Nuclear reactor core shroud fixing device

Abstract

The fixation device is installable on the outer circumferential surface of the reactor core shroud and in contact with the inner circumferential surface of the pressure vessel. The fixation device includes a base configured to contact the outer circumferential surface of the reactor core shroud. The fixation device also includes a radial expansion device including an actuator, a stationary support portion fixed to the base, and a movable contact portion. The radial expansion device is configured to allow the actuator to move the movable contact portion along the stationary support portion to radially compress the movable contact portion against the inner circumferential surface of the pressure vessel.

Description

[Technical field]

[0001] FIELD OF THE DISCLOSURE This disclosure relates generally to nuclear reactors, and more particularly to an apparatus for securing a core shroud within a nuclear reactor. [Background technology]

[0002] Boiling water reactors have a core shroud that keeps the fuel in proper alignment with the control rod drives so that the unit can be safely shut down. The core shroud has circumferential welds that hold the cylinders of the shroud together to maintain alignment. Some of these circumferential welds show signs of needing some kind of repair to allow continued safe operation.

[0003] No. 5,402,570 discloses a method for repairing a boiling water reactor shroud using a number of tie rods that apply a vertical compressive force to the shroud. No. 5,809,100 discloses a method and tool for measuring the preload applied to tie rods that apply a force to a reactor core shroud.

[0004] FIG. 1 shows a perspective view of a shroud including a larger shroud cylinder 114 positioned above a smaller shroud cylinder 112. The shroud further includes a disk portion (middle ring) that connects the larger shroud cylinder 114 to the smaller shroud cylinder 112 through two circumferential welds conventionally named welds H2 and H3. FIG. 1 further shows a number of blocks 110 that are bolted across the H2 and H3 welds to secure the smaller shroud cylinder 112 and the larger shroud cylinder 114 relative to each other. These repairs include (i) a number of these blocks, for example approximately 12, and (ii) a bolt 116 that passes completely through the cylinders 112, 114 to secure the block 110 to the cylinders 112, 114. Both features (i) and (ii) are required to achieve axial and radial support for the shroud cylinders during operation. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] U.S. Patent No. 5,402,570 [Patent Document 2] U.S. Patent No. 5,809,100 Summary of the Invention

[0006] A fixation device is provided for placement on an outer circumferential surface of the reactor core shroud and in contact with an inner circumferential surface of the pressure vessel. The fixation device includes a base configured to contact the outer circumferential surface of the reactor core shroud. The fixation device also includes a radial expansion device including an actuator, a stationary support portion secured to the base, and a moveable contact portion. The radial expansion device is configured to allow the moveable contact portion to move along the stationary support portion by the actuator to radially press the moveable contact portion against the inner circumferential surface of the pressure vessel.

[0007] A method is provided for installing a retention device on an outer circumferential surface of a reactor core shroud and in contact with an inner circumferential surface of a pressure vessel, the method including securing a base of the retention device to the outer circumferential surface of the reactor core shroud and moving an actuator of the retention device to urge a moveable contact portion of the retention device along a stationary support portion of the retention device to radially press the moveable contact portion against the inner circumferential surface of the pressure vessel.

[0008] The invention will now be described with reference to the following drawings, in which: [Brief description of the drawings]

[0009] [Figure 1] 1 shows a perspective view of a conventional arrangement of fixed portions of a nuclear reactor core shroud relative to one another. [Diagram 2] 1 shows a reactor core shroud assembly. [Diagram 3] 1 illustrates a perspective view of a fastening device according to an embodiment of the present invention installed on an outer circumferential surface of a shroud. [Figure 4] 1 illustrates a perspective view of a fastening device according to an embodiment of the present invention installed on an outer circumferential surface of a shroud. [Figure 5a] FIG. 5 shows an exploded view of the fixation device shown in FIGS. 3 and 4. [Figure 5b] FIG. 5 shows an exploded view of the fixation device shown in FIGS. 3 and 4. [Figure 6] FIG. 2 shows a plan view of a fixed device with a stationary part and a movable part connected together by an actuator. [Figure 7a-7d] 13 shows how the actuator moves axially to change the radial extent of the fixation device. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0010] Detailed Description One problem with the prior art is that they are very costly to implement due to the length of time required for installation and the expense of hardware that must be manufactured for repairs. Another problem with the prior art shown in Figure 1 is that the through holes in the shroud cylinder affect the stiffness of the shroud and the bolts that appear inside the shroud cylinder affect the fluid flow inside the shroud cylinder.

[0011] The present disclosure provides lateral support to the fixture to reduce the number of repairs required to provide the fixture for safe operation. The fixture of the present disclosure utilizes blind hole underwater threading to allow the base of the fixture to be oriented and bolted to the core shroud without creating a through hole, reducing bypass flow inside the core shroud associated with repairs that go completely through the shroud wall.

[0012] Advantages of the fixation device of the present disclosure can be as follows. - providing axial and radial support for the welds on the core shroud with the installation of a minimum number of fasteners (there may be only four fasteners of the present disclosure spaced circumferentially at 90 degree increments, as opposed to the approximately twelve shown in FIG. 1); - mounting the fixing device of the present disclosure on the shroud does not increase the bypass flow rate within the shroud; and - The tapered wedge shape of the fixture of the present disclosure allows the lateral support of the fixture to be adjusted during installation for optimal fit to the pressure vessel and allows for thermal expansion during operation (Belleville washers may be utilized to provide compliance to the lateral support).

[0013] FIG. 2 shows a schematic of a reactor core shroud assembly 10 surrounded by a reactor pressure vessel in a nuclear reactor. The shroud assembly 10 is centered on a vertically extending longitudinal central axis CA. As used herein, the terms circumferential, radial, axial, and derivatives thereof are defined relative to the central axis unless otherwise specified. The shroud assembly 10 includes a shroud 12 surrounded by a plurality of jet pump assemblies 14 provided in an annular space known as a downcomer radially between the shroud 12 and the pressure vessel and surrounding the nuclear fuel assemblies. Each jet pump assembly 14 includes two jet pumps 16 connected by a beam 20 to a riser pipe 18. Water enters the riser pipe 18, passes through the beam 20, and is then conveyed downwardly by a motive nozzle 24 into an inlet mixer 22.

[0014] 3 and 4 show perspective views of a fastening device 30 according to an embodiment of the present invention installed on the outer circumferential surface 32 of the shroud 12 above the jet pump assembly 14. FIG 4 shows how the fastening device 30 is radially clamped between the outer circumferential surface 32 of the shroud 12 and the inner circumferential surface 34 of the pressure vessel 36.

[0015] 3 and 4, the shroud 12 includes a first cylindrical portion 38 and a second cylindrical portion 40 having a smaller outer diameter than the first cylindrical portion 38. The first cylindrical portion 38 is positioned above the second cylindrical portion 40. The shroud 12 further includes a disc portion 42 connecting the first cylindrical portion 38 to the second cylindrical portion 40. The disc portion 42 extends radially inward from the first cylindrical portion 38 to the second cylindrical portion 40. The portions 38, 40, 42 together define the outer circumferential surface 32 of the shroud 12, where the portion 38 defines the first outer circumferential surface portion 32a, the portion 40 defines the second outer circumferential surface portion 32b, and the disc portion 42 defines the third outer circumferential surface portion 32c. Portions 38 , 42 have the same outer diameter, which is greater than the outer diameter of portion 40 .

[0016] The disk portion 42 is secured to the first cylindrical portion 38 by a first weld 44, known as an H2 weld, and to the second cylindrical portion 40 by a second weld 46, known as an H3 weld. Over time, the welds 44, 46 will show signs of needing some repair to allow continued safe operation of the shroud 12. The fastener 30 is applied to the shroud 12 to radially secure the portions 38, 40, 42 relative to one another to allow continued operation of the shroud 12.

[0017] 3 and 4, the fastener 30 includes a stepped base 48 configured to contact the outer circumferential surface 32 of the shroud 12. The base 48 includes a first radial contact portion 50 having a contact surface 50a configured to radially contact the first outer circumferential surface portion 32a and to radially contact the third outer circumferential surface portion 32c. The base 48 also includes a second radial contact portion 52 radially offset from the first radial contact portion 50 having a contact surface 52a configured to radially contact the second outer circumferential surface portion 32b. The base 48 further includes an axial contact portion 54 connecting the radial contact portions 50, 52 having a contact surface 54a configured to axially contact the lower radially extending surface 42a of the disc portion 42.

[0018] The securing device 30 further includes a radial expansion device 56 configured to bear against the inner circumferential surface 34 of the pressure vessel 36 to radially clamp the securing device 30 between the outer circumferential surface 32 of the shroud 12 and the inner circumferential surface 34 of the pressure vessel 36. The radial expansion device 56 includes a first portion in the form of a stationary support portion 58 rigidly fixed to the base 48 and a second portion in the form of a movable contact portion 60 that can be moved radially relative to the base 48 by an actuator 62 of the radial expansion device 56. As described further below, the stationary support portion 58 includes an inclined surface 64 (FIGS. 5a, 5b) and the movable contact portion 60 includes an inclined surface 66 (FIGS. 5a, 5b). The portions 58, 60 are configured such that axial movement of the movable contact portion 60 by the actuator 62 moves the movable contact portion 58 radially relative to the base 48 and the stationary support portion 58. Axial movement of the movable contact portion 60 changes the radial distance between the contact surface 50 a of the base 48 and the contact surface 56 a of the radial expansion device 56 .

[0019] The base 48 includes a plurality of holes 50b, 52b passing radially therethrough. More specifically, the first radial contact portion 50 includes two holes 50b passing radially therethrough, and the second radial contact portion 52 includes two holes 52b passing radially therethrough. The holes 50b pass from the contact surface 50a to the outer surface 50c of the first radial contact portion 50, and the holes 52b pass from the contact surface 52a to the outer surface 52c of the second radial contact portion 50. Each hole 50b, 52b receives a respective fastener 68, 70, which in a preferred embodiment is a bolt, for mounting the retainer 30 to the shroud 12. Each fastener 68, 70 includes a respective threaded shank 68a, 70a (FIGS. 5a, 5b) that extends through each of the holes 50b, 52b into a respective one of a plurality of blind threaded holes 72, 74 formed in the shroud 12 such that the threads of the shank 68a, 68b and the threads of the holes 72, 74 intermesh to secure the fastener 30 to the shroud 12. In particular, the first cylindrical portion 38 includes two threaded holes 72 formed therein that extend from the outer circumferential surface portion 32a into the first cylindrical portion 38 without breaking the inner circumferential surface 38a of the portion 38, and the second cylindrical portion 40 includes two threaded holes 74 formed therein that extend from the outer circumferential surface portion 32b into the second cylindrical portion 40 without breaking the inner circumferential surface 40a of the portion 40.

[0020] 5a and 5b show exploded views of the fixation device 30. As shown in Figs. 5a and 5b, the base 48 is integrally formed as a single piece with the stationary support portion 58. The stationary support portion 58 projects radially from the outer surface 50c of the first radial contact portion 50 and forms a first wedge portion 51 including three walls 76, 78, 80 extending from the outer surface 50c that define a receptacle 82 configured to receive the first end 60a of the mobile contact portion 60. The first and second walls 76, 78 define identical side walls of the support portion 58, and the third wall 80 defines a bottom wall. The third wall 80 includes an axially extending threaded hole 84 that passes completely through the axial direction and is configured to receive a portion of the actuator 62. Each of the walls 76, 78 has a tapered irregular prismatic shape such that the walls 76, 78 become gradually wider in the radial direction as the walls 76, 78 extend downwardly toward the bottom wall 80. Together, the walls 76, 78, 80 form the inclined surface 64 of the support portion 58. In particular, each wall 76, 78 includes a respective inclined surface portion 76a, 78a that extends further away from the first radial contact portion 50 as the surface portion 76a, 78a extends downwardly. In the embodiment shown in Figures 5a, 5b, the wall 80 also includes an inclined surface portion 80a that coincides with and is coextensive with the surface portions 76a, 78a. However, in alternative embodiments, the surface portion 80a may have a different shape.

[0021] The movable contact portion 60 includes a first end 60a for axial and vertical movement within the receptacle 82 and a second end 60b configured to contact the inner circumferential surface 34 of the pressure vessel 36 by a contact surface 56a. The first end 60a includes an insert portion 86 axially slidable within the receptacle 82, between the side walls 76, 78, toward and away from the bottom wall 80. The insert portion 86 includes a radially elongated slot 87 passing axially therethrough, configured to receive the actuator 62. The insert portion 86 is laterally connected to a second wedge portion 89 between side walls 88, 90 of the second wedge portion 89 that form the inclined surface 66. The walls 88, 90 each include an inclined surface portion 88a, 90a that forms the inclined surface 66. The walls 88, 90 are shaped such that the ramp surface 66 has a complimentary shape to the ramp surface 64 such that the ramp surface 64 maintains contact with the ramp surface 66 throughout axial and radial movement of the movable contact portion 60 by the actuator 62. The contact portion 60 further includes an arm 92 extending radially from the wedge portion 89 to a stop 94 provided at the second end 60b which includes the contact surface 56a which defines the radially outermost surface of the fixation device 30.

[0022] The actuator 62 is formed as a bolt including a head 96 and a threaded body 98. The actuator 62 is centered at the insert 86 between the walls 88, 90 by a washer 100. The washer 100 includes a square base 102 configured to slide in contact with the lateral surfaces of the walls 88, 90 while the actuator 62 is moving between the radial ends of the elongated slot 87. The actuator 62 is able to move axially within the elongated slot 87 because the radial length of the slot 87 is greater than the diameter of the body 98. The base 102 contacts a radially inner surface of an outer wall 104 of the wedge portion 89 when the actuator 62 is at its radially outermost position. The outer wall 104 extends laterally between the side walls 88, 90. During axial movement of the actuator 62, the base 102 moves radially along the walls 88, 90 toward and away from the outer wall 104. The washer 100 further includes a cylindrical centering wall 106 extending axially upwardly from the base 102 that is configured to receive the head 96 for adjusting the actuator 62 .

[0023] Figure 6 shows a top view of fixation device 30 with portions 58, 60 connected together by actuator 62. In Figure 6, actuator 62 is at its radially innermost position such that radially innermost contact surface 86a of insert portion 86 contacts outer surface 50c of first radial contact portion 50. As shown in Figure 6, wedge portion 89 radially overlaps wedge portion 51.

[0024] 7a-7d show how the actuator 62 moves axially within the bore 84 and radially within the elongated slot 87 to vary the radial expansion of the fixation device 30. FIGS. 7a and 7b show the actuator 62 in a first position with the fixation device 30 in a radially contracted position, with FIG. 7a showing a side view of the fixation device 30 and FIG. 7b showing a vertical cross-sectional view of the fixation device 30. In the position shown in FIGS. 7a-7b, the second wedge portion 89 is located at the top of the wedge portion 51, the actuator 62 is located such that the bottom of the body portion 98 is in the bore 84 and the top of the body portion 98 is located within the elongated bore 87, spaced away from both the radially inner edge 87a and the radially outer edge 87b of the elongated bore 87.

[0025] Figures 7c and 7d show the actuator 62 in a second position with the fixation device 30 in a radially expanded position, with Figure 7c showing a side view of the fixation device 30 and Figure 7d showing a vertical cross-section of the fixation device 30. In the position shown in Figures 7c and 7d, the second wedge portion 89 is located near the bottom of the wedge portion 51, the actuator 62 is in a position such that the bottom of the body 98 is below the hole 84 and the top of the body 98 is located within the elongated hole 87 in contact with the radially inner edge 87a.

[0026] 7a, 7b, where the fixation device 30 has a minimum radial length L1, to the position in Figures 7c, 7d, where the fixation device 30 has a maximum radial length L2, the actuator 62 rotates such that the body 98 moves axially downward within the bore 84 due to the engagement of the helical threads of the body 98 with the helical threads of the bore 84. As the actuator 62 moves axially downward, the inclined surface 66 of the movable contact portion 60 slides along the inclined surface 64 of the stationary support portion 58, causing the movable contact portion 60 to move radially outwardly away from the base 48. Movement of the movable contact portion 60 radially outwardly away from the base 48 enables the contact surface 56a of the stop 94 to bear against the inner circumferential surface 34 of the pressure vessel 36, such that the contact surface 50a of the first radial contact portion 50 is pressed radially against the first outer circumferential surface portion 32a of the shroud 12 and the contact surface 52a of the second radial contact portion 52 is pressed radially against the second outer circumferential surface portion 32b of the shroud 12.

[0027] The method of installing the fasteners 30 to the shroud 12 first includes machining threaded holes 72, 74 in the shroud 12. The method then includes aligning the holes 50b, 52b in the base 48 with the respective holes 72, 74 in the shroud, and installing fasteners 68, 70 through the respective holes 50b, 52b and into the respective holes 72, 74 such that the base 48 is secured to the shroud 12. The actuator 62 then moves axially downward such that the inclined surface 66 of the movable contact portion 60 moves along the inclined surface 64 of the stationary support portion 58 to press the movable contact portion 60 radially outward against the inner circumferential surface 34 of the pressure vessel 36. The fasteners 30 are spaced apart from one another circumferentially on the shroud 12 to support the alignment of the portions 38, 40, 42 of the shroud 12 relative to one another and relative to the vessel 36.

[0028] Thermal expansion of the shroud 12 and pressure vessel 36 may be counteracted using a series of Belleville washers on the actuator 62 that can be either below or above the moving contact portion 60. The location of the washers can be determined as thermal analysis is completed to determine the different growth characteristics.

[0029] In the above specification, the present invention has been described with respect to certain exemplary embodiments and examples thereof. However, it is apparent that various modifications and changes can be made thereto without departing from the broader spirit and scope of the invention as set forth in the following claims. The specification and drawings are therefore to be regarded in an illustrative rather than a restrictive sense. Particularly in a boiling water reactor shroud, any connection between two shroud cylinders assembled by disk sections and weld lines can be secured by utilizing the security device and method of the present invention. [Explanation of symbols]

[0030] 10 Shroud Assembly 12 Shroud 30 Fixation device 32 Outer circumferential surface of shroud 34 Inner circumferential surface of pressure vessel 36 Pressure Vessels 38 First cylindrical section 40 Second cylindrical section 42 Disc part 48 Base 50 first radial contact portion 51 First wedge part 52 second radial contact portion 54 Axial contact area 56 Expansion Unit 58 Stationary support part 60 Movable contact parts 62 Actuator 89 Second wedge section 100 Washer

Claims

1. 1. A fastener for installation on an outer circumferential surface of a nuclear reactor core shroud and in contact with an inner circumferential surface of a nuclear reactor pressure vessel, the fastener comprising: a base (48) configured to contact an outer circumferential surface of a reactor core shroud; A radial expansion device (56) including an actuator (62), a stationary support portion (58) fixed to a base (48), and a movable contact portion (60). a radial expansion device (56) configured to allow the movable contact portion (60) to move along the stationary support portion (58) by an actuator (62) to radially press the movable contact portion (60) against the inner circumferential surface of the pressure vessel; A radial expansion device (56) is configured such that axial movement of the actuator (62) radially moves the movable contact portion (60); the movable contact portion (60) includes an elongated hole (87) formed therein, such that as the actuator (62) moves the movable contact portion (60) radially, the actuator (62) is capable of moving radially within the elongated hole (87); The elongated hole (87) is a radially elongated slot; the radial length of the elongated slot is greater than the diameter of the body of the actuator (62), thereby allowing the actuator (62) to move radially within the elongated slot; the stationary support portion (58) includes an axially extending bore (84) formed therein, an actuator (62) movable within the axially extending bore (84) to radially move the movable contact portion (60); A fixation device in which an actuator (62) moves axially within an axially extending hole (84) and radially within an elongated slot in a stationary support portion (58) to vary the radial extent of the fixation device.

2. The fixation apparatus of claim 1 further comprising a plurality of fasteners for passing through the base (48) and into the reactor core shroud.

3. 3. The fixing device according to claim 1 or 2, wherein the stationary support portion (58) comprises a first wedge portion (51), and the movable contact portion (60) comprises a second wedge portion (89), the first wedge portion (51) being slidable along the second wedge portion (89) by an actuator (62) to radially press the movable contact portion (60) against the inner circumferential surface of the pressure vessel.

4. 4. The fixation device of claim 3, wherein the first wedge portion (51) includes a first inclined surface (64) and the second wedge portion (89) includes a second inclined surface (66) complementary to the first inclined surface (64).

5. 5. The fixing device according to claim 1, wherein the actuator (62) is a bolt extending through the movable contact portion (60) and through the stationary support portion (58).

6. 6. The fastening device of claim 1, wherein the base (48) includes a first radial contact portion (50) having a contact surface configured to radially contact a first outer circumferential surface portion of the reactor core shroud and a second radial contact portion (52) having a contact surface configured to radially contact a second outer circumferential surface portion of the reactor core shroud, the first radial contact portion (50) being radially offset from the second radial contact portion (52).

7. The fixing device according to any one of claims 1 to 6, wherein the stationary support portion (58) and the movable contact portion (60) are connected together by an actuator (62).

8. 1. A method for installing a fastener on an outer circumferential surface of a nuclear reactor core shroud and in contact with an inner circumferential surface of a pressure vessel, the method comprising: Securing a base portion (48) of a fastener to an outer circumferential surface of a reactor core shroud; moving an actuator (62) of the fastening device to urge a movable contact portion (60) of the fastening device along a stationary support portion (58) of the fastening device to radially press the movable contact portion (60) against the inner circumferential surface of the pressure vessel; the movable contact portion (60) including an elongated hole (87) therein, and movement of the actuator (62) includes moving the actuator (62) radially within the elongated hole (87); The elongated hole (87) is a radially elongated slot; the radial length of the elongated slot is greater than the diameter of the body of the actuator (62), thereby allowing the actuator (62) to move radially within the elongated slot; the stationary support portion (58) includes an axially extending bore (84), and movement of the actuator (62) moves the actuator (62) axially within the axially extending bore (84); The method, wherein the actuator (62) moves axially within an axially extending hole (84) and radially within an elongated slot of the stationary support portion (58) to vary the radial extent of the fixation device.

9. 9. The method of claim 8, wherein securing the base (48) to the outer circumferential surface of the reactor core shroud includes machining blind holes in the shroud and installing fasteners through the base and into the blind holes.

10. 10. The method of claim 8 or 9, wherein the stationary support portion (58) comprises a first wedge portion (50), the movable contact portion comprises a second wedge portion (52), and the movement of the actuator (62) comprises sliding the second wedge portion (52) along the first wedge portion (50) by the actuator (62).

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