Combination of top nozzle and guide thimble for nuclear fuel assembly

Abstract

A combination of a top nozzle and a guide thimble of a nuclear fuel assembly and, more particularly, a structure for joining an inner-extension tube, the top nozzle and the guide thimble. When an inner-extension tube head, which is provided as a means for facilitating removal of the top nozzle of the nuclear fuel assembly from the guide thimble, is removed from an inner-extension tube body to separate the top nozzle from the nuclear fuel assembly, the inner-extension tube body is prevented from undesirably rotating, so that the guide thimble and the inner-extension tube body can maintain the joined state.

Claims

1. A combination of a guide thimble and a top nozzle for a nuclear fuel assembly, the combination comprising: a guide thimble, and a top nozzle coupled to the guide thimble, the top nozzle comprising: a flow plate located above the guide thimble, with a coupling through hole formed through the flow plate; an outer guide post coupled at a lower end thereof to the coupling through hole of the flow plate; an inner-extension tube disposed in the outer guide post in such a way that a lower end of the inner-extension tube passes through the coupling through hole of the flow plate; and an inner-extension tube head coupled to an upper end of the inner-extension tube and to an upper end of the outer guide post, wherein: the inner-extension tube includes at least one rotation-preventing protrusion provided on a circumferential outer surface of the inner-extension tube such that the rotation-preventing protrusion is disposed within the coupling through hole of the flow plate; a circumferential inner surface of the coupling through hole of the flow plate includes a first part disposed at a position corresponding to the rotation-preventing protrusion of the inner-extension tube and a second part; and the rotation-preventing protrusion and the first part, collectively, prevent the inner-extension tube from rotating when the inner-extension tube head is rotated.

2. The combination as set forth in claim 1, wherein the rotation-preventing protrusion has a planar surface, and the first part is formed as a planar surface.

3. The combination as set forth in claim 1 further comprising: a first coupling protrusion provided on a circumferential outer surface of the inner-extension tube head; a first coupling groove formed in a circumferential inner surface of the inner-extension tube so that the first coupling protrusion is inserted into the first coupling groove; a second coupling protrusion provided on the circumferential outer surface of the inner-extension tube; and a second coupling groove formed in a circumferential inner surface of the outer guide post so that the second coupling protrusion is inserted into the second coupling groove.

4. The combination as set forth in claim 3, wherein the inner-extension tube has at least one longitudinal slit extending a predetermined length from an upper end of the inner-extension tube.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The above and other objects, features and advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which:

(2) FIG. 1 is a schematic view of a conventional nuclear fuel assembly;

(3) FIG. 2 is a partial sectional view of a conventional top nozzle;

(4) FIG. 3 is a sectional view of a conventional guide thimble;

(5) FIG. 4 is a perspective sectional view showing the joining of a top nozzle to a guide thimble according to the present invention;

(6) FIG. 5 is a perspective sectional view of an outer guide post according to the present invention;

(7) FIG. 6 is a perspective view of an inner-extension tube according to the present invention;

(8) FIG. 7 is a perspective sectional view of a flow plate according to the present invention;

(9) FIG. 8 is a perspective view showing an inner-extension tube according to a first embodiment of the present invention;

(10) FIG. 9 is a perspective view showing an inner-extension tube according to a second embodiment of the present invention;

(11) FIG. 10A is a perspective sectional view showing an embodiment of the flow plate according to the present invention;

(12) FIG. 10B is a bottom view showing a joint structure between the flow plate and the inner-extension tube of the first embodiment of the present invention;

(13) FIG. 10C is a bottom view showing a joint structure between the flow plate and the inner-extension tube of the second embodiment of the present invention;

(14) FIG. 11 is a perspective view showing an inner-extension tube body according to a third embodiment of the present invention;

(15) FIG. 12 is a perspective sectional view of a flow plate according to the third embodiment of the present invention;

(16) FIG. 13 is a perspective sectional view of an outer guide post according to a fourth embodiment of the present invention;

(17) FIG. 14 is a perspective view of an inner-extension tube body according to the fourth embodiment of the present invention; and

(18) FIG. 15 is a perspective sectional view of an inner-extension tube head according to the fourth embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

(19) The present invention is provided to achieve the above-mentioned object.

(20) The present invention is characterized by the constructions of an outer guide post 120, an inner-extension tube 150, a flow plate 160 and a guide thimble 3.

(21) (Therefore, a hold-down plate and a hold-down spring which are elements of a top nozzle but are not directly related to the present invention will not be explained herein.)

(22) Hereinafter, a joint structure among the above elements will be described in detail. FIG. 4 illustrates the joint structure.

(23) Referring to FIG. 5, the shape of the outer guide post 120 is that of a hollow cylinder that has open upper and lower ends. An external thread 102 is formed on a predetermined portion of an outer surface of a lower end of the outer guide post 120 and is used to couple the outer guide post 120 to the flow plate 160.

(24) The diameter of a predetermined portion of an upper end of the outer guide post 120 larger than those of other portions of the outer guide post 120. Thus, an annular retaining part 104 is formed by this difference in diameter so that the outer guide post 120 can be joined with a hold-down plate (not shown) by means of the annular retaining part 104.

(25) As shown in FIG. 6, the inner-extension tube 150 includes an inner-extension tube body 130 and an inner-extension tube head 140.

(26) The shape of the inner-extension tube body 130 is that of a hollow cylinder that has open upper and lower ends in the same manner as that of the outer guide post 120. Furthermore, the inner-extension tube body 130 is longer than the outer guide post 120 and is disposed in the outer guide post 120 in such a way that an upper end of the inner-extension tube body 130 is level with that of the outer guide post 120 while its lower end protrudes outwards from that of the outer guide post 120.

(27) An external thread 132 is formed on a predetermined portion of the lower end of the inner-extension tube body 130, in other words, on the portion of the inner-extension tube body 130 that protrudes outwards from the outer guide post 120. The external thread 132 is used to couple the inner-extension tube body 130 to the guide thimble 3 which will be explained in detail later herein.

(28) The inner-extension tube head 140 is coupled to the upper end of the inner-extension tube body 130. The shape of the inner-extension tube head 140 is that of a cork stopper of which upper and lower ends are the same in shape but are different in diameter. Further, the inner-extension tube head 140 has a hollow structure which is open on upper and lower ends thereof. An internal thread 142 is formed on a predetermined portion of an inner surface of the lower end of the inner-extension tube head 140 so that it is threaded over a circumferential outer surface of an upper end 134 of the inner-extension tube body 130.

(29) Here, an annular retaining part 144 is formed by a difference in diameter between the upper and lower ends of the inner-extension tube head 140. The maximum diameter of the annular retaining part 144 is equal to or larger than the diameter of the upper end of the outer guide post 120 so that the inner-extension tube head 140 can be placed on the upper end of the outer guide post 120 rather than being inserted thereinto. Furthermore, the inner-extension tube head 140 has a thin film structure such that it is crimped into a depression of the outer guide post 120, thus preventing the inner-extension tube head 140 from becoming loose.

(30) A rotation-preventing surface 136 is formed on the lower portion of the inner-extension tube body 130.

(31) The rotation-preventing surface 136 is disposed above the external thread 132 that is formed on the lower end of the inner-extension tube body 130. The rotation-preventing surface 136 is a planar surface formed by cutting out a portion of an annular flange provided above the external thread 132. The rotation-preventing surface 136 functions to prevent the inner-extension tube body 130 from rotating when the inner-extension tube head 140 is separated from the inner-extension tube body 130.

(32) As shown in FIG. 7, a coupling through hole 162 is formed at a predetermined position in the flow plate 160 so that the outer guide post 120 is coupled to the inner-extension tube 150 via the screw coupling the upper peripheral surface on the hole 162.

(33) An female screw is formed on a circumferential inner surface of the coupling through hole 162. The diameter of the annular retaining part is the same as the outer diameter of the outer guide post 120. A lower surface of the annular retaining part is put into close contact with an upper surface of the annular flange provided on the inner-extension tube body 130.

(34) The diameter of an upper end of the coupling through hole 162 is the same as the outer diameter of the outer guide post 120. An internal thread is formed on a circumferential inner surface of the upper end of the coupling through hole 162 so that the lower end of the outer guide post 120 is threaded into the coupling through hole 162.

(35) A rotation-preventing portion 166 is formed on a lower end of the inner surface of the coupling through hole 162. The rotation-preventing portion 166 is disposed at a position corresponding to the rotation-preventing surface 136 of the inner-extension tube body 130 and has a polygonal shape corresponding to the shape of the rotation-preventing surface 136.

(36) A threaded portion 31 is provided on an upper end of the guide thimble 3 (refer to FIG. 3). The shape of the threaded portion 31 of the guide thimble 3 is that of a hollow cylinder that has an open upper end. The diameter of the threaded portion 31 is the same as that of the inner-extension tube 150. An internal thread is formed on the inner surface of the threaded portion 31.

(37) The lower end of the inner-extension tube body 130 is threadedly coupled to the threaded portion 31 of the guide thimble 3.

(38) A process of disassembling the top nozzle from the guide thimble will be explained with reference to FIG. 6.

(39) First, the inner-extension tube heads 140 are rotated with respect to the corresponding inner-extension tube bodies 130 and removed therefrom. Thereafter, the top nozzle that includes the outer guide posts 120, the flow plate 160, hold-down springs (not shown) and the hold-down plate (not shown) is separated from the nuclear fuel assembly.

(40) When each inner-extension tube head 140 is separated from the corresponding inner-extension tube body 130, the thread-coupling between the inner-extension tube body 130 and the threaded portion 31 of the corresponding guide thimble 3 may become loose. However, by virtue of the rotation-preventing surface 136 formed on the inner-extension tube body 130 and the rotation-preventing portion 166 provided on the flow plate 160, the undesirable separation of the inner-extension tube body 130 from the threaded portion 31 of the guide thimble 3 can be prevented.

(41) FIGS. 8 through 10 illustrate first and second embodiments of the rotation-preventing surface of the inner-extension tube body and the rotation-preventing portion of the flow plate.

(42) Unlike the inner-extension tube body 130 described above, several planar surfaces may be formed on the annular flange of the inner-extension tube body 130 to provide a plurality of rotation-preventing surfaces 135.

(43) In detail, as shown in FIG. 8, the rotation-preventing surfaces 135 are formed on the annular flange at positions spaced apart from each other at angular intervals of 90. Planar cut portions 133 are formed on opposite sides of each rotation-preventing surface 135.

(44) The planar cut portions 133 formed between the rotation-preventing surfaces 135 function to prevent interference between fuel rods and the annular flange of the inner-extension tube body when the fuel rods, which are disposed adjacent to the guide thimble in spacer grids of the nuclear fuel assembly, are inserted longitudinally into or drawn out from the nuclear fuel assembly.

(45) The second embodiment of the inner-extension tube body 130 is illustrated in FIG. 9.

(46) As shown in FIG. 9, rotation-preventing surfaces 138 are formed on an annular flange of the inner-extension tube body 130 at positions spaced apart from each other at angular intervals of 90. Recesses 139 are formed in the annular flange on opposite sides of each rotation-preventing surface 138 at positions spaced apart from each other by a predetermined distance. The recesses 139 conduct the same role as that of the planar cut portion 133 described in the first embodiment.

(47) The flow plate 160 which is coupled to the inner-extension tube body 130 of the first or second embodiment may be used in such a way that flow plates that correspond to the respective flanges of the inner-extension tube bodies of the first and second embodiments are separately provided. However, as shown in FIG. 10A, the lower end of the coupling through hole of the flow plate 160 preferably has a shape corresponding to the annular flange of the inner-extension tube body 130 that is provided only with the rotation-preventing surfaces 135 without any planar cut portion 133. In this case, as shown in FIG. 10B, when the inner-extension tube body 130 is coupled to the flow plate 160, the rotation-preventing surfaces 135 are put into close contact with the flow plate 160, but portions of the annular flange other than the rotation-preventing surfaces 135 are spaced apart from the flow plate 160 rather than making contact with it.

(48) Furthermore, the flow plate 160 that has the shape of FIG. 10A can be coupled not only to the inner-extension tube body of the first embodiment but also to that of the second embodiment. As shown in FIG. 10C, the general shape of the annular flange of the inner-extension tube body of the second embodiment is the same as that of the annular flange of the inner-extension tube body of the first embodiment, except for a difference in shape between the planar cut portions 133 and the recesses 139. Therefore, the flow plate of FIG. 10A can be used for the inner-extension tube body of the second embodiment.

(49) FIGS. 11 and 12 illustrate a third embodiment with regard to the rotation-preventing surface of the inner-extension tube body and the rotation-preventing portion of the flow plate.

(50) In this embodiment, a plurality of rotation-preventing protrusions 137 are provided on the inner-extension tube body 130 and arranged in the circumferential direction at the same positions as those of the rotation-preventing surfaces 136 of the inner-extension tube body 130 described above.

(51) Rotation-preventing recesses 167 that have shapes corresponding to the rotation-preventing protrusions 137 are formed in the flow plate 160 at the same positions as those of the rotation-preventing portions 166 of the flow plate 160 described above.

(52) The joint structure between the inner-extension tube body and the inner-extension tube head of the inner-extension tube or between the inner-extension tube body and the outer guide post may be embodied by force-fitting rather than thread-coupling (in a fourth embodiment).

(53) Referring to FIG. 13, an outer guide post 220 is shaped like a hollow cylinder that has upper and lower ends that are open. An external thread 202 is formed on a predetermined portion of an outer surface of a lower end of the outer guide post 220. The external thread 202 is used to join the outer guide post 220 with a flow plate 160 which will be explained later herein.

(54) The diameter of a predetermined portion of an upper end of the outer guide post 220 is larger than at other portions of the outer guide post 220. Thus, an annular retaining part 204 is formed by this difference in diameter so that the outer guide post 220 can be joined with a hold-down plate (not shown) by means of the annular retaining part 204.

(55) A coupling groove 206 is formed in an inner surface of the outer guide post 220. The coupling groove 206 is used to join the outer guide post 220 with an inner-extension tube which will be explained later herein.

(56) The inner-extension tube includes an inner-extension tube body 230 and an inner-extension tube head 240.

(57) As shown in FIG. 14, the shape of the inner-extension tube body 230 is that of a hollow cylinder that has open upper and lower ends in the same manner as that of the outer guide post 220. Furthermore, the inner-extension tube body 230 is longer than the outer guide post 220 and is disposed in the outer guide post 220 in such a way that an upper end of the inner-extension tube body 230 is level with that of the outer guide post 220 while its lower end protrudes outwards from that of the outer guide post 220.

(58) An external thread 232 is formed on a predetermined portion of the lower end of the inner-extension tube body 230, in other words, on the portion of the inner-extension tube body 230 that protrudes outwards from the outer guide post 2220. The external thread 232 is used to join the inner-extension tube body 230 with the guide thimble 3 which will be explained in detail later herein.

(59) Furthermore, a circumferential coupling protrusion 234 is provided on the inner-extension tube body 230. The coupling protrusion 234 is disposed on a circumferential outer surface of the inner-extension tube body 230 at a position corresponding to the coupling groove 206 formed in the inner surface of the outer guide post 220 so that when the inner-extension tube body 230 is inserted into the outer guide post 220, they are coupled to each other by the coupling groove 206 and the coupling protrusion 234.

(60) A plurality of longitudinal slits 236 of a predetermined length are formed in the upper end of the inner-extension tube body 230. The longitudinal slits 236 make it possible for the upper end of the inner-extension tube body 230 to move elastically so that when the inner-extension tube body 230 is inserted into the outer guide post 220, the coupling protrusion 234 can be easily hooked into the coupling groove 206.

(61) Further, a coupling groove 238 is formed in an inner surface of the inner-extension tube body 230. The coupling groove 238 is used to join the inner-extension tube body 230 with an inner-extension tube head 240, and it will be explained later herein.

(62) A rotation-preventing surface 239 is formed on the lower portion of the inner-extension tube body 230.

(63) The rotation-preventing surface 239 is disposed above the external thread 232 that is formed on the lower end of the inner-extension tube body 230. The rotation-preventing surface 239 is a planar surface formed by cutting out a portion of an annular flange provided above the external thread 232. The rotation-preventing surface 239 functions to prevent the inner-extension tube body 230 from rotating when the inner-extension tube head 240 is separated from the inner-extension tube body 230.

(64) As shown in FIG. 15, the inner-extension tube head 240 is coupled to the upper end of the inner-extension tube body 230 and has the shape of a cork stopper whose upper and lower ends are the same in shape but different in area. The inner-extension tube head 240 has a hollow structure which is open on upper and lower ends thereof.

(65) A coupling protrusion 242 extends in a circumferential direction and is provided on a circumferential outer surface of a lower portion of the inner-extension tube head 240. The coupling protrusion 242 is disposed at a position that corresponds to the position of the coupling groove 238 of the inner-extension tube body 230 when the inner-extension tube head 240 is joined with the inner-extension tube body 230.

(66) A fitting protrusion 244 which extends in a circumferential direction is provided on the circumferential outer surface of the lower portion of the inner-extension tube head 240. The fitting protrusion 244 functions to apply pressure to the inner surface of the inner-extension tube body 230 outwards when the inner-extension tube 250 is coupled to the outer guide post 220, thereby strengthening the coupling between them.

(67) A plurality of through holes 246 are formed in the circumferential surface of the lower end of the inner-extension tube head 240. The through holes 246 allow a tool or the like to be inserted thereinto to facilitate removal of the inner-extension tube head when disassembling. A taper 248 is formed in the lower end of the inner-extension tube head 240, thus making it easy to insert the inner-extension tube head 240 into the inner-extension tube body 230 or remove it therefrom.

(68) The joining between the top nozzle and the guide thimble using the force-fitting structure seldom causes the inner-extension tube body to be rotated. Therefore, this joint structure can be used regardless of the presence of the rotation-preventing surface

(69) As described above, in the present invention, a rotation-preventing surface is formed on an inner-extension tube, and a rotation-preventing portion is formed on a flow plate. Thus, when a top nozzle is separated from a guide thimble, the inner-extension tube is prevented from undesirably rotating, thereby preventing the inner-extension tube from becoming loosened from the guide thimble. Therefore, a separate rotation-preventing member is not required. Furthermore, because the area of the contact portion between elements that rotate can be minimized, the assembly or disassembly of the top nozzle can be facilitated, thus reducing the time required to assemble or disassemble the structure.

(70) Although the preferred embodiments of the present invention have been disclosed 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.