Patent Number: 060552883
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the drawings in detail and in particular to FIG. 1 there is shown a conventional nuclear reactor vessel 10 which may be employed in commercial pressurized water reactors for commercial electric power generation. The reactor vessel 10 generally has a vessel shell 12 with a removable head 14 fastened thereto by a plurality of nuts 16 on studs 18. The vessel shell 12 has at least one reactor inlet nozzle 20 and at least one reactor outlet nozzle 22 in fluid flow communication with a nearby steam generator (not shown). In commercial electric power generating plants, a reactor vessel 10 will normally be coupled with two, three or four steam generators and receive primary coolant from reactor coolant pumps associated with the steam generators. Thus, there will normally be one inlet nozzle 20 and one outlet nozzle 22 for each steam generator. A core barrel 24 is suspended from an internal ledge (not shown) and is radially supported at a lower support plate 26 by radial supports 28 attached to the reactor vessel shell 12. The core barrel 24 supports a plurality of fuel assemblies (illustrated by fuel assembly 36) in a core region 30 which is generally defined by a lower core plate 32 and an upper core plate 34. The core region 30 may support up to 150 or more fuel assemblies surrounded and supported by a baffle assembly 38 including a substantially vertical baffle plate 40 (which may be comprised of a plurality of smaller plates which are welded or bolted together) fastened to generally horizontal former plates 42 by baffle plate/former plate bolts 50. Bolts 50 are shown in FIG. 1 as protruding into the core region 30 for purposes of illustration but Generally do not protrude as shown in commercial reactor vessels 10. The former plates 42 may be welded or bolted to the core barrel 24. The primary coolant fluid flows into the reactor vessel 10 from a reactor coolant pump (not shown) through the reactor vessel inlet 20, downwardly through an annulus know as a "downcomer" between the reactor vessel shell 12 and the core barrel 24, upwardly through holes 46 in the lower plate 26, through the core region 30 into an upper plenum 48, and out through reactor vessel outlet 22 to a steam generator (not shown) and then back to the reactor coolant pump. The principal functions of the recirculating coolant fluid are to absorb heat generated by the fuel assemblies 36 and to cool the reactor vessel 10 and its internal structure. FIGS. 2 and 3 illustrate a baffle plate 40/former plate 42 joint design having a modified baffle/former bolt 50 which permits the primary coolant to flow past a modified bolt head portion 52 to cool the bolt 50 and to wash the undersurface 54 of the bolt head portion 52. The undersurface 54 may be the underside of a flanged section of the head portion 52 (as is shown and is preferable) or may be the undersurface of an unflanged head (as shown in U.S. Pat. No. 4,069,102, although not shown herein) or the undersurface of a separate washer (not shown). The baffle plate 40 has a countersunk hole 56 having a diameter extending to a smaller diameter bolt hole 58, and the former plate 42 has a bolt hole 60 aligned with the baffle plate hole 58. The bolt 50 has a shank 62 extending from the undersurface 54 of the bolt head portion 52 into the aligned bolt holes 58,60 to a threaded distal end 64 which threadedly engages the former plate 42. When the baffle assembly 38 is fastened together, the underside 54 of the bolt head portion 52 seats against countersunk surface 64 of the baffle plate 40 and forms a crevice therewith. The bolt head portion 52 has one or more spaced apart peripheral slots 68 which, together with the baffle plate countersunk surface 64, defines fluid flow passageways such as orifices 70 between the countersunk hole 56 and the baffle plate bolt hole 58. The orifices 70 may be sized to permit the primary coolant to flow into and out of the bolt hole 58 and past the bolt head portion underside 54 so that any steam which may form will be vented, any solids which may tend to deposit near the crevice under the bolt head portions 52 will be washed away and the undersurface 54 of the bolt head portion 52 will be cooled. Alternatively and/or additionally, the fluid flow passageways may be formed by one or more holes through the head portion 52 externally of the shank 62 as illustrated by holes 72. FIGS. 4-9 illustrate other embodiments of the present invention where one or more fluid flow passageways may be machined into the baffle plate 40 and the former plate 42 while the baffle assembly 38 is assembled in the reactor vessel. FIGS. 4-7 illustrate embodiments of the present invention wherein two slots 80,82 and 84,86 are machined into the baffle plate 40 by any suitable means such as electro-machining. The slots 80,82 have the same cross-sectional areas whereas slots 84,86 have different cross-sectional areas for providing different flow rates. The slots 80,82 and slots 84,86 extend to slots 90, 92 and 94,96 (having far sides 91, 93 and 95, 97, respectively) in the former plate 42, which permits the primary coolant to flow from one side of the baffle plate to the other side. These slots are sized, positioned and designed to prevent excessively high hydraulic forces of the flowing primary coolant from impinging upon the adjacent fuel assemblies 36. Advantageously, the slots 90,92 and 94,96 may be cut very narrowly and substantially as long as the shank 62 for cooling the bolts 50. FIGS. 8-9 illustrate another embodiment of the present invention wherein two spaced apart slots 100-102 are machined into the baffle plate 40 for controlling the flow of primary coolant from one side of the baffle plate 40 to the other. Slot 100 extends to a slot 104 (having a far side 105) machined into the former plate 42 whereas slot 102 is only partially machined through the baffle plate 40 (extending to an interior far side 103). FIGS. 8-9 also illustrate a design modification where, in addition to one or more fluid flow passageways illustrated by FIGS. 2-9, a fluid flow passageway extending internally of the head portion 52 and the shank 62, as shown by phantom hole 110, may be formed through the bolt 52 for communicating between the countersunk hole 56 and the bolt hole 60 in the former plate 42. Advantageously, an internal hole 110 may extend toward the distal end 64 of the bolt for washing the threads. However, and as has been noted above, bolts having internal passageways in their shanks are difficult to ultrasonically inspect and the internal passageways in the shank may affect the strength of the bolt. FIGS. 8-9 also illustrate an embodiment wherein elongated (above and below the former plates 42) fluid flow passageways, such as phantom slots 106,108, are machined in the baffle plate 40. Advantageously, such a design permits cooling fluid flow from one side of the baffle plate 40 to the other without having to machine or drill holes through the relatively heavier former plates 42. For example, the baffle plates 40 may be on the order of two inches thick whereas the former plates 42 may be on the order of four inches thick. Also, the cross-sectional areas of the slots 106,108 may differ. FIGS. 10 and 11 illustrates a locking cup 110 which may be employed to engage is the slots 99, 100, 101 or 102 in the baffle plate 40 shown in FIG. 8 in order to prevent loosening of the baffle/former bolts 50. As is shown, the locking cup 110 may be a peripheral bowl-shaped extension 114 integral with or welded to the flange on the bolt head portion 52, which may be wedged or otherwise deformed as shown at 116 and 118 to tightly engage fluid flow slots 100 and 102, respectively. In other embodiments, the locking cup 110 may be separate from the bolt head portion 52 in accordance with previously employed locking cup designs. In addition, the locking cup 110 may be crimped into an irregularly shaped feature of the baffle plate. See, e.g., the locking design of U.S. Pat. No. 4,683,108, which is incorporated by this reference. To backfit a reactor vessel 10, the reactor is taken out of service, the vessel is submerged in a pool, e.g., of refueling water, the top 14 removed, the fuel assemblies 36 removed, and the existing bolts removed. The existing bolts may then be replaced by modified bolts 50 illustrated in FIGS. 2-4 and, if preferred, locked on with a locking cup. In some cases it may be desirable to enlarge the countersunk hole 56 and the bolt holes 58,60 to provide for more cooling or lower flow velocitites. The reactor vessel 10 may then be refueled and restarted. In alternative situations where the fluid flow passageways are formed by machining the baffle plate 40, the baffle plate 40 (and in some cases the former plate 42) may be machined after the existing bolts are removed, the same or similar bolts 50 installed and the reactor vessel 10 is then refueled. While various present preferred embodiments 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.