Steam vent tube for boiling water reactor fuel assembly

Abstract: In a fuel bundle (B) for a boiling water reactor having one or more part length rods (22) in the two phase region, a steam vent tube (T1) is introduced overlying the part length rods (22). This steam vent tube has openings (44,46) and devices distributed along its length to encourage steam flow interior of the tube and remove liquid flow from the interior of the tube. The steam flow within the vent tube (T1) eliminates the interface drag between the steam interior of the tube and the surrounding water steam mixture. The presence of the steam vent tube as a high velocity steam escape path enables remaining portions of the fuel bundle to contain a higher liquid moderator fraction with flatter axial voids and power distributions during the operating state of the fuel bundle. The presence of high velocity escaping steam combined with distributed apertures along the length of the steam vent tube promotes resistance to fluid oscillations within the fuel bundles. Variations of the steam vent tubes are disclosed including steam vent tubes overlying multible clustered part lenght rods. There results a fuel bundle design in which the entire upper cross section of the fuel bundle is devoted to steam generation and coolant outflow.

Description: STEAM VENT TUBE FOR BWR FUEL ASSEMBLY This invention relates to nuclear fuel bundles having part length rods. BACKGROUND OF THE INVENTION Ueda Japanese Patent Showa 52-50498 disclosed the use of part length fuel rods for the creation of an improved fuel to moderator ratio in the upper two phase region of a fuel bundle, especially in the cold shut down state of the reactor. A fuel bundle was disclosed in which clustered part length fuel rods defined a large, central, generally conical shaped void in the upper two phase region of the fuel bundle. Two embodiments were disclosed in the Ueda reference. A first embodiment includes a large conical water rod occupying the large central conical void defined collectively by the part length rods. A second, and apparently preferred embodiment, disclosed the conical region otherwise unoccupied. Regarding this latter design, testing has established that while nuclear improvements in the upper two phase region in the cold state might be realized, adverse heat transfer performance, especially in terms of adverse critical power may be realized by large central void regions in a boiling water reactor fuel bundle. Specifically, the large defined void results in vapor being concentrated to the region. Unfortunately, surrounding portions of the two phase region tends to flow into the steam vent area. This results in the diversion of significant amounts of liquid coolant away from the heated rod surfaces adjacent the void, this liquid coolant being entrained in the accelerated steam flow within the large defined conical void. There results a reduced flow adjacent the full length fuel rods which surround the large void. This reduced flow rate has a corresponding reduced critical power on the rod surfaces adjacent the void. overall fuel bundle efficiency is reduced. In EP-A-0,336,203 entitled Two-Phase Pressure Drop Reduction BWR Assembly Design, corresponding to US Patent Application Serial No. 07/176,975, filed April 4, 1988), we maintained the nuclear benefits and removed the adverse thermal hydraulic effects by using generally smaller open flow channels dispersed across the two phase region of the fuel bundle lattice. The dispersed flow channels realized the natural tendency of the vapor phase of the two phase mixture to migrate ("drift") toward the low resistance flow paths where the realized flow was primarily vapor. It has been found that such dispersed flow paths are favorable for a BWR fuel assembly since preferential diversion of vapor away from the fueled rods has combined nuclear, stability, and thermal-hydraulic advantages. An object of the present invention is substantially to overcome the above-mentioned problem in respect of JP-A-52,050,498 in a manner different to that disclosed in EP-A-0,336,203. SUMMARY OF THE INVENTION According to the present invention there is provided, as defined in claim 1, a fuel bundle for a boiling water reactor having: a plurality of vertically aligned spaced apart fuel rods for forming a fuel rod group within said fuel bundle for generation of a fission reaction in the presence of water moderator, a lower tie plate for admitting water moderator through said lower tie plate to the Interstitial volume between said fuel rods and supporting said vertically aligned and spaced apart fuel rods, an upper tie plate for permitting water and steam to be discharged from the top of the fuel bundle and maintaining vertically aligned and spaced apart full length said fuel rods in upstanding spaced apart side-by-side relation, a surrounding fuel channel for confining moderator flow along a path over the fuel rods and from said lower tie plate to said upper tie plate; at least one of said fuel rods being a part length rod extending from the lower tie plate vertically less than the full length to the upper tie plate ending interior of the fuel bundle at a disposition wherein the upper end of at least one said part length rod defines with respect to surrounding said full length fuel rods an empty volume overlying said partial length rod(s) under said upper tie plate, said fuel bundle being characterised by: at least one steam vent tube overlying at least one of said part length rods; means supporting said steam vent tube in said volume overlying said part length rod(s), said steam vent tube being supported in the volume of said fuel bundle between an end of said part length rod(s) and said upper tie plate; said steam vent tube defining an opening disposed to said end of said part length rod(s) for the receipt of vapor moderator within said void overlying said part length rod(s); said steam vent tube further defining an opening disposed to said upper tie plate and away from the end of said part length rod(s) for the discharge of steam moderator from said fuel bundle. The invention may alternatively be defined in the terms of claim 10 as a process for improving the outflow of generated steam in a full bundle for a boiling water reactor. The fuel bundle according to the invention may be further characterised in having means for deflecting water away from the steam vent tube near said end of said part length rod(s). Because of the opening of the steam vent tube overlying the end of the part length rod, the steam vent tube preferentially admits initial vapor flow to the interior of the tube and thereafter rapidly vents the vapor to discharge at the upper tie plate. An advantage of the steam vent tube is that it forms a dividing wall flow interface between rapidly flowing, upwardly venting steam interior of the tube and the surrounding two phase steam water mixture exterior of the steam vent tube. There results an isolated steam flow channel having a reduction in flow friction and a local increase in the volumetric flux of vapor outflow. The turbulent interface between the upwardly flowing steam and the surrounding two phase mixture is removed. Consequently, entrainment between the upwardly flow vapor and the bounding two phase liquid vapor region is removed. A further advantage of the steam vent tube is that it establishes a localized high velocity steam vent within the interior upper two phase region of the fuel bundle. As a result, less of the upper cross sectional area of the fuel bundle within the two phase region is required for upward vapor flow. The upper volume of the fuel bundle not occupied by vapor flow can instead be occupied by two phase liquid vapor mixture having a higher water content. There results a fuel bundle capable of maintaining a higher overall liquid moderator content. Flatter axial voids and power distributions result. Yet an additional advantage of the steam vent tube is that the entire upper portion of the fuel bundle is utilized either for the generation of steam or out flow of liquid and vapor moderator. Large flow blocking water rods for increasing the fuel to moderator ratio - but also otherwise obstructing flow area - are not required. A fuel bundle concept is disclosed in which the steam vent tube in classifying water to the exterior of the vent tube functions to both provide improved moderator fraction while maintaining (and not obstructing) flow area. An additional advantage of the disclosed concept is that it provides additional moderator in a manner which is the inverse of moderator provided by the more typically used large water rods. By the expedient of providing a concentrated isolated flow path for the upward preferential flow of vapor, the remaining portions of the fuel bundle in the upper two phase region can have increase liquid moderator content - equivalent in may cases to that moderator content supplied by a large water rod. At the same time, all areas of the upper two phase region of the bundle are available for moderator flow. As a result, no portion in the fuel bundle need be dedicated to flow obstructing water rods. The steam vent tube may include a plurality of apertures in the side walls of said steam vent tube configured to preferentially admit steam interior of said steam vent tube. The steam vent tube may include a plurality of apertures in the side walls of said steam vent tube configured to preferentially discharge water from the interior of said steam vent tube. The steam vent tube may be manifolded by either or both of the just-mentioned forms of apertures being distributed along the length of the steam vent tube. An advantage of the manifolding of the steam vent tube is that a natural resistance to hydraulic oscillations is present in the arrangement. Lower frequency natural resonant oscillations within the surrounding two phase region of the fuel bundle are not in phase with higher frequency natural resonant oscillations within the steam vent tube. The result is that the two different resonant frequency systems tend to dampen one another. The tendency of the fuel bundle to resonate at one frequency is reduced. DESCRIPTION OF EMBODIMENTS Embodiments of the invention will now be described in detail with reference to the drawings in which: Fig. 1 is a perspective view of a nuclear fuel bundle for a boiling water reactor, the bundle being shown in section in the upper two phase region of the bundle adjacent the upper tie plate and illustrating four steam vent tubes, each overlying a part length rod, with the two part length rods and steam vent tubes in the foreground being exposed by breaking away the channel and tubes that would otherwise obstruct view; Fig. 2 is a perspective view of one of the partial length rods of Fig 1 illustrating the bottom rod opening, the upper discharge adjacent the tie plate, and the wall openings extending between the bottom opening and top discharge for the preferential admittance of steam interior of the vent tube and the preferential ejection of water from the interior of the vent tube the rods and steam vent tube being broken away to less than their full vertical height; Fig. 3 is an alternate perspective view broken away similar to Fig. 2 of one of the partial length rods of Fig. 1 illustrating an embodiment where the steam vent tube overlying the part length rod has an increased diameter with respect to the underlying steam vent tube; Fig. 4 is a perspective view of a fuel bundle having four part length rods placed in a side by side grouping with a flow diverter place immediately over the end of the part length rods with the opening of the overlying circular sectioned steam vent tube being provided with a circuitous entry path for the preferential admission of vapor and corresponding exclusion of water from entry into the tube; Fig. 5 is a perspective view of an alternate embodiment of the steam vent tube, the tube being enlarged to overly a group of nine part length rods, the tube here being of a square cross section for the full occupation of the lattice spaces overlying the part length rods; Figs. 6A and 6B are side elevation sectional views and a plan view respective of an alternative system arrangement of the steam vent tube of this invention with a single large central steam vent tube occupying a large central conical vacancy created by a plurality of part length rods of differing lengths, the steam vent tube here illustrated being circular in section; Fig. 7 is a steam vent tube illustrating lower peripheral passages for permitting the preferential influx of steam; and, Fig. 8 is a steam vent tube illustrating upper peripheral passages for permitting the preferential ejection of water from the interior of the steam vent tube. IN THE SPECIFICATIONS Referring to Fig. 1, a fuel bundle B utilizing this invention is illustrated. The fuel bundle is, for the most part, conventional. The reader will note that the preferred embodiment of the invention as set forth in Fig. 1 does not include water rods. The presence of the steam vent tubes of this invention may well obviate the need for such water rods. The fuel bundle here illustrated includes full length fuel rods 20 and part length fuel rods 22. Full length fuel rods 20 are conventional and extend the full distance between lower tie plate 14 to upper tie plate 16. The reader will understand that the channel C has been broken away so that the fuel rods are exposed. Part length rods 22 are also present. These part length rods extend from lower tie plate 14 and terminate short of upper tie plate 16. Spacers are also utilized. Typically, as here shown, seven such spacers are evenly placed throughout the fuel bundle. Part length rods 22 are here shown terminating just above spacer S5. Thereafter, at spacer S6 and S7, a steam vent volume is defined overlying the end of the part length rods. It is into this steam vent volume that the steam vent tubes T1-T4 of this invention are placed. The reader will understand that the length of fuel rod used here is exemplary; lengths of fuel rods other than those specifically shown may be used. Referring to Fig. 2, one of the steam vent tubes placed within the fuel bundle B of Fig. 1 is illustrated. Specifically, paired full length rods 20 are shown sectioned from an interior portion of the bundle extending upwardly on either side of a part length rod 22. Part length rod 22 terminates at spacer S5 just above the spacer at end 40. Overlying, or connected to end 40 of the part length rod is steam vent tube T1 having a deflector 19 which is here shown conical in shape. Deflector 19 is separated a small interval from steam vent tube T1 for the purpose of deflecting water off the end of the rod and permitting the preferential entry of steam into vent tube T1. Tube T1 includes a lower opening 42 for receiving steam off the end of part length rod 22 at end 40. The tube extends upwardly and through the upper tie plate 16 at a discharge end 44 for venting steam through the upper tie plate. Suspension of the steam vent tube occurs from upper tie plate 16 and spacers S6 and S7 (see Fig. 1). Optimally, and shown adjacent the end 40 of partial length rod 22 in the steam vent tube, there can be provided openings 48, 46. These openings can have functions for either ejecting water from the interior of the steam vent tube T1, or for admitting steam. The function of these openings will hereinafter be described with respect to Figs. 7 and 8. Referring to Fig. 3, an alternate embodiment of the steam vent tubes is illustrated at T5. A partial length rod 22 is again shown between full length rods 20 terminating at end 40. Overlying the part length rod 22, there is provided a steam vent tube T5 and deflector 19. Comparing this view with that shown in Fig. 2, it can be seen that the diameter of the tube T5 exceeds the diameter illustrated of both the rods 20, full length rods 20, and the part length rods 22. All other constructions remain, including lower opening 42, upper opening 44, and vents 48, 46. Referring to Fig. 4, another steam vent tube T6 is illustrated. Tube T6 is shown at its lower end only between lower spacer S5 and upper spacer S6. As can be seen in the view of Fig. 4, four part length rods 22A, 22B, 22C and 22D are placed inside in a side-by-side array. Rods all terminate at an upper end 40 which end 40 is clustered interior of the fuel bundle. Thus, the volume overlying the ends of the part length rods 40 and the upper tie plate constitutes an interval which would normally be occupied by four full length rods 20. A steam trapping entry is provided by a first complete cone 50 and a second cone truncated at the apex, this second cone being designated 52. Portions of the complete cone 50 are broken away to expose the truncated apex 54. As before, apertures 48,46 interrupt and manifold the sidewalls of the steam vent tube T6. An explanation of the entry of steam and the rejection of water at the respective cones 50,52 is instructive. As is well known, in a two-phase liquid vapor mixture, the liquid occupies a higher density. Therefore, upon impact with lower cone 50, liquid will typically continue along the outside of the tube T6 (see the schematically illustrated liquid at vector 60). The low density steam is another matter. Typically, the steam will find its way around and into the truncated apex 54 of truncated cone 52 (see schematic vectors 62). Stated in ordinary terms, the steam has a density which will admit of its followinq circuitous flow paths. Once the steam is interior of the steam vent tube T6, upward flow will continue. Thus far, all steam vent tubes illustrated have been of circular section. Referring to Fig. 5, a square section T7 steam vent tube is illustrated. Referring to Fig. 5, a plurality of full length rods 20 surrounds a cluster of nine partial length rods 22. Partial length rods 22 all terminate at upper ends 40. Since the rods 22 are in a 3 x 3 array, it will be understood that overlying the ends 40 of the part length rods there exists a steam vent volume that may be suitably occupied by a square sectioned steam vent tube T7. Steam vent tube T7 has a generally conical bottom 70 flaring to a square sectioned tube structure 72. As before, tube structure 72 continues upwardly through the upper tie plate, it being realized that the top portion of the tube T7 is not illustrated. As before, apertures 48,46 are present. Referring to Fig. 6A, a steam vent tube placed within a generally conical void within a fuel bundle is shown. A plan view of the steam vent tube T8 is illustrated in Fig. 6B. Referring to Fig. 6B, a 9 x 9 array of full length rods is illustrated. This 9 x 9 array has 21 part length rods 22 terminating underlying the steam vent tube T8. Referring to Fig. 6A, it can be seen that the central length rod 22 terminates at a first elevation 91. Second part length rods 22 terminate at second elevations 92. Finally, third groups of path length rods 22 terminate at third elevations 93. The rods are placed in a stepped configuration similar to that illustrated in Ueda, Japanese Patent Showa 52-50498 so as to define from lower spacer S5 to upper tie plate 16, a generally conical volume. This volume is filled with a step tapered tube T8. Tube T8 includes a flare portion 100 at the upper end of short partial length rod 22. Thereafter, a cylinder 102 connected by truncated cone 104, connects to cylinder 106. A second truncated cone 108 connects to cylinder 110. Cylinder 110 extends upwardly to and possibly through tie plate 16. As before, alternating openings 46,48 are shown utilized. Referring to Fig. 7, a vent 46 is illustrated which vent 46 in the side of a vent tube T permits the preferential entry of vapor along an illustrated path 120. Aperture 46 includes an upper opening 122 and is disposed outwardly from the linear wall 124. As can be seen, vapor with its low density can follow the circuitous path 120. Water with its higher density will pass by aperture 122 as illustrated in vector 126. It will be realized that the aperture shown in Fig. 7 is shown in a planar wall. This aperture can just as easily be adapted to circular walls. Referring to Fig. 8, a vent 48 is illustrated which vent 48 is for the ejection of water interior of the wall 124. Specifically, water will normally flow against the wall. Since aperture 132 of opening 48 is downwardly disposed, water tracking along wall 124 will be ejected from the interior of the steam vent tube T. It will be realized that the placement of the respective apertures 46,48 and their respective dimensions will be a function of the size of the steam vent tube, the total volume of vapor to flow within the tube as well as the total volume of water to be ejected from the tube. Thus, the designer will place these apertures and dimension them in accordance with the design parameters of the fuel bundle involved. It will be realized that the preferred embodiments have been shown without a water rod. Those having skill in the art will understand that a water rod can be added. Further, the steam vent tubes of the invention have been shown fastened to the spacers and tie plates. The steam vent tube can as well be fastened to the end of the part length rod.

Claims: A fuel bundle (B) for a boiling water reactor having: a plurality of vertically aligned spaced apart fuel rods (20,22) for forming a fuel rod group within said fuel bundle for generation of a fission reaction in the presence of water moderator, a lower tie plate (14) for admitting water moderator through said lower tie plate to the Interstitial volume between said fuel rods and supporting said vertically aligned and spaced apart fuel rods, an upper tie plate (16) for permitting water and steam to be discharged from the top of the fuel bundle and maintaining vertically aligned and spaced apart full length said fuel rods (20) in upstanding spaced apart side-by-side relation, a surrounding fuel channel (C) for confining moderator flow along a path over the fuel rods and from said lower tie plate to said upper tie plate; at least one of said fuel rods being a part length rod (22) extending from the lower tie plate vertically less than the full length to the upper tie plate ending interior of the fuel bundle at a disposition wherein the upper end of at least one said part length rod defines with respect to surrounding said full length fuel rods an empty volume overlying said partial length rod(s) under said upper tie plate, said fuel bundle being characterised by: at least one steam vent tube (T1-T8) overlying at least one of said part length rods; means (16,56,57) supporting said steam vent tube in said volume overlying said part length rod(s), said steam vent tube being supported in the volume of said fuel bundle between an end (40) of said part length rod(s) and said upper tie plate; said steam vent tube defining an opening (42) disposed to said end (40) of said part length rod(s) for the receipt of vapor moderator within said void overlying said part length rod(s); said steam vent tube further defining an opening (44) disposed to said upper tie plate and away from the end of said part length rod(s) for the discharge of steam moderator from said fuel bundle. The fuel bundle of claim 1 wherein said fuel bundle has a plurality of said part length rods spaced apart one from another. The fuel bundle of claim 1 or 2 wherein said steam vent tube overlies a plurality of said part length rods. The fuel bundle of any one of claims 1 to 3 wherein said steam vent tube includes a plurality of apertures (46) in the side walls of said steam vent tube configured to preferentially admit steam interior of said steam vent tube. The fuel bundle of any one of claims 1 to 4 wherein said steam vent tube includes a plurality of apertures (48) in the side walls of said steam vent tube configured to preferentially discharge water from the interior of said steam vent tube. The fuel bundle of claim 4 or claim 5 wherein said apertures are distributed along the length of said steam vent tube. The fuel bundle as claimed in any one of claims 1 to 6 wherein said steam vent tube overlying said part length rod(s) is square. The fuel bundle as claimed in any one of claims 1 to 6 wherein said steam vent tube overlying said part length rod(s) is round. The fuel bundle as claimed in any preceding claim, and having means (19,50) for deflecting water away from the steam vent tube near said end (40) of said part length rod(s). A process for improving the outflow of generated steam in a fuel bundle for a boiling water reactor including the steps of: providing a plurality of vertically aligned spaced apart fuel rods (20,22) for forming a fuel rod group within said fuel bundle for generation of a fission reaction in the presence of water moderator; providing a lower tie plate (14) for admitting water moderator through said lower tie plate to the interstitial volume between said fuel rods and supporting said vertically aligned and spaced apart fuel rods; providing an upper tie plate (16) for permitting water and steam to be discharged from the top of the fuel bundle and maintaining said vertically aligned and spaced apart fuel rods in upstanding spaced apart side-by-side relation; providing a surrounding fuel channel (C) for confining moderator flow along a path over the fuel rods and from said lower tie plate to said upper tie plate; providing at least one (22) of said fuel rods extending from the lower tie plate vertically less than the full length to the upper tie plate ending interior of the fuel bundle at a disposition wherein the upper end of the part length rod(s) defines with respect to said surrounding fuel rods an empty volume overlying said partial length rod under said upper tie plate, said process being characterised by: providing at least one steam vent tube (T1-T8) overlying at least one of said part length rods; supporting (S6,S7,16) said steam vent tube in said volume overlying said part length rod, said steam vent tube being supported in the volume of said fuel bundle between an end (40) of said part length rod and said upper tie plate; defining an opening (42) in said steam vent tube disposed to said end (40) of said part length rod for the receipt of steam moderator within said void overlying said part length rod; defining an opening (44) in said steam vent tube disposed to said upper tie plate for the discharge of steam moderator from said fuel bundle; and reacting said fuel bundle in the presence of water moderator to form in the upper two phase region of said fuel bundle adjacent said part length rod a two phase mixture of vapor and liquid moderation whereby steam finds a preferential flow path in said steam vent tube from the top of said part 5length rod to said upper tie plate.

Inventors: DIX GARY ERROL, DIX, GARY ERROL
Applicants: GEN ELECTRIC, GENERAL ELECTRIC COMPANY
Application year: 1992
Main classification: G21C3
IPC codes: G21C3