Patent Number: 053751531
Section: description

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS The present invention provides a coolant vent structure to be used alone in a bundle, in conjunction with a lower water rod, or in conjunction with a combination rod of water and fuel elements, or in conjunction with a part length fuel rod. In the latter case, the arrangement is particularly advantageous for improving CHF performance while retaining the benefits of a part-length fuel rod. The coolant vent duct is thus located above a part-length fuel rod. The duct can include a hollow tube extension that has one or more inlet openings at a bottom thereof and one or more outlet openings at a top thereof. Accordingly, some of the reactor coolant, that is a two phase mixture, passes inside the hollow tube extension. Due to the structure of the coolant vent fuel rod, at least a partial separation of the steam and liquid of the two phase coolant that surrounds the tube is achieved by the hollow tube extension. This allows coolant with a higher steam content to bypass the upper active portions of the fuel assembly. The structure of the present invention reduces the enthalpy rise maldistribution problem by providing active flow subchannels outside of and adjacent to the coolant vent duct that are much smaller than in the case of a part-length fuel rod without a duct. The coolant vent duct also provides an isolated inactive flow path inside of the duct so as to retain a significant part of the pressure drop reduction that occurs with a part-length rod. The coolant vent duct achieves a significant part of the part length rod pressure drop reduction because it provides most of the flow area gain that one achieves with a part-length fuel rod. The coolant vent duct can function as a steam extraction device. Specifically, the geometry of the inlet region of the coolant vent duct can be configured to achieve significant separation of steam from water such that the coolant with the increased steam content flows inside the hollow duct while the remaining coolant with increased liquid water content continues up the active flow channels. This coolant with increased liquid water content enables generally better cooling of the nuclear fuel rods and thus enables improved CHF performance. With either a part-length rod alone or with the coolant vent duct of the present invention, the improvement in pressure drop relative to having full length fuel rods is controllable via the number of such rods and the length of the fuel section of the rods. The trade off on whether to adjust the number or the length is made so as to yield favorable neutronics performance. The coolant vent duct of the present invention allows for fine tuning the pressure drop by using the flow holes in the coolant vent duct to meter the amount of flow that will go inside the coolant vent duct. Referring now to the Figures, and specifically to FIGS. 1 and 2, an embodiment of a combination coolant vent duct and part length fuel rod 10 of the present invention is illustrated. As illustrated, a part-length fuel rod 12 is provided that includes a fuel portion 14 that is located within the cladding 16 of the rod. The upper end of the part-length fuel rod 12 includes an insulator disk 18 that can be preferably constructed from Al.sub.2 O.sub.3. Additionally, the part-length fuel rod 12 and includes a connector member 20 that is received by and extends from an end 22 of the part-length fuel rod 12. The connector member 20 allows a coolant vent duct 23 having an extension tube 24 to be coupled to the part-length fuel rod 12 and disposed axially thereto. In the preferred embodiment illustrated, a horizontal pin 25 is provided for proper rotational alignment. The duct 23 extends vertically to the vicinity of the upper spacer 22 shown in FIG. 1, or to some point above the upper spacer (see FIG. 8). The duct could extend all the way up to the upper tie plate (not shown) in which case there would be a means for fixing the upper end such as an upper end cap 41 and locating pin 42 (see FIG. 8). The connector member 20 may be an assemblage of smaller pieces. Among other functions the connector member 20 can serve as an upper end cap to the part length fuel rod 12 below and as a mechanical connection between the part length fuel rod 12 and the coolant vent duct 23. The extension tube 24 includes a hollow interior 26 that provides a coolant flow path. The duct 23 also includes a transition portion 28 and an upper portion 30 formed together or bonded together. The upper portion 30 has an outer perimeter, or diameter, that is greater than the outer perimeter of the part-length fuel rod 12. Because the outer perimeter of the upper portion 30 and the tube 24 is greater than the outer perimeter of the part-length fuel rod 12, a more radial uniform enthalpy distribution among the active flow channels is achieved. The transition portion 28 has a lower section 32 that has a substantially constant outer perimeter, and has a tapered section 34 thereabove. The tapered section 34 and lower section 32 are formed together or attached together. The lower section 32 has an outer perimeter approximately equal to the outer perimeter of the part-length fuel rod 12. The outer perimeter of the transition portion 28 increases in the tapered section 34 to a point where the upper portion 30 begins. Preferably, at the end of the transition portion 28, there is a sharp break 36 that functions to assist in stripping liquid film off the outside surface of the coolant vent fuel rod 10. As illustrated in FIG. 2, the duct 23 can include a plurality of inlet openings 38 located in the transition portion 28. The inlet openings 38 allow a fluid to enter into the interior 26 of the extension tube 24. In a preferred embodiment, four inlet openings 38 are provided; one inlet opening 38 being aligned with each of the four subchannels that surround the combination coolant vent fuel rod 10. At an upper portion of the extension tube 24, outlet openings 40 are provided. These outlet openings can be a grouping of openings around the tube circumference as shown in FIG. 8. The outlet openings 40 allow fluid to exit the enclosed area 26 defined by the extension tube 24. The principle section of the extension tube 24 could be round, square, or some other shape that provides an open interior. The extension tube surface can have a local distortion 45 at certain elevations to facilitate mechanical interfacing with the spacers 22, 25 and mating with the upper portion 30 or upper end cap 41 if any. This invention has inlet openings 38 either in the extension tube 24 or the transition portion 28 and outlet openings 40 to enable upwards flow of the reactor coolant inside of the duct 23. There may be additional openings over the length of the duct either to admit flow into or exhaust flow from the interior 26 of the duct. The openings could also be simply open end or ends of the duct such as at the top 48 of extension tube 24 shown in FIG. 1. The mechanical connection of the lower part length fuel rod 12 and the upper tube 24 is a desirable feature but not a mandatory feature of this invention. When there is no connection the hollow tube 24 is located above a part length rod and has some means for admitting flow into the region inside the tube at the bottom and for exhausting flow at the top. The spacers provide lateral restraint when this invention is either connected or unconnected to the part length rod. When not connected to the part length rod below, vertical restraint of this invention would be provided by either the spacers or the upper tie plate. For example, locking tabs could be added to the outside surface of the tube or the tube could be distorted locally at one or more spacer elevations in such a way that if the tube is rotated 45.degree. it would lock into place and could not be removed vertically unless rotated again. In simple form, a hollow tube is placed axially above a part length fuel rod, and serves to add hydraulic resistance in a region that would otherwise be a large open subchannel. The addition of hydraulic resistance reduces subchannel flow and thereby provides a means to correct the enthalpy rise maldistribution problem of the prior art (i.e., fuel bundles with part length fuel rods). With a simple hollow tube form of this invention either the top or bottom (or both) ends of the tube might be deformed in such a way as to exert control over the amounts of liquid water and steam flows entering the tube. A transition piece and holes in the sides of the tube may or may not be present. In the embodiment illustrated in FIG. 2, a liquid diversion structure 50 is provided. In FIG. 2, the liquid diversion structures 50 extends outwardly from the transition portion 28, in juxtaposition to the inlet opening 38. In the embodiment illustrated, the liquid diversion structure 50 has a "V" shaped fence. Of course, other shapes and structures can be used to create a liquid diversion. The liquid diversion structure 50, as illustrated, is so constructed and arranged that liquid, such as droplets 52 in the two phase mixture, that are near the surface as well as the liquid film on the surface, are diverted away from the inlet opening 38 while steam 54 is allowed to enter the inlet opening 38. The V-shaped fence 50 diverts the liquid film flowing up the surface of the part length rod 12 below around and away from the inlet opening 38. It also provides an obstacle for steam and liquid drop flow near the surface of the combination coolant vent fuel rod 10 that is headed for the inlet opening. This obstacle is more easily negotiated by the steam than drops. This maximizes the steam quality of the fluid flowing into the interior 26 of the extension tube 24. By causing steam to enter the extension tube 24, and therefore, exit at the outlet openings 40 or 48 of the extension tube 24, one is able to maximize the liquid available for cooling the active fuel rods while maintaining sufficient flow inside the extension tube 24 to achieve the desired reduction and bundle pressure drop. The liquid diversion structures 50 provide at least some separation of vapor from liquid at the inlet opening 38 of the extension tube 24. This allows one to achieve a separation of the two phases as soon as practical after the fuel section of the part-length fuel rod 12. The increased diameter or size of the upper portion 30 of the extension tube 24, with respect to the transition portion 28, also facilitates flow separating inlet designs by allowing the liquid diverting structure 50, or other protrusion, to be located in the vicinity of flow inlet openings 38 without compromising fuel loadability into the bundle. For example, as illustrated in FIG. 2, the V-shaped fence 50 does not protrude beyond the outer circumference of the extension tube 24. Hence, coolant vent duct loading is not compromised. As set forth above, pursuant to the present invention, the transition portion 28 preferably has increasing diameter as one moves upwards and ends with a sharp break 36 to help strip off the liquid film on the outside surface of the rod. Additional film stripping means could be included over the length of the hollow tube 24 to minimize the liquid flow on the surface. The tube extension 24 could be truncated at the top spacer or it could be continued up to the upper tie plate with multiple outlet holes above the last spacer to minimize the hydraulic resistance of the flow leaving the coolant vent duct 10 (see FIG. 8). Referring now to FIG. 3, an embodiment of the coolant vent fuel rod 110 of the present invention is illustrated. In this embodiment, inlet openings 138 are located in a transition portion 128 of this invention. In this embodiment, the openings 138 are located in a lower section 132 below a tapered section 134 of the transition portion 128. As illustrated, in this embodiment, several inlet openings 138 are located one after the other. Each opening 138 includes a liquid diversion structure 150 located in juxtaposition to the opening. Again, the structure 150 limits the liquid that enters the openings 138. In all other aspects, the coolant vent duct 110 is similar to the embodiment illustrated in FIGS. 1 and 2. By having several inlet openings 138 one after the other, the openings provide a larger total inlet flow hole area if this is necessary to achieve sufficient flow inside the coolant vent fuel rod to achieve the necessary reduction and bundle pressure drop. Referring now to FIGS. 4 and 5, a further embodiment of a coolant vent duct 210 of the present invention is illustrated. In this embodiment, within an enclosed interior 226 defined by an extension tube 224, a structure 227 is located for separating water droplets from steam in the two phase mixture. Once separated, at least a portion of the separated water can be transferred to the surface of adjacent fuel rods increasing the critical heat flux (CHF) capability. To this end, openings 240 are provided along the length of the extension tube 224 to facilitate the transfer of the water that is separated to adjacent fuel rods. The structure 227 for separating the water from the steam can be, as illustrated in FIG. 5, a device for imparting centrifugal force to the two phase mixture. In the embodiment illustrated in FIG. 5, the device includes a louver 227 that is formed into the wall of the extension tube. Referring now to FIGS. 6 and 7, a further embodiment of the coolant vent fuel rod 310 is illustrated. In this embodiment, a structure 327 for separating steam from liquid is a twisted ribbon 327 located within an interior 326 defined by an extension tube 324. As in FIG. 4, separated water exits through one or more vent holes facilitating its transfer to adjacent fuel rods. In addition to the internal devices for imparting centrifugal force illustrated in FIGS. 4-7, other devices and surfaces for causing agglomeration of water droplets can be used. For example, a spacer can be used. Surfaces that cause a rapid change in direction or configurations can also facilitate separation through gravity, surface tension, and other natural forces. Referring now to FIGS. 8 and 9, there is another embodiment of the coolant vent duct. In this embodiment a connector/transition piece 414 is provided having a central flared section 415. A part length fuel rod 412 mounts axially thereon a coolant vent duct 416. Inlet holes 427 are located at the bottom end of the extension tube 416. The connector/transition piece 414 provides separation of the liquid water and steam. A liquid film 462 is shown moving up the part length rod 412 surface and onto the surface of the connector/transition piece 414. Once on the transition piece 414, this film is brought in inward direction 464 by a reduction in transition piece diameter 464a and then is rapidly redirected in radially outward direction 465. A sharp break 466 in the surface contour causes the film to separate and to continue to flow in a radially outwards direction as a thin sheet of liquid water 468. The liquid sheet of water 468 and approaching liquid drops 470 collide and outwards radial momentum is imparted to the liquid drops such that downstream of the separator device, liquid 481 is moving radially outwards while steam vapor 482 is moving inward towards the inlet holes 427. Above the outwardly flared section 415 of the transition piece 414 the diameter is again reduced 474 so that a naturally forming eddy flow 476 behind the separating sheet of liquid 468 intersects the separating sheet 468 in a more parallel fashion as opposed to a more perpendicular fashion, at the film separation point, the break 466. A second flared portion 478 of the transition connector piece 414 repeats the process for any residual liquid. If the initial flared portion 415 is not present, the second flared portion 478 becomes the primary means of directing liquid outward and away from the inlet holes 427. The initial diameter reduction 464a at the bottom end of the transition connector piece is done smoothly so that the liquid film remains attached to the surface until it reaches the intended separation point, the break 466. The diameter is reduced ahead of the flared section so that the flared section presents a greater frontal area to the main flow stream and therefore has a greater interaction with it. The flared surfaces are rounded or curved as opposed to being straight conical or tapered surfaces so as not to impede the flowing film by trapping it in any sharp concave corners, upstream of the intended separation point 466. As shown in FIG. 9, the transition connector piece is a surface of revolution about a vertical axis down the vents. While this is easy to fabricate, sections other than round sections, for example square sections, would likely function in a satisfactory manner. Also the flared section might be sectioned into several pieces that are displaced axially from one another. In the embodiments shown in FIGS. 8 and 9, connector/transition piece 414 which functions to separate liquid water and steam is mounted at one end to a part length fuel rod 412 and at its other end is connected to the coolant vent duct 416. As contrasted to functioning, in part, as a mechanical connection between the part length fuel rod and the coolant vent duct, the connector/transition piece can instead serve as an upper end cap of the part length fuel rod. In a further embodiment of the present invention, a reflex upper end cap or fitting 2201, is connected to the part length fuel rod and is shown in FIG. 16. The reflex upper end fitting is shown connected to a part length fuel rod without using either a connector/transition piece and/or a coolant vent duct. The reflex upper end fitting has a flared section which is truncated at a sharp break. As coolant/moderator flows through the fuel assembly and along the part length fuel rod, a liquid film is formed on the surface of the part length fuel rod. The liquid film moves up the part length fuel rod and onto the reflex upper end fitting and is then directed towards surrounding fuel rods. In the case of light water reactors, upwards flowing liquid water drops in the two phase flow collide with the continuous liquid film sheet as it is moving towards the surrounding fuel rods and is imparted with an outwards radial momentum, and together with the liquid film sheet impinge upon the surrounding fuel rods. The reflex upper end fitting thus increases the amount of liquid coolant/moderator on and near the surface of the surrounding fuel rods while steam vapor flows into the large open subchannel above the top of the part length fuel rod. Reflex upper end fitting 2201 shown in FIG. 16 is the portion of connector/transition 414 shown in FIG. 9 from and including sharp break 466 upstream to the part length fuel rod 412. FIG. 16 shows reflex upper end fitting 2201 in which corresponding elements in each of FIGS. 9 and 16 have the same reference numbers. Reflex upper end fitting 2201 is connected to part length fuel rod 412 and in the present embodiment is shown without a coolant vent duct. As shown in FIG. 16, a single flared section 415 is truncated at sharp break 466 and liquid film 462 is shown moving up the surface of part length fuel rod 412 onto the surface of the reflex upper end cap 2201. Once on the reflex upper end cap, liquid film 462 is brought in inward direction 464 by a reduction in diameter 464a and then is rapidly redirected in radially outward direction 465. Sharp break 466 in the surface contour causes liquid film 462 to separate from the reflex upper end fitting 2201 and to continue to flow in a radially outwards direction towards the surrounding subchannels and fuel rods as a thin sheet of liquid water 468. Liquid sheet of water 468 and approaching liquid drops 470 collide in the flow stream and outwards radial momentum is imparted to liquid drops 470 such that downstream of the reflex upper end fitting 2201, liquid 481 is moving radially outwards towards the surrounding fuel rods 2220 and surrounding subchannels while steam vapor 482 concentrates in the large open subchannel above the reflex upper end fitting. After impact with water drops, sheet 468 is no longer continuous and begins to break up thereby enabling vapor to pass through it to the large open subchannel above the reflex upper end fitting. Eddy flow 2210 is formed behind sheet 468 and intersects the sheet 468 in a parallel fashion at sharp break 466. The movement of liquid onto and around the surrounding fuel rods 2220, and the concentration of vapor in the large open subchannel above the part length fuel rod and away from surfaces of the surrounding fuel rods, results in an enthalpy reduction for the flow on and near the surface of the surrounding fuel rods. In order to facilitate the insertion and/or removal of a part length fuel rod having a reflex upper end fitting into a fuel assembly as well as to facilitate fuel assembly fabrication, it may be desirable that the maximum outside diameter of the reflex upper end fitting not exceed that of the part length fuel rod to which it is connected. As discussed previously, in order for liquid film 462 which flows upwards on the surface of part length fuel rod 412 to separate from the surface of the end fitting as a thin continuous sheet, the reflex upper end fitting has an outwardly flared section 415 which ends in sharp break 466. In order to both facilitate fuel rod loading into a fuel assembly as well as to have the liquid film separate as a thin continuous sheet from the surface of the end fitting, the shape of the reflex upper end fitting includes a reduction in diameter 464a, upstream of outwardly flared section 415 and sharp break 466. As a practical matter, with respect to the embodiments illustrated in FIGS. 1-3, 8 and 9, the separation of steam from water at the coolant vent duct inlet openings may be less than 100%, i.e., 100% steam may not be flowing into the interior of the coolant vent duct. Therefore, if desirable, the concepts illustrated in FIGS. 1-3, 8 and 9 and 4-7 can be combined. Accordingly, even though liquid diversion means are located on an outer wall of the extension tube, additional flow separation means can be enclosed inside the coolant vent ducts as set forth in the embodiments illustrated in FIGS. 4-7. Further embodiments of connector/transition pieces are shown in FIGS. 10-15. These embodiments can be used alone to mount beneath a coolant vent duct or can be used such as shown in FIG. 8 to mount a coolant vent duct 416 to a part length fuel rod 12. All these embodiments can be used to mount a coolant vent duct 416 to a water rod or a water/fuel rod or the like. FIG. 10 shows a transition/connector 500 having a hollow perforated tube portion 504 mounted axially above a substantially solid connector portion 506. The hollow tube portion 504 can be fashioned having a top socket portion 510 which interfits inside the coolant vent duct 416. The socket portion 510 has a top open end 512. The connector section 506 has a bottom socket portion 516 for mechanical insertion and connection to a rod therebelow. The transition section 506 has a taper 520 in upward direction, an elongate neck portion 522 terminating in a sharp expansion 524 with a sharp break 526 at the intersection with the hollow tube portion 504. The connector 506 would behave similarly as the lower one half of the connector 414 as shown in FIG. 9. FIG. 10 shows the hollow tube portion 504 having a plurality of holes 530 thereinto. The holes 530 can merely be openings around a circumference of the cylindrical tube wall. Optionally, the holes can have a cylindrical protruding rim 534 or a beveled protruding rim 536. These protruding rims 534, 536 or variations thereof could be applied to any of the embodiments if desired. The transition connector 600 shown in FIG. 11 is substantially the same as the transition connector 500 shown in FIG. 10 except that a helical fin is attached to and wound around a connector section 604. The connector section 604 provides a first taper 608, a neck section 610 and a sharp expansion 612 terminating in a sharp break 614 at an intersection between the connector section 604 and a hollow perforate tubular section 620 thereabove. The transition/connector 600 provides a open top socket 622 similar to the socket 510 in FIG. 10. A bottom socket portion 626 can be provided for connection to a rod below. Another embodiment of the transition/connector is shown as transition/connector 700 in FIG. 12. This embodiment is similar to FIGS. 10 and 11 and can provide an open top socket 704 at a top end thereof and a mechanical connection socket 706 at a bottom end thereof. The transition connector is substantially hollow. A helical pad 708 is formed or wrapped around an elongate neck 710 of the transition/connector 700. Arranged along the helical pad 708 are openings 712. The elongate neck section 710 terminates at an upper end in a sharp expansion 720 and at a lower end in a taper 722. The opening 712 communicate into the hollow neck section 710 where steam vapor can be carried upward through the elongate neck section 710 and out of the open top socket 704 into a coolant vent duct 416 arranged thereabove (not shown). Another embodiment of a transition/connector 800 is shown in FIG. 13. In this embodiment a substantially hollow neck section 802 has an open top socket portion 804 for interfitting into a coolant vent duct 416 thereabove (not shown). A bottom socket 806 is provided for mechanical interconnection with a rod below. Adjacent the socket 806 is a sharp expansion 808 and a sharp break 810 into the hollow neck section 802. A plurality of openings 816, in this embodiment shown as square, are provided. Above each opening is a cone-shaped depression 820 which tapers down in upward direction to the surface of the elongate neck section 802. Another embodiment of a transition/connector 900 is shown in FIG. 14. In this embodiment, a perforated hollow tube section 902 having a top socket with an open top 904 thereabove is mounted axially above a connecting portion 906 having a plurality of helical veins 908 arranged protruding from a tapered neck section 910. The tapered neck section tapers inwardly from its bottom to a central area and outwardly thereafter up into hollow tube portion 902. An attachment socket 914 is provided below the neck section 910. The hollow tube portion 902 is provided with openings 920 for inlet of steam to progress upwardly inside the tube portion 902, out of the top socket 904 into a coolant vent duct 416 mounted thereabove (not shown). Another embodiment of a connection portion 950 is shown in FIG. 15. In this embodiment, the connector 414 of FIG. 9 is elongated and means for separating the upwards flowing liquid surface film is a ridge 952. The embodiments for the connector/transition pieces shown in FIGS. 10-15 could also be used in combination with the embodiments of FIGS. 4-7, thus providing liquid diversion means on both an outside and on an inside of the transition/connection piece or the coolant vent duct. The present inventive coolant vent duct need not be associated with a part-length fuel rod. It could be used in conjunction with a water rod or some combination of water and fuel elements. A bundle containing one or more coolant vent ducts can use these ducts in conjunction with water rods, water channels, and the like. The inlet elevation of the coolant vent duct can be at any elevation along the active length of the bundle where there is available steam to be separated from liquid coolant. A coolant vent duct bundle can contain one or more individual coolant vent ducts whose design need not all be identical. For example, each coolant vent duct in the bundle might have a different inlet elevation. The shape of inlet and outlet openings can be various. Examples of inlets are round holes, rectangular holes, or openings of some other regular or irregular shape. The surface of the duct can be distorted in the vicinity of the inlets to enhance separation of steam from liquid, or to reduce pressure drops. It should be understood that various changes and modifications to the presently preferred embodiments described herein will be apparent to those skilled in the art. Such changes and modifications can be made without departing from the spirit and scope of the present invention and without diminishing its attendant advantages. It is therefore intended that such changes and modifications be covered by the appended claims.