JP4317652B2 - Substructure of fuel assembly for nuclear reactor and thimble screw for fuel assembly - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a fuel assembly, and particularly to a substructure of a fuel assembly for a pressurized water reactor.
[0002]
[Prior art]
Currently, there is a pressurized water reactor as a nuclear reactor widely used for power generation, and the fuel assembly used for it is generally a canless fuel assembly without a wrapper pipe. When this structure is outlined, an upper nozzle and a lower nozzle each having a plurality of coolant through holes are connected by a plurality of control rod guide tubes extending in parallel with each other. That is, the upper end of the so-called guide thimble of the control rod guide tube is mechanically coupled to the upper nozzle, and the lower end of the guide thimble is also mechanically coupled to the lower nozzle. Such a guide thimble accepts one elongated control rod of the control rod cluster one by one. Depending on the core loading position of the fuel assembly, the control rod is not in that position and will not be accepted. In this case, a non-fuel core structure such as a thimble plug or a flammable poison rod is accepted. A plurality of support grids are attached to such a guide thimble, and fuel rods are received in the grid openings and elastically supported.
[0003]
Of the structure of the fuel assembly outlined above, the structure of the connecting portion between the guide thimble and the lower nozzle will be described more specifically with reference to FIGS. First, referring to FIG. 9, an end plug 3 with an internal thread is attached to the lower end of a hollow tube-shaped guide thimble 1, and a bottomed cylindrical body called an insert 5 is put on these. The lower nozzle 7 located below the guide thimble 1 has a stepped through attachment hole 9 through which a fastening bolt called a thimble screw 11 is inserted. The thimble screw 11 is provided with a through-hole 13 so as to allow the cooling water to flow during use in the core. In this case, the screw at the tip of the thimble screw 11 is screwed into the inner screw of the end plug 3 to firmly clamp the insert 5 and connect the lower nozzle 7 and the guide thimble 1. A pin groove is formed in the head portion of the thimble screw 11, and a non-rotating pin 15 is inserted and fixed to the lower nozzle 7 by welding. On the other hand, a lower support grid 17 is fixed to the insert 5. A counterbore 19 is formed at the lower end of the through-hole 13 so that the anti-rotation pin 15 does not interfere with the flow of cooling water.
[0004]
[Problems to be solved by the invention]
In the conventional structure described above, the through water passage hole 13 of the thimble screw 11 guides the coolant into the guide thimble 1 in the core, and the introduced coolant is a non-fuel core structure attached thereto. Cool down. It also functions as a drain hole for discharging the internal coolant. Furthermore, the control rod is urgently inserted into the guide thimble during an emergency while the reactor is operating, but it also functions as a throttle for limiting the outflow speed of the internal coolant in order to reduce the drop impact at that time. In other words, the larger the diameter of the through water passage hole 13 is better in order to ensure the above-mentioned cooling function, but there is a conflicting property that the smaller diameter is better in order to ensure the throttling function. For this reason, there is a problem that the diameter range allowed for the through water passage hole 13 is small, and one function is often sacrificed to some extent.
In addition, the rotation stop pin is located at the lower entrance of the through water passage hole 13. Although not so frequently, there is a risk that both of the above-mentioned functions cannot be ensured if foreign substances wake up in the coolant and flow into the through water passage hole 13 and become clogged.
Therefore, the subject of this invention is providing the fuel assembly provided with the lower structure which can achieve the above-mentioned cooling water introduction function and throttle function simultaneously.
[0005]
[Means for Solving the Problems]
In order to solve the above-described problems, the present invention provides an upper nozzle, a lower nozzle, a plurality of control rod guide tubes that are connected to both nozzles and extend in parallel with each other, and a plurality of control rod guide tubes that are provided at intervals in the guide tube A fuel assembly having a plurality of fuel rods inserted and supported individually through a support opening of the support grid and a thimble screw that connects the guide tube to the lower nozzle is formed with a water passage hole therethrough. And a check valve-like function that allows upward flow and restricts downward flow is provided on the inlet side or the outlet side of the water hole of the thimble screw. Provide substructure.
Further, a thimble screw for connecting the control rod guide tube of the nuclear reactor fuel assembly and the lower nozzle, a screw surface screwed into the guide tube and a head having a diameter larger than the mounting hole of the lower nozzle. A fuel assembly thimble screw having a through water passage hole extending in the axial direction and having a valve portion that allows upward flow on the inlet side or outlet side of the water flow hole and restricts downward flow. provide.
In other words, a configuration is formed that exhibits a valve function that allows normal coolant upward flow but restricts the downward flow, and this configuration is preferably a water passage hole that penetrates the thimble screw in the axial direction. It is obtained by defining a slightly wider space at the inlet or outlet and disposing a small diameter valve ball in the space.
[0006]
DETAILED DESCRIPTION OF THE INVENTION
Embodiments of the present invention will be described below with reference to the accompanying drawings. Throughout the drawings, the same parts are denoted by the same reference numerals.
First, the overall structure of the fuel assembly 20 according to the present invention will be described with reference to FIG. The upper nozzle 21 is a bottomed box-like structure having a substantially square horizontal cross section, and a plurality of coolant flow holes and guide thimble mounting holes are formed in an end plate corresponding to the bottom plate. In addition, a pressing spring 21a is attached to the upper part. The lower nozzle 23 has a top plate portion or end plate having a substantially square plan view, and a plurality of coolant flow holes and guide thimble mounting holes are formed therein. Then, leg portions 23a are integrally formed to protrude from the bottom surfaces of the four corners of the end plate. These upper nozzle 21 and lower nozzle 23 are connected to the upper and lower ends of a plurality of hollow tube-shaped guide thimbles 25 using the aforementioned mounting holes. As in the case of the guide thimble 1, an end plug with an internal thread is fixed to the lower end portion of the guide thimble 25. In this guide thimble 25, one upper support lattice 27 and seven intermediate support lattices 28 are attached at intervals, and the lower support lattice 29 is also attached in a conventional connection structure. . It should be understood that the number of intermediate support grids 28 can be appropriately increased or decreased. The fuel rods 26 are inserted and supported one by one through the aligned lattice openings of the upper support lattice 27, the intermediate support lattice 28, and the lower support lattice 29, and the fuel assembly 20 is thus formed.
[0007]
Next, the structure of the thimble screw 30 according to the present invention for connecting the guide thimble 25 and the lower nozzle 23 will be described with reference to FIGS. The entire structure of the thimble screw 30 is shown in FIG. 2, and is roughly composed of three parts, that is, a head part 31, a central shaft part 33, and a valve part 35 at the tip. The detailed structure will be described. A pin groove 37 for a detent pin is formed in the head 31 and a counterbore hole 39 is also formed. The central shaft 33 is formed with a screw surface 41 that is screwed into an end plug (not shown) of the guide thimble 25, and a guide hole 43 having a relatively large diameter is continuously bored in the counterbore 39. Yes. The valve portion 35 is formed by a small outer diameter cylindrical valve box 45 integrally connected to the central shaft portion 33 and an inner small diameter valve ball 47. As specifically shown in FIG. 3, an opening 49 is formed in the valve box 45, which extends in a gate shape as shown in FIG. The hollow portion 51 in the valve box 45 communicates with the guide hole 43 through a small-diameter water hole 53, and a truncated inverted conical surface 55 is formed at the upper end of the water hole 53.
[0008]
When the fuel assembly 20 incorporating the thimble screw 30 is used, during normal operation, the reactor coolant, that is, light water flows upward from below, and as shown by the arrow F, the counterbore 39, the guide hole 43, the through hole, It flows in the order of the water hole 53 and the hollow portion 51 and flows into the lower portion of the guide thimble 25 from the opening portion 49. In this case, the valve ball 47 is pushed up by the water flow and floats up, sticking to the ceiling of the valve box 45, and does not hinder the flow of the coolant, so that a sufficient cooling function is obtained. On the other hand, when a control rod (not shown) is rapidly inserted into the guide thimble 25, the coolant flows as indicated by an arrow F shown in FIG. Accordingly, the valve ball 47 contacts the truncated inverted conical surface 55 by the action of its own weight and downward flow, but a minute coolant penetrates due to the difference in the geometric shape. In this way, the aperture function is achieved.
[0009]
Next, the structure of another thimble screw according to the present invention will be described with reference to FIGS. Although the entire structure of the thimble screw 60 is shown in FIG. 5, it has a head portion 61 and a shaft portion 63, and the valve portion is formed in the head portion 61. A screw surface 65 that is screwed into an end plug of the guide thimble 25 (not shown) is formed in the shaft portion 63, and a water passage hole 67 is drilled through the shaft portion 63 in the axial direction. On the other hand, a counterbore 69 is formed in the head 61 and is closed by a screw plug 71 screwed together. A truncated conical surface 73 is formed on the upper surface of the non-rotating plug 71, and a truncated conical surface 75 is formed on the upper surface of the counterbore hole 69, and a small diameter valve ball 77 is inserted therebetween. Further, a guide hole 79 is formed adjacent to the lower end portion of the water passage hole 67, and a through hole 81 is also formed in the non-rotating plug 71. Thus, the valve ball 77 placed in the counterbore 69 can move in the vertical direction and exhibits a valve function as described later. On the other hand, the non-rotating plug 71 is provided with a pin groove 83 across the through-hole 81, and a non-illustrating non-illustrating pin is inserted and fixed by welding after the assembly is completed.
[0010]
When the fuel assembly 20 incorporating the thimble screw 60 is used, similarly to the case of the thimble screw 30, the reactor coolant flows upward from below during normal operation. The counterbore 69 flows in the order of the counterbore 69, the guide hole 79, and the water passage hole 67, and flows into the lower portion of the guide thimble 25. In this case, the valve ball 77 is pushed by the water flow and floats, and is attached to the ceiling portion of the counterbore 69. However, since the guide hole 79 is provided, the flow of the coolant is not hindered and sufficient cooling function is achieved. Is obtained. On the other hand, when a control rod (not shown) is rapidly inserted into the guide thimble 25, the coolant flows as indicated by an arrow F shown in FIG. Therefore, the valve ball 77 is in contact with the truncated inverted cone surface 73, but a minute coolant penetrates due to the difference in the geometric shape. In this way, the aperture function is achieved.
[0011]
Next , another examination example of the thimble screw studied by the present inventor will be described with reference to FIG. In FIG. 8, the thimble screw 90 has a head portion 91 and a shaft portion 93. On the outer surface of the shaft portion 93, a screw surface 95 that is screwed into an end plug of the guide thimble 25 (not shown) is formed, and a water passage hole 97 having a diameter a extends in the axial direction. The diameter a is selected so as to adjust the inflow amount of the coolant. On the other hand, a counterbore hole 99 having a diameter b is formed in the head portion 91 and communicates with the water passage hole 97 through a guide hole 98 having an intermediate diameter. Further, a detent pin 94 having a diameter c extends in the radial direction and is provided in the counterbore hole 99. From the relationship between the diameters a, b, and c , the inflow of foreign matter larger than the diameter a of the water passage hole 97 is blocked at the inlet of the thimble screw 90.
[0012]
【The invention's effect】
As described above, according to the present invention, the amount of the coolant flowing into the control rod guide tube is regulated by the size of the water passage hole of the thimble screw, and the amount of the coolant flowing out through the thimble screw is the valve portion. since is restricted by the shape of the suitable size respectively maintain, Ki and cooling function and the stop function de be suitably retained, the both functions is always ensured.
[Brief description of the drawings]
FIG. 1 is an overall elevation view of a fuel assembly according to an embodiment of the present invention.
FIG. 2 is a partial elevation view showing a main part of the embodiment.
3 is a cross-sectional plan view taken along line III-III in FIG.
FIG. 4 is a partial elevational sectional view for explaining the operation of the embodiment.
FIG. 5 is a partial elevation view showing a main part of another embodiment of the present invention.
6 is a cross-sectional plan view taken along line VI-VI in FIG.
FIG. 7 is a partial elevational sectional view for explaining the operation of the another embodiment.
FIG. 8 is a partial elevational view showing a main part of still another example of a thimble screw studied by the present inventor according to the present invention .
FIG. 9 is a partial elevational sectional view showing the structure of a conventional one.
FIG. 10 is a bottom view corresponding to FIG. 9;
[Explanation of symbols]
20 Fuel assembly 21 Upper nozzle 23 Lower nozzle 25 Guide thimble 26 Fuel rod 27 Upper support lattice 28 Intermediate support lattice 29 Lower support lattice 30 Thimble screw 31 Head 33 Central shaft portion 35 Valve portion 39 Counterbore 41 Screw surface 43 Guide Hole 45 Valve box 47 Valve ball 49 Opening portion 51 Hollow portion 53 Water passage hole 60 Thimble screw 61 Head 63 Shaft portion 65 Screw surface 67 Water passage hole 69 Counterbore hole 71 Non-turn plug 77 Valve ball 79 Guide hole 81 Through hole

Claims (2)

The upper nozzle, the lower nozzle, a plurality of control rod guide tubes that are connected to both nozzles at both ends and extend in parallel with each other, a plurality of support grids provided at intervals in the guide tubes, and a grid opening of the support grid In a fuel assembly having a plurality of fuel rods inserted and supported in the thimble screw that connects the guide tube to the lower nozzle, a water passage hole is formed therethrough , allowing upward flow and restricting downward flow. A substructure for a fuel assembly for a reactor , wherein a check valve-like function is provided on an inlet side or an outlet side of the water passage hole of the thimble screw . A thimble screw for connecting a control rod guide tube and a lower nozzle of a fuel assembly for a nuclear reactor, and having a threaded surface screwed into the guide tube and a head having a diameter larger than a mounting hole of the lower nozzle. A thimble screw for a fuel assembly that includes a through water passage hole extending in the axial direction and has a valve portion that allows upward flow and restricts downward flow on the inlet side or the outlet side of the water passage hole .

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