported on the vessel body 12. The upper support plate 42 is further supported by peripheral support columns (shown as peripheral support column 46) and interior support columns (shown as interior support column 48) extending from the upper core support plate 40. As employed herein, "peripheral" support columns refer to those columns adjacent the core barrel 24 and "interior" columns refer to the other columns in the interior of the array. The interior support column 48 shown in FIG. 1 generally illustrates a well known column design having a cylindrical vertically extending length with downwardly extending support legs 50. The interior support columns normally are aligned with passageways 52 in the upper core support plate 40 and passageways 54 in the upper support plate 42 which permit incore instrumentation 56 to extend from the head 14 into the core 30. Peripheral support columns having a similar design generally have vertical rods rather than cylinders because they are not employed to protect the instrumentation 56. The passageways 52 may have diameters of up to about six inches or more. This type of design also permits some of the coolant to flow through the passageways 52 and into the lower region of the upper plenum 32. Another known support column design has a vertically oriented cylinder with flanges which contact the support plates 40 and 42 in the manner that the peripheral support column 46 shown in FIG. 1 contact the support plates 40 and 42. However, unlike the support column 46 of FIG. 1 (which has a series of peripheral slots 62 in its upper section 64 and an unslotted lower section 66), the earlier design provided a series of peripheral slots along the entire length of the cylinder which introduced coolant into the lower region as well as the upper region of the upper plenum. As is shown in FIG. 1, the upper plenum 32 also contains guide tubes 70 having bottom slots 72 extending therethrough. These tubes 70 permit control rod drives 74 to position the rod control clusters 76 in the core 30. The slots 72 are sized so that most of the coolant in the core region 30 flows into the upper plenum 32 through the bottom slots 72. The upper portion of the guide tubes 70 are unslotted so that streams of coolant flow into the lower region of the upper plenum 32 below the level of the hot leg nozzle 34 and mix together in the interior of the array as they flow toward the hot leg nozzle 34. FIG. 2 generally shows the relative locations of the peripheral support columns 46, the interior support columns 48 and the guide tubes 70 (shown in FIG. 1) in one quadrant of the upper plenum 32. The relative locations of columns 46 and 48 and tubes 70 will be the same in the other quadrants. FIG. 3 generally shows the calculated temperature of streams of coolant flowing from a core region 30 in which the fuel assemblies have been loaded in a low leakage pattern with the lowest enriched fuel in the central portion of the core and the highest enriched fuel in the surrounding portion of the core with low enriched peripheral fuel assemblies 28 (shown in FIG. 1) adjacent the core shroud 24. As may be seen in FIG. 3, the coolant temperature at the peripheral locations will vary from about 577 to about 591 degrees Fahrenheit and the highest coolant temperatures in the interior will vary from about 608 to about 635 degrees Fahrenheit. The inventors of the present invention have hypothesized that hot leg streaming from reactor vessels having low leakage loading patterns can occur due to the flow of relatively cool coolant through the lower portions of peripheral support columns toward the lower portion of the hot leg nozzles 34 without sufficiently mixing with the bulk of the coolant flowing from the interior region of the upper plenum. Thus, they introduce relatively cold coolant flowing from passageways in the upper core support plate 40 above the peripheral fuel rods 28 into the upper regions of the upper plenum 32 so that the relatively cold coolant can mix with the hotter coolant from the interior portions of the upper plenum 32. Accordingly, reactor vessels 12 of the present invention have peripheral hollow support columns with slots 62 in their upper portions 64 and unslotted lower portions 66. Preferably, the slots 62 comprise at least about 50% of the peripheral area of the upper section 64, and most preferably from abut 50% to about 66% of the peripheral area of the upper section 64, to provide a large flow area for introducing the relatively cold coolant into the bulk of the coolant at the lowest practical velocity. As used above and hereafter in the following claims of invention, the "upper portion" of a peripheral support column refers to the upper half of a support column and the "lower portion" of a support column refers to the lower half of a support column. Preferably, however, the slots 62 of the peripheral support columns 46 do not extend below the bottom of the hot leg nozzle 34. As is shown in FIG. 1, the slots 62 of the peripheral support columns 46 are preferably above the slots 72 of the guide tubes 70. The inventors conducted several analytical hot leg steaming studies using computational fluid dynamics to test their hypothesis using the core exit temperatures of FIG. 3 as input to a standard four loop upper plenum. A top slotted peripheral support column design embodying the present invention was compared with a standard unslotted peripheral support column design and a full slotted peripheral support column design. For each case, the mixing effectiveness was measured in terms of the range of coolant temperatures in the hot leg cross-section at the resistance temperature detectors (Temp. Range) and the standard deviation of the coolant temperatures in the hot leg cross-section at the resistance temperature detectors (Temp. Dev.). The mixing effectiveness of each design increases as each parameter decreases. The comparison is shown in the following table: ______________________________________ PERIPHERAL TEMP. TEMP. COLUMN DESIGN RANGE (.degree.F.) DEV. (.degree.F.) ______________________________________ No Slots Along Length 5.41 1.41 Slots Along Entire Length 5.97 1.41 Slots Only In Upper Portion 3.61 0.96 ______________________________________ As this table shows, the column design of the present invention (wherein only the upper portion of the peripheral columns are slotted) enhances thermal mixing by about 30-40% in the hot leg. While a present preferred embodiment of the present invention has been shown and described, it is to be understood that the invention may be otherwise variously embodied within the scope of the following claims of invention.