A further understanding of the invention can be gained from the following description of the preferred embodiments when read in conjunction with the 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 now to the drawings, and particularly to
Spaced radially, inwardly from the reactor vessel 12 is a generally cylindrical core barrel 18 and within the barrel 18 is a former and baffle system, hereinafter called a “baffle structure” 20, which permits transition from the cylindrical barrel 18 to a squared-off, stepped periphery of the reactor core 14 formed by the plurality of fuel assemblies 16 being arrayed therein. The baffle structure 20 surrounds the fuel assemblies 16 of the reactor core 14. Typically, the baffle structure 20 is made of plates 22 joined together by bolts (not shown). The reactor core 14 and the baffle structure 20 are disposed between upper and lower core plates 24,26 which, in turn, are supported by the core barrel 18.
The upper end of the reactor pressure vessel 12 is hermetically sealed by a removable hemispherical closure head 28 upon which are mounted a plurality of control rod drive mechanisms 30. Again for simplicity, only a few of the many control rod drive mechanisms 30 are shown. Each drive mechanism 30 selectively positions a rod cluster control mechanism 32 above and within some of the fuel assembly 16.
A nuclear fission process carried out in the fuel assemblies 16 of the reactor core 14 produces heat which is removed during operation of the PWR 10 by circulating a coolant fluid, such as light water, through the core 14. More specifically, the coolant fluid is typically pumped into the reactor pressure vessel 12 through a plurality of inlet nozzles 34 (only one of which is shown in
Due to the existence of pressure relief holes (not shown) in the core barrel 18, coolant fluid is also present between the barrel 18 and baffle structure 20 and at a higher pressure than exists within the reactor core 14. However the baffle structure 20, together with the core barrel 18, do separate the coolant from the fuel assemblies 16 as the fluid flows downwardly through the annular region 36 between the reactor vessel 12 and core barrel 18.
As briefly mentioned above, the reactor core 14 is composed of a large number of elongated fuel assemblies 16. Turning to
Each fuel rod 48 of the fuel assembly 16 includes nuclear fuel pellets 54 and the opposite ends of each fuel rod are closed by upper and lower end plugs 56,58 to hermetically seal the rod. Commonly, a plenum spring 60 is disposed between the upper end plug 56 and the pellets 54 to maintain the pellets in a tightly stacked tandem array within the fuel rod 48. The fuel pellets 54 composed of fissile material are responsible for creating the reactive power which generates heat in the core 14 of the PWR 10. As mentioned, the coolant fluid is pumped upwardly through each of the fuel assemblies 10 of the core 14 in order to extract heat generated therein for the production of useful work.
To control the fission process, a number of control rods 62 of each rod cluster control mechanism 32 are reciprocally moveable in the guide thimbles 44 located at predetermined positions in the fuel assembly 16. However, not all of the fuel assemblies 16 have rod cluster control mechanisms 32, and thus control rods 62, associated therewith. Though typically, the fuel assemblies that accommodate control rods are of the same design as other fuel assemblies within the core that do not have control rods associated therewith. Specifically, each rod cluster control mechanism 32 is associated with a top nozzle 52 of the corresponding fuel assembly 16. The control mechanism 32 has an internally threaded cylindrical member 64 with a plurality of radially extending arms 66. Each arm 66 (also known as flukes) is interconnected to one or more control rods 62 such that the control mechanism 32 is operable to move the control rods 62 vertically in the guide thimbles 44 to thereby control the fission process in the fuel assembly 16, all in a well known manner.
This invention enhances fuel assembly dimensional stability to support aggressive fuel management (increase burn-up and resident time in-core) and decreases the probability of incomplete rod cluster control assembly insertion, handling accidents and other consequences of fuel assembly bow. Fuel assembly dimensional stability is enhanced by improving the skeleton lateral stiffness; with an additional bulge joint between guide thimble and spacer grid support sleeve, as shown in
Therefore, the double bulge connection provides a significant benefit to the skeleton and fuel assembly lateral stiffness to support aggressive fuel management with increase burn-up and in-core resident time. The double bulge skeleton design does not adversely affect the other fuel assembly characteristics, e.g., pressure drop, etc. This design modification may be easily implemented for any PWR or VVER fuel assembly utilizing the bulge connection.
The fuel assembly distortion resistance depends on the fuel assembly lateral stiffness. The fuel assembly lateral stiffness is a combination of the fuel rod bundled stiffness and the skeleton stiffness.
The fuel rod bundled stiffness mainly depends upon the fuel rod geometry and the spacer grid spring forces. Unfortunately, the grid spring forces decrease during in-core radiation and the fuel rod bundled stiffness degrades. Increasing as-built spring forces does not provide a long term benefit in the assembly lateral stiffness.
The skeleton stiffness depends on the number, location and geometry of the guide thimbles and their capability to work together. The skeleton stiffness does not change significantly during irradiation. The number, location and geometry of the guide thimble are typically prescribed by the core internals design and cannot be change for existing nuclear plants. As mentioned above, the guide thimbles are connected to each other by spacer grids. Typically, spacer grid designs include a support sleeve 68 to provide the interface with the guide thimbles 44. The support sleeve is needed, especially when the support grid and guide thimble are made from dissimilar materials that are difficult to metallurgically join, e.g., Zircaloy and Inconel, to provide room for a mechanical connection. The sleeve may be attached to the guide thimble utilizing friction forces (interference fittings), welds or bulge connections. The skeleton lateral stiffness significantly depends on this connection. Therefore, a reasonable way to enhance skeleton stiffness is to improve the connection between the guide thimble and the spacer grid sleeve.
Traditionally, many fuel assembly designs utilize bulge expansion joints to connect the guide thimble to the spacer grid sleeve. Typically, one bulge is used to connect the guide thimble to the corresponding spacer grid sleeve at each spacer grid location as explained above. It is known that the bulge connection “free play” decreases the skeleton lateral stiffness.
The skeleton lateral stiffness maybe defined as follows:
Alpha=the guide tube connection factor; NGT=the number of guide tubes and instrumentation tubes; IGT=guide tube or instrumentation tube moment of inertia; a=distance from the center of the guide tube to the bend principal axis; and FGT=guide tube cross-sectional area.
The guide thimble connection factor for the single bulge connection is approximately 0.3. It is noted that the bulge connection “free play” may be significantly decreased. However, the technical options to provide this improvement are limited when taking into account manufacturing limitations (for example: available manufacturing methods) and design limitations (for example: pressure drop limit).
In order to enhance the skeleton lateral stiffness this invention introduces a second bulge well removed from the first bulge to connect the guide thimble to the spacer grid as presented in
Accordingly, the double bulge connection provides a significant improvement to the skeleton and fuel assembly lateral stiffness to support aggressive fuel management with increased burn-up and in-core resident time. The double bulge skeleton design does not adversely affect the other fuel assembly characteristics (for example: pressure drop, etc.). This design modification may be easily implemented for any PWR or VVER fuel assembly utilizing the bulge connection.
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 full breath of the appendant claims and any and all equivalents thereof.