Abstract:
An intermediate end plug assembly for a segmented fuel rod can stably support the fuel rod to the end of its cycle even if an interval between the fuel rods becomes narrow due to application of a dual-cooled fuel rod, and reduce excess vibration induced by flows of interior and exterior channels of the dual-cooled fuel rod for obtaining high burnup and output. To this end, the fuel rod has a segmented structure so as to make its length short. A lower intermediate end plug includes at least one channel hole, through which a coolant flows into an internal channel of the fuel rod, so that a possibility of causing departure from nuclear boiling ratio (DNBR) of the dual-cooled fuel rod is reduced.

Description:
CROSS-REFERENCES TO RELATED APPLICATION 
     This patent application claims the benefit of priority under 35 U.S.C. §119 from Korean Patent Application No. 10-2008-117334 filed on Nov. 25, 2008, the contents of which are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates, in general, to an end plug for a dual-cooled fuel rod and, more particularly, to an intermediate end plug assembly for a segmented fuel rod, capable of stably supporting the fuel rod to the end of its cycle even if an interval between the fuel rods becomes narrow due to application of a dual-cooled fuel rod, and reducing excess vibration induced by flows of interior and exterior channels of the dual-cooled fuel rod for obtaining high burnup and output. 
     2. Description of the Related Art 
     A nuclear fuel assembly is charged in the core of a pressurized water reactor. This nuclear fuel assembly is composed of a plurality of fuel rods, in each of which a cylindrical uranium sintered compact (or a cylindrical uranium pellet) is inserted. 
     The fuel rods can be divided into two types, cylindrical and annular, according to shape. The annular fuel rods are called dual-cooled fuel rods. 
     In comparison with the pellet of the cylindrical fuel rod, the pellet of the annular fuel rod has a low internal temperature due to a thinner thickness and a wider heat transfer area, and thus a relatively higher safety margin. 
       FIG. 1  is a schematic front view illustrating a conventional cylindrical nuclear fuel assembly. Referring to  FIG. 1 , the nuclear fuel assembly  10  includes fuel rods  11 , spacer grids  15 , guide thimbles  13 , an upper end fitting  17  and a lower end fitting  16 . 
     Each fuel rod  11  has a structure in which a uranium sintered compact or a uranium pellet (not shown) generating high-temperature heat through nuclear fission is enclosed by a zirconium alloy cladding tube. 
     Each fuel rod  11  has upper and lower end plugs  18  and  19  coupled to lower and upper portions thereof so as to prevent inert gas filled between the cladding tubes thereof from leaking out. 
     Meanwhile, the fuel rod  11  is a structure having a considerably long length compared to the diameter thereof. When this structure having a great elongation ratio is put under the flow of a coolant, the fuel rod  11  causes flow-induced vibrations due to the flow of the coolant. Thus, in order to reduce this flow-induced vibration, the structure called a spacer grid  15  is installed in a predetermined section with respect to the entire length of the fuel rods  11  so as to support the fuel rods  11 , thereby preventing the fuel rods  11  from being vibrated by the flow of the coolant. 
     However, in the case of the dual-cooled fuel rod designed to charge nuclear fuel into an annular space defined by a dual tube of inner and outer tubes, the spacer grid taking charge of an important function of inhibiting the vibration of the fuel rods caused by the flow of the coolant has no choice but to support only the outer tube of each fuel rod due to its structure. Due to the limitation of this supporting structure, in the case of the inner tube having the elongation ratio of about 400 or more, only opposite ends of each fuel rod are supported by the upper and lower end plugs. 
     Of course, in the case of the dual-cooled fuel rod, a uranium dioxide (UO 2 ) pellet exists between the inner and outer tubes. Thus, the vibration of the inner tube is expected to be inhibited to a certain extent. However, in considering the fuel rod having the elongation ratio of about 400 or more, it is easily expected that a vibration amplitude of the inner tube is remarkably great, as compared to the outer tube having numerous support points formed in an axial direction of the fuel rod by the spacer grid. 
     Further, in the case of the typical fuel rod as shown in  FIG. 2 , since a coolant channel is formed outside the fuel rod, a phenomenon in which the flow of the coolant in the reactor core is restricted mainly occurs due to foreign materials in the reactor core. Thus, since cooling performance of the fuel rod is sufficiently maintained if the foreign materials are screened to a certain extent before they enter the reactor core, such a phenomenon is mainly overcome by additionally installing an apparatus for filtering the foreign materials on the lower end fitting or a support for filtering the foreign materials above the lower end fitting. 
     However, in the case of the dual-cooled fuel rod which obtains economical effects by lowering a central temperature of the nuclear fuel to secure stability of the nuclear fuel at a very high burnup together with high output, another problem occurs. This is because, in the case of the dual-cooled fuel rod, the coolant channel is formed outside the fuel rod, but the coolant flows in the fuel rod in order to increase the cooling performance, so that the coolant channel is formed inside the fuel rod. This coolant channel formed inside the fuel rod has an advantage in that it increases the cooling performance and thus the output of the nuclear fuel, but it has a disadvantage in that, if it is blocked, the fuel rod is greatly exposed to a danger of departure from nuclear boiling ratio (DNBR). Particularly, since the internal coolant channel of the dual-cooled fuel rod has a very narrow flow cross-sectional area, it is in high danger of being blocked although only small foreign materials get into it. 
     In this manner, if the internal coolant channel of the dual-cooled fuel rod is blocked and as a result a smooth flow of the coolant is obstructed, the coolant does not flow in the fuel rod, and thus is stagnant. Thereby, the dual-cooled fuel rod is exposed to the danger of DNBR. However, since the internal coolant channel does not exist in the conventional fuel rod, a new resolution to the dual-cooled fuel rod must be found. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention has been made keeping in mind the above problems occurring in the related art, and embodiments of the present invention provide an intermediate end plug assembly for a segmented fuel rod, in which the segmented fuel rod has a half or quarter of a length of a typical fuel rod, and intermediate end plugs mounted on upper and lower ends thereof, so that a dual-cooled fuel rod having the segmented fuel rods supported at an intermediate position thereof is enabled unlike the typical fuel rod, and thus vibration displacement induced by a coolant flowing in an internal channel of the dual-cooled fuel rod is reduced. 
     Further, there is provided an intermediate end plug assembly for a segmented fuel rod, in which, when a channel for a coolant is blocked at a lower end of an entire fuel rod (i.e. upstream of the coolant), the coolant can be complemented through a complementary channel hole formed in a lower intermediate end plug located above the segmented fuel rod, thereby reducing a possibility of causing departure from nuclear boiling ratio (DNBR) attributable to the failure to supply the coolant to the entire internal channel of a dual-cooled fuel rod. 
     To sum up, the segmented fuel rod has the complementary channel holes in the upper and lower intermediate end plugs thereof in order to provide a path along which the coolant flows through the complementary channel holes formed in the upper and lower intermediate end plugs when the internal channel for dual cooling is blocked at the lower end of the entire fuel rod, thereby reducing the possibility of causing the DNBR in the internal channel, and simultaneously in order to form support points for supporting the fuel rod through the segmented fuel rods, thereby inhibiting flow-induced vibration of an inner cladding tube of the fuel rod to promote structural soundness of the entire fuel rod. 
     According to an aspect of the present invention, there is provided an intermediate end plug assembly for a segmented fuel rod, which includes: a first end plug having a cylindrical body, in a center of which a through-hole is formed in a longitudinal direction, the cylindrical body having an annular groove in one of circular upper and lower surfaces thereof between an outer circumference thereof and an outer circumference of the through-hole, and an annular protrusion protruding along an outer circumference thereof and a first flat coupling face inside the protrusion on the other surface thereof; and a second end plug having a cylindrical member, which has a body and a through-hole having diameters identical to those of the respective body and through-hole of the first end plug, the body of the second end plug having a groove identical to the groove of the first end plug in one of circular upper and lower surfaces thereof, and a cylindrical insert having an annular space along an outer circumference thereof so as to correspond to the annular protrusion of the first end plug and a second flat coupling face on an upper surface of the insert on a second surface thereof. The protrusion of the first end plug is inserted into the annular space of the second end plug, so that the first and second coupling faces come into close contact with each other. 
     Here, the protrusion of the first end plug may have “L” shaped coupling recesses, each of which is partially open, and the insert of the second end plug has latches fitting into the respective coupling recesses on an outer circumference thereof, such that the first end plug can be simply and rapidly coupled with the second end plug. The coupling recesses and the latches may be equal to each other in number, and the latches may be formed on the outer circumference of the inset of the second end plug so as to correspond to a position where the coupling recesses are formed. 
     Particularly, each coupling recess may include a seat having a locking step protruding in a shape of a hill, so that the coupling recesses can be prevented from being released from the latches. 
     Meanwhile, in comparison of inner and outer annular faces located inside and outside grooves of the first and second end plugs, the former may protrude higher than the latter. 
     At least one of the first and second end plugs may have at least one complementary channel hole communicating with the through-hole. 
     According to another aspect of the present invention, there is provided a dual-cooled fuel rod, which includes: a segmented upper fuel rod adopting the first end plug having the aforementioned configuration as a lower intermediate end plug thereof; and a segmented lower fuel rod adopting the second end plug having the aforementioned configuration as an upper intermediate end plug thereof. Thus, the dual-cooled fuel rod is formed by the coupling of two segmented fuel rods, each of which has half an elongation ratio compared to that of a conventional dual-cooled fuel rod. 
     According to another aspect of the present invention, there is provided a dual-cooled fuel rod, which includes: a segmented upper fuel rod adopting the second end plug as a lower intermediate end plug thereof; and a segmented lower fuel rod adopting the first end plug as an upper intermediate end plug thereof. In other words, the first end plug can be freely selected as the upper or lower intermediate end plug, and thus does not need to be limited to only the lower intermediate end plug. 
     Here, the dual-cooled fuel rod may further include at least one segmented intermediate fuel rod between the segmented upper and lower fuel rods. The segmented intermediate fuel rod may include the first and second end plugs at respective opposite ends thereof. This means that the dual-cooled fuel rod can be configured of at least three segmented fuel rods. Thus, the dual-cooled fuel rod can freely adjust the elongation ratio of each segmented fuel rod. 
     Accordingly, each of the segmented upper, lower and intermediate fuel rods may have an elongation ratio ranging from 100 to 200. 
     Meanwhile, at least one of the first and second end plugs may have at least one complementary channel hole which communicates with the through-hole and is inclined toward the segmented upper fuel rod. 
     Further, each of the segmented upper, lower and intermediate fuel rods may have a plenum spring and a spacer installed in an inner annular space thereof. 
     According to embodiments of the present invention, the intermediate end plug assembly for a segmented fuel rod is configured to be applied to each dual-cooled fuel rod having a segmented structure, so that the inner cladding tube of the segmented dual-cooled fuel rod coupled through the upper and lower intermediate end plugs can be supported at numerous support positions. As the number of support positions of the inner cladding tube increases, the displacement resulting from the flow-induced vibration can be remarkably reduced. Since the reduction of the displacement resulting from the flow-induced vibration improves support performance of the fuel rod, a possibility of causing damage resulting from fretting attrition can also be remarkably reduced. 
     Further, since the complementary channel holes are formed in the upper and lower intermediate end plugs, and the coolant flows into the internal channel of the inner cladding tube through the complementary channel holes, the cooling performance of the dual-cooled fuel rod can be maintained in the case in which the coolant is not supplied to the internal channel of the inner cladding tube when the channel is blocked at the lower end of the segmented lower fuel rod, so that the possibility of causing the DNBR of the dual-cooled fuel rod can be reduced. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other objects, features and advantages of the present invention will be more clearly understood from the following detailed description when taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a schematic front view illustrating a conventional cylindrical nuclear fuel assembly; 
         FIG. 2  is a cross-sectional view cutting out the cylindrical nuclear fuel assembly of  FIG. 1  in a transverse direction; 
         FIG. 3  is a perspective view illustrating a first end plug according to an embodiment of the present invention; 
         FIG. 4  is a perspective view illustrating a second end plug according to an embodiment of the present invention; 
         FIG. 5  is a sectional view taken along the line A-A′ of the first end plug of  FIG. 3  and the line B-B′ of the second end plug of  FIG. 4 ; 
         FIG. 6  is an assembled perspective view illustrating first and second end plugs according to an embodiment of the present invention; 
         FIG. 7  is a perspective view illustrating a dual-cooled fuel rod configured of segmented upper and lower fuel rods having respective first and second end plugs according to an embodiment of the present invention; 
         FIG. 8  is a cross-sectional perspective view taken along the line A-A′ of the dual-cooled fuel rod of  FIG. 7 ; 
         FIG. 9  is a perspective view illustrating a dual-cooled fuel rod configured of segmented upper and lower fuel rods having respective second and first end plugs according to another embodiment of the present invention; and 
         FIG. 10  is a perspective view illustrating a dual-cooled fuel rod configured of a segmented intermediate fuel rod in addition to the segmented upper and lower fuel rods of the dual-cooled fuel rod of  FIG. 7  according to another embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Reference will now be made in greater detail to an exemplary embodiment of the invention with reference to the accompanying drawings. 
       FIGS. 3 through 6  illustrate first and second end plugs configured as upper and lower intermediate end plugs for a segmented fuel rod according to an embodiment of the present invention. 
     As illustrated in  FIGS. 3 and 5 , the first end plug  100  includes a cylindrical body  110 , in the center of which a through-hole  112  is formed in a longitudinal direction. Here, one of circular upper and lower surfaces of the body  110  is provided with an annular groove  114  between an outer circumference thereof and an outer circumference of the through-hole  112 , and the other is provided with an annular protrusion  116  protruding along an outer circumference thereof, and a first flat coupling face  118  inside the protrusion  116 . 
     As illustrated in  FIGS. 4 and 5 , the second end plug  200  is configured as a cylindrical member having a body  210  and a through-hole  212  having the same respective diameters as the body  110  and through-hole  112  of the first end plug  110 . Here, one of circular upper and lower surfaces of the body  210  is provided with a groove  214  identical to the groove  114  of the first end plug  100 , and the other is provided with a cylindrical insert  217  having an annular space  216  along an outer circumference thereof so as to correspond to the annular protrusion  116  of the first end plug  100 , and a second flat coupling face  218  on an upper surface of the insert  217 . 
     The first and second end plugs  100  and  200  having the aforementioned configuration are coupled with each other in such a manner that the protrusion  116  of the first end plug  100  enters the annular space  216  of the second end plug  200 , so that the first and second coupling faces  118  and  218  are engaged with each other. After the first and second end plugs  100  and  200  are coupled, they are subjected to resistance welding or laser welding, and thereby are further firmly coupled. 
     Meanwhile, in an exemplary embodiment of the present invention, the coupling between the first and second end plugs  100  and  200  is based on a quick-connection mode. 
     The quick-connection mode based coupling is accomplished by forming “L” shaped coupling recesses  130 , each of which is partially open, in the protrusion  116  of the first end plug  100 , and latches  230  fitted into the respective coupling recesses  130  on an outer circumference of the insert  217  of the second end plug  217 . Thus, the latches  230  are inserted into inserting openings  132  that are the open ends of the coupling recesses  130  first, and then are turned along flat seats  134  extending from the inserting openings  132  perpendicular to the inserting openings  132 . Thereby, the first and second end plugs  100  and  200  are rapidly and simply coupled with each other. A rotating angle required to fully insert the latches  230  into the seats  134  is properly about 15° in consideration of actual workability, so that an angle of circumference of each seat  134  is preferably limited to about 15°. 
     The number of coupling recesses  130  is identical to that of the latches  230 . Further, the latches  230  are formed on the outer circumference of the insert  217  of the second end plug  200  so as to correspond to positions where the coupling recesses  130  are formed. The number of coupling recesses  130  and the number of latches  230  can be set to one or two or more, and preferably four in consideration of the strength of the protrusion  116  of the first end plug  100 . 
     Particularly, each of the seats  134  of the coupling recesses  130  is provided with a locking step  136  protruding in the shape of a hill so as to prevent the coupling between the coupling recesses  130  and the latches  230  from becoming loosened. In this manner, when the locking steps  136  slightly protruding in the shape of a hill are formed on the respective seat  134  adjacent to the inserting openings  132 , the latches  230  can be inhibited from returning to their original state before being coupled after being seated on the coupling recesses  130 . At this time, the top of each locking step  136  is smoothly formed as in a curved or flat plane, and the latches  230  can be prevented from being damaged to their surfaces while riding across the locking steps  136 . 
     Meanwhile, in the case of inner and outer annular faces  120  and  122  located inside and outside the groove  114  of the first end plug  100 , the former preferably protrudes higher than the latter. This relationship is equally applied to inner and outer annular faces  220  and  222  located inside and outside the groove  214  of the second end plug  200 . This is attributable to the fact that the segmented dual-cooled fuel rod is configured of concentric inner and outer cladding tubes, the inner annular faces  120  and  220  of the first and second and plugs  100  and  200  are welded to the inner cladding tube first, and then the outer annular faces  122  and  222  of the first and second end plugs  100  and  200  are welded to the outer cladding tube. In other words, the inner annular faces  120  and  220  are configured to protrude higher than the outer annular faces  122  and  222 , so that the inner annular faces  120  and  220  can be welded to the inner cladding tube while avoiding interfering with the outer annular faces  122  and  222 . 
     At least one of the first and second end plugs  100  and  200 , particularly one used as a lower intermediate end plug of a segmented upper fuel rod  600 , which will be described below, is preferably provided with at least one of complementary channel holes  140  and  240  communicating with the through-holes  112  and  212 . For example, when an internal channel  540  of the segmented lower fuel rod (i.e. upstream of the coolant) is blocked, the coolant can be complemented into the internal channel  540  through the complementary channel hole  140  formed in the lower intermediate end plug located above the segmented lower fuel rod, thereby reducing a possibility of causing departure from nuclear boiling ratio (DNBR) attributable to the failure to supply the coolant to the entire internal channel  540  of the dual-cooled fuel rod  500 . 
     Meanwhile, as illustrated in  FIGS. 7 and 8 , the dual-cooled fuel rod  500  is configured of a segmented upper fuel rod  600  having the lower intermediate end plug as the first end plug  100 , and a segmented lower fuel rod  700  having the upper intermediate end plug as the second end plug  200 . With the configuration of the segmented upper and lower fuel rods  600  and  700 , the dual-cooled fuel rod  500  can be configures by coupling the two segmented fuel rods  600  and  700  halving the elongation ratio compared to that of a conventional dual-cooled fuel rod. Here, the segmented upper fuel rod  600  refers to a segmented fuel rod having the upper end plug  610  at an upper end thereof, i.e. disposed at the uppermost portion of the dual-cooled fuel rod  500 . Similarly, the segmented lower fuel rod  700  refers to a segmented fuel rod having the lower end plug  710  at a lower end thereof, i.e. disposed at the lowermost portion of the dual-cooled fuel rod  500 . Further, the intermediate end plugs designate end plugs used for the segmented fuel rods  600 ,  700  and  800 , excluding the upper and lower end plugs  610  and  710 . 
     Further, as illustrated in  FIG. 9 , the dual-cooled fuel rod  500  can be configured of a segmented upper fuel rod  600  having the lower intermediate end plug as the second end plug  200 , and a segmented lower fuel rod  700  having the upper intermediate end plug as the first end plug  100 . In other words, both the first end plug  100  and the second end plug  200  can be freely selected as the upper or lower intermediate end plug, and thus do not need to be limited to the respective lower or upper intermediate end plug. 
     Furthermore, as well illustrated in  FIG. 10 , at least one segmented intermediate fuel rod  800  having the first and second end plugs  100  and  200  at opposite ends thereof is disposed between the segmented upper fuel rod  600  and the segmented lower fuel rod  700 . This structure means that the dual-cooled fuel rod  500  can be configured of three or more segmented fuel rods  600 ,  700  and  800 . Accordingly, the dual-cooled fuel rod  500  can freely adjust the elongation ratios of the segmented fuel rods  600 ,  700  and  800 . 
     According to the aforementioned configuration, the elongation ratios of the segmented upper fuel rod  600 , segmented lower fuel rod  700 , and segmented intermediate fuel rod  800  can be adjusted to a range from 100 to 200. 
     Meanwhile, at least one of the first and second end plugs  100  and  200  is provided with at least one of complementary channel holes  140  and  240  communicating with the through-holes  112  and  212 . Particularly, the first or second end plug  100  or  200  used as the lower intermediate end plug of the segmented upper fuel rod  600  is effectively provided with the complementary channel hole  140  or  240 . Thus, the coolant introduced into the complementary channel holes  140  and  240  enters the internal channel  540  of the dual-cooled fuel rod  500  through the through-holes  112  and  212 . 
     It is effective to form the complementary channel holes  140  and  240  so as to be inclined toward the segmented upper fuel rod  600 . This is because the coolant flows from the top to the bottom in the reactor core. At this time, the reason the inclined direction of the complementary channel holes  140  and  240  is set on the basis of the segmented upper fuel rod  600  is because the inclined directions are opposite to each other depending on whether the first and second end plugs  100  and  200  are used as the upper intermediate end plug or the lower intermediate end plug. This can be easily understood if  FIG. 5  is turned 180°. 
     Further, a plenum spring  510  and a spacer  520  can be installed in inner annular spaces of the segmented upper fuel rod  600 , segmented lower fuel rod  700 , and segmented intermediate fuel rod  800 . The plenum spring  510  inhibits vibration of the annular pellet fuel rod, and allows for lengthwise growth of the annular pellet caused by irradiation growth. The spacer  520  is interposed between the plenum spring  510  and the annular pellet so as to prevent the plenum spring  510  from coming into direct contact with the annular pellet, and adjusts a contracted length of the plenum spring  510  such that a proper spring force is applied to the annular pellet. 
     Although an exemplary embodiment of the present invention has been described for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.