Patent Application
20110164719
1. Field of the Invention
The present invention relates generally to nuclear reactors and, more particularly, is concerned with a debris filter bottom nozzle for a pressurized water reactor nuclear fuel assembly.
2. Related Art
During manufacturing, subsequent installation and repair of components comprising a nuclear reactor coolant circulation system, diligent effort is made to help assure removal of all debris from the reactor vessel and its associated systems, which circulate coolant throughout the primary reactor coolant loop under various operating conditions. Although elaborate procedures are carried out to help assure debris removal, experience shows that in spite of the safeguards used to effect such removal, some chips and metal particles still remain hidden in the system. Most of the debris consists of metal turnings, which were probably left in the primary system after steam generator repair or replacement.
In particular, fuel assembly damage due to debris trapped at the lower most grid has been noted in several reactors in recent years. Debris enters through the fuel assembly bottom nozzle flow holes from the coolant flow openings in the lower core support plate when the plant is started up. The debris tends to become lodged in the lower most support grid of the fuel assembly within spaces between the “egg crate” shaped cell walls of the grid and the lower end portions of the fuel rod tubes. The damage consists of fuel rod tube perforations caused by frettings of the debris in contact with the exterior of the fuel rod tube or cladding. Debris also becomes entangled in the nozzle plate holes and the flowing coolant causes the debris to gyrate which tends to cut through the cladding of the fuel rods.
Several different approaches have been proposed and tried for carrying out the removal of debris from nuclear reactors. Many of these approaches are discussed in U.S. Pat. No. 4,096,032 to Mayers et al. Others are illustrated and described in various patents cross referenced in U.S. Pat. No. 4,900,507 and in U.S. Publication US2005/0157836, assigned to the instant Assignee. While all of the approaches described in the cited patent, published application and cross references operate reasonably well and generally achieve their objectives under the range of operating conditions for which they were designed, a need still exists for a further improved approach to the problem of debris filtering in nuclear reactors, to obtain an improved reduction in debris that passes up through the flow holes of the bottom nozzle.
The present invention provides a debris filter bottom nozzle for a nuclear fuel assembly designed to satisfy the aforementioned need. The bottom nozzle of the present invention includes a substantially horizontal adapter plate extending approximately transverse to the axis of the fuel rods and having an upper face directed toward the lower most grid of the nuclear fuel assembly. The upper face of the adapter plate has a plurality of holes defined therethrough for the passage of coolant fluid from a lower face to the upper face of the adapter plate. The flow through holes may be similar to those described in U.S. published Application 2005/0157836. Each of the coolant flow through holes extends substantially in the axial direction of the fuel rods, and are in fluid communication with unoccupied spaces in the lower most grid of the fuel assembly. A skirt circumscribes the lower face of the adapter plate and a corrugated undulating screen extends across a lower portion of the skirt to substantially cover a bottom portion thereof and form a plenum between the lower face of the adapter plate and the screen. Preferably, holes are provided in slanted portions of the corrugated screen and in the skirt for the passage of coolant into the plenum wherein the coolant changes direction to pass through the flow through holes in the adapter plate.
In one embodiment, the corrugations in the screen have a predetermined amount of elasticity and are compressed so that they exert a force on opposite walls of the skirt on which an end of the corrugations abut. Preferably, the screen is constructed of a material that has substantially the same coefficient of thermal expansion as the skirt and desirably, the corrugated undulating screen is supported by pins that extend between opposite walls of the skirt. Generally, the pins are flush with the outside surface of the walls of the skirt and pass through holes in the undulating screen. Preferably, the tops and bottoms of the undulations in the screen have no flow through holes and are rounded and desirably, the flow holes in the screen are formed as slots smaller than or equal to the length of debris that the screen is intended to trap and the width of the slots is less than or equal to a diameter of the debris. Desirably, the slots are arranged horizontally.
A further understanding of the invention can be gained from the following description of the preferred embodiments when read in conjunction with accompanying drawings in which:
In the following description, like reference characters designate like or corresponding parts throughout the several views of the drawings. Also, in the following description, it is to be understood that such terms as “forward”, “rearward”, “left”, “right”, “upwardly”, “downwardly” and the like are words of convenience and are not to be construed as limiting terms.
Referring to the drawings and particularly to
The fuel assembly 10 further includes a plurality of transverse grids 20 axially spaced along, and mounted to, the guide thimble tubes 18 and an organized array of elongated fuel rods 22 transversely spaced and supported by the grids 20. Also, the assembly 10 has an instrumentation tube 24 located in the center thereof and extending between, and mounted to, the bottom and top nozzles 12 and 16. With such an arrangement of parts, fuel assembly 10 forms an integral unit capable of being conveniently handled without damaging the assembly of parts.
As mentioned above, the fuel rods 22 in the array thereof in the assembly 10 are held in spaced relationship with one another by the grids 20 spaced along the fuel assembly length. Each fuel rod 22 includes nuclear fuel pellets 26 and is closed at opposite ends by upper and lower end plugs 28 and 30. The pellets 26 are maintained in a stack by a plenum spring 32 disposed between the upper end plug 28 and the top of the pellet stack. The fuel pellets 26, composed of fissile material, are responsible for creating the reactive power of the reactor. A liquid moderator/coolant such as water or water containing boron, is pumped upwardly through a plurality of flow openings in the lower core plate 14 to the fuel assembly. The bottom nozzle 12 of the fuel assembly 10 passes the coolant upwardly through the guide tubes 18 and along the fuel rods 22 of the assembly in order to extract heat generated therein for the production of useful work.
To control the fission process, a number of control rods 34 are reciprocally movable in the guide thimbles 18 located at predetermined positions in the fuel assembly 10. Specifically, a rod cluster control mechanism 36 is positioned above the top nozzle 16 and supports the control rods 34. The control mechanism has an internally threaded cylindrical member 37 with a plurality of radially extending flukes or arms 38. Each arm 38 is interconnected to a control rod 34 such that the control rod mechanism 36 is operable to move the control rods vertically in the guide thimbles 18 to thereby control the fission process in the fuel assembly 10, all in a well known manner.
As mentioned above, fuel rod damage due to debris trapped at or below the lower most one of the grids 20 supporting the fuel bearing regions of the fuel rods has been found to be a problem. Leaking fuel rods due to debris has been identified by the Institute of Nuclear Power Operations as one of four leaking mechanisms that needs to be considered to achieve zero fuel failure by 2010. Reactor manufacturers such as Westinghouse Electric Company LLC, Pittsburgh, Pa., currently offer a number of features for minimizing the adverse affects of such debris, i.e., debris filter bottom nozzles, protective lower most grids, extended fuel rod end plugs, fuel rod coating, etc. The existing debris filter bottom nozzle design (such as that described in U.S. published Application 2005/0157836) alone is not sufficient to prevent debris from passing into the core that has a potential for resulting in a fuel rod leak. The current debris filter bottom nozzle design blocks approximately 70% by weight of the metallic particles that could result in fuel clad failure from passing through the nozzle flow passages. The current debris filter bottom nozzle flow through holes have a diameter of approximately 0.19 inch (0.48 cm), which will not prevent long pieces of wire from passing through the flow passages in the bottom nozzle adapter plate since the flow holes are straight in the direction of flow. Therefore, to prevent the occurrence of fuel cladding damage, it is highly desirable to minimize such debris that passes through the bottom nozzle flow holes or the interfaces between the outlets of the bottom nozzle flow holes and the adjoining structures.
The present invention provides an improved bottom nozzle 12 which, in addition to supporting the fuel assembly 10 on the lower core support plate 14, also contains features which function to filter out potentially damaging sized debris from the coolant flow passed upwardly through the bottom nozzle. The bottom nozzle 12 includes support means, for example, the skirt 40 shown in
The screen is a metal sheet which is bent into a corrugated undulating form having a thickness approximately equal to or between 0.026 inches (0.066 cm) and 0.060 inches (0.152 cm) and should exhibit sufficient flexibility and elasticity at least equal to approximately half of the pitch of the corrugated screen folding form to allow for screen installation without additional permanent deformation (or permanent set) and provide a force on opposite walls of the skirt after installation. A central opening in the screen accommodates the extension tube 56 which can be flared at its lower end to avoid the bypass of debris at the interface of the extension tube 56 and the screen 58. The area between the screen 58, the interior side of the skirt 40 and the underside of the adapter plate 46 defines a plenum 62. Flow through holes 64 in the skirt 40 and slots 66 in the screen 58, which can best be appreciated from
The folded length of the screen 58 is sized to exceed the available room inside the skirt 40 so that the screen 58 is deformed during installation to restrict screen displacement. The perforation and bend characteristics are chosen to ensure that a predetermined size of metallic particles cannot pass through the screen. For this purpose, it is assumed that the metallic particles of interest have a cylindrical shape that can be characterized by an outer diameter (OD) and length (L). The perforation slot geometry should be consistent with those dimensions, i.e., slot length (Ls) should be less than or equal to L and slot width (Ds) should be less than or equal to OD. However, it should be appreciated that the geometry of the slot may vary and the number of slots should be sufficient to satisfy pressure drop requirements. As shown in
The pins 70 that are used to secure the screen 58 inside the skirt 40 can be secured within the holes 76 using a threaded joint between the pin and a pin nut (not shown) in a countersunk portion of the skirt holes 76. The pin nut can be secured by mechanical deformation of the nut head (similar to the guide thimble screw lock cup deformation presently employed).
As previously mentioned, it is desirable that the screen be slightly deformed during pin installation to restrict the screen's displacement during operation. Preferably, the fuel assembly is fabricated in accordance with the current process up to the point where the lower nozzle 12 is installed and secured. At that point, the screen 58 is inserted within the skirt 40 taking advantage of its flexibility and the pins are installed through the holes 76 in the skirt then threaded through the holes 72 in the screen 58 and secured with the pin nuts. The remainder of the process for assembling the fuel assembly 10 remains the same as is currently employed.
Accordingly, coolant emerging through the flow holes 50 in the lower core support plate 14 enters the lower nozzle through the slots 66 in the slanted sides of the corrugated undulating screen 58 and the flow through holes 64 in the skirt 40 and turns upward and through the flow through holes 48 to enter the lower most grid 20. In that way, debris that could likely damage the fuel cladding will settle out into the crevices 78 within the corrugations of the screen 58 before the coolant leaves the nozzle 12.
While specific embodiments of the invention have been described in detail, it will be appreciated by those skilled in the art that various modifications and alternatives to those details could be developed in light of the overall teachings of the disclosure. Accordingly, the particular embodiments disclosed are meant to be illustrative only and not limiting as to the scope of the invention which is to be given the breath of the appended claims and any and all equivalents thereof.
