Closure head attachment

Abstract

A fixture is provided for mounting a closure head during manufacture of a reactor pressure vessel closure head. The closure head has a central section for passage of control rod guide tubes therethrough and a peripheral section including a circumferential array of through holes corresponding to the locations of bolt holes through which the closure head will be bolted to the reactor pressure vessel in use. The fixture includes a support ring providing a circumferential array of upstanding projections matingly engageable through respective holes in the circumferential array of holes in the peripheral section of the closure head for mounting the closure head to the support ring. The fixture also includes a lower frame for holding the support ring and the closure head mounted thereon off the ground.

Description

[Technical Field]

[0001] The present invention relates to a fixture for attaching a closure head to a nuclear reactor pressure vessel during its manufacture. [Background technology]

[0002] Nuclear power plants convert thermal energy from the nuclear decay of fissile material contained in fuel assemblies within the reactor core into electrical energy. Water-cooled nuclear power plants, such as pressurized water reactors (PWRs) and boiling water reactors (BWRs), contain a reactor pressure vessel (RPV) that houses the reactor core / fuel assemblies, and turbines that generate electricity from steam produced by the heat from the fuel assemblies.

[0003] A conventional nuclear power plant can have an output of approximately 1600 MWe. However, the footprint and capital costs of a conventional nuclear power plant are high.

[0004] To reduce the required footprint and / or reduce capital costs, small or medium modular reactors (SMRs) can be used. Small or medium modular reactors can produce up to about 700 MWe. They are called modular reactors because they are assembled from modular units manufactured in a factory, then transported to and installed on site. The economies of scale achievable from modular construction mean that SMRs can be an economically viable option over conventional nuclear power plants, which have high capital costs.

[0005] An RPV typically includes a body defining a cavity for containing the reactor core / fuel assemblies and a closure head for closing an upper opening to the cavity. The closure head generally forms part of an integrated head package (IHP) and further includes multiple control rod drive mechanisms within the IHP structure. Each control rod drive mechanism may include multiple drive rods that extend through the closure head and connect to control rods housed within the reactor core. The control rods are provided to absorb neutron radiation within the reactor core and control nuclear reactions within the core. The drive rods within each control rod drive mechanism are driven by a power source that controls an elevator used to vertically raise the control rod within the reactor core.

[0006] The RPV closure head is a complex, heavy component that requires significant manufacturing operations. For example, the closure head weighs approximately 40 tons, and when combined with the rest of the IHP, the total weight is approximately 120 tons. This presents challenges when moving the closure head during manufacturing operations in the factory, not only because of the closure head's weight, but also because of the need to avoid damage during the manufacturing process. In particular, the control rod latch assembly housing and control rod guide tube (also known as the adapter tube) that pass through the closure head can easily be damaged, which, if damaged, can result in significant rework, delays, or scrap. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Publication No. 6-347581 Summary of the Invention [Problem to be solved by the invention]

[0008] The present invention has been made in view of the above points. [Means for solving the problem]

[0009] In a first aspect, the present disclosure provides a fixture for attaching a closure head during manufacture of a reactor pressure vessel closure head, the closure head having a central section for passage of control rod guide tubes therethrough and a peripheral section including a circumferential row of through holes corresponding to the locations of bolt holes through which the closure head will be bolted to the reactor pressure vessel in use, the fixture comprising: a support ring providing a circumferential row of upstanding projections matingly engageable through respective holes in the circumferential row of holes in the peripheral section of the closure head for mounting the closure head to the support ring; and A lower frame for holding the support ring and the closure head mounted thereon off the ground.

[0010] The holes in the peripheral section provide a convenient means for holding the closure head on the support ring. Typically, the fixture is a transport fixture along which the closure head is moved during manufacturing operations, for example, using a guided vehicle or via a crane. In this case, the fixture is particularly useful as a means for safely moving the head from location to location within a factory. Typically, the central section of the closure head is dome-shaped and is attached to the support ring with the convex side of the closure head facing downward.

[0011] The upstanding protrusions can form fastening points for fixing lifting gear, so that when the closure head is mounted on the support ring, lifting performed using the lifting gear fixed to the fastening points lifts the closure head and optionally the support ring and lower frame. For example, the fastening points can be threaded holes formed in the upstanding protrusions for fixing eyebolts of the lifting gear. The first group of upright protrusions may be provided by a first portion of a support ring that is rigidly coupled to the lower frame, and the second group of upright protrusions may be provided by a plurality of second portions of the support ring that are liftably removable from the first portion, with the first and second groups of upright protrusions forming first and second fastening points, respectively, for securing lifting gear, such that when the closure head is attached to the support ring, lifting performed using the lifting gear secured to the first fastening points lifts the closure head, the support ring, and the lower frame, while lifting performed using the lifting gear secured only to the second fastening points lifts only the closure head and the second portion of the support ring. In this way, by selecting either the first or second group of protrusions for the lifting operation at a given location, the closure head can be lifted with or without the lower frame, depending on the type of manufacturing operation being performed. Conveniently, the second portions of the support ring can be received in respective recesses formed in the first portion. When the closure head lifted by the second group of protrusions is returned to the lower frame, the second portion is simply re-entered into its respective recess. Typically, the first group of upright protrusions can be formed by a first subgroup of an integer N adjacent upright protrusions, and the second group of upright protrusions can be formed by a second subgroup of N adjacent upright protrusions, with the first and second subgroups arranged in an alternating pattern around the support ring. For example, N can be 3 or 4, which can provide a stable lifting configuration.

[0012] Conveniently, the fastening points (particularly the first fastening points when the first and second groups of upstanding protrusions form the first and second fastening points, respectively) can be used to clamp the closure head to a fixture when it is moved from location to location during a manufacturing operation. They can also be used to clamp the closure head to a fixture when a particular manufacturing operation requires the closure head to be attached to a lower frame and the operation involves tilting or moving the closure head. For example, the closure head can be attached to a lower frame, and the lower frame can then be attached to a manipulator to rotate the closure head laterally up to 90° while performing a cladding operation.

[0013] Advantageously, the first and second fastening points can be configured differently such that a lifting gear that can be secured to the first fastening point cannot be secured to the second fastening point, and vice versa. For example, if the fastening points are threaded holes formed in the upright protrusions for fastening the eyebolts of the lifting gear, the threaded holes formed in the first group of upright protrusions can have a different thread and / or a different diameter than the threaded holes formed in the second group of upright protrusions. In this way, the fixture provides a "poka-yoke" type of configuration that prevents the closure head from being accidentally lifted with the fixture when it is not needed for the next manufacturing operation, or without the fixture when it is needed for the operation.

[0014] The lower frame can be configured to shield the control rod guide tubes that pass through the central section from accidental damage. For example, the lower frame can have a guard portion that extends from the base of the lower frame to the support ring, thereby helping to protect the closure head. The guard portion can be a solid wall or can have an opening formed therein so that the underside of the closure head is visible from outside the fixture. Conveniently, the lower frame can have at least one opening formed therein to allow a human or manipulator to access the underside of the closure head.

[0015] The lower frame may, for example, be generally cylindrical in shape to fit the generally circular peripheral section of the closure head, but other shapes of the lower frame are also possible, such as a generally prismatic cylinder with a square cross section.

[0016] The fixture can have rotational structures, such as trunnions at the upper and lower ends of the lower frame, configured to facilitate rotation of the lower frame and support ring. This can be useful when a particular manufacturing operation requires the closure head to be rotated while attached to the fixture. A lower frame having a generally prismatic, cylindrical shape can be advantageous for such operations because the fixture can more easily balance on its base as it is rotated.

[0017] In a second aspect, the present disclosure provides a fixture system including the fixture of the first aspect as a first fixture and a removable second fixture for holding the first fixture off the ground, for example, to allow better access to the end of a control rod guide tube passing through the central section of the closure head. The removable second fixture can include a second support ring for mounting the base of the first fixture thereon and a second lower frame for holding the second support ring and the first fixture mounted thereon off the ground. The second support ring can provide an additional circumferential row of upstanding protrusions that can fit through and engage respective holes in the circumferential row of holes in the peripheral section of the closure head to mount the closure head on the second support ring. In this way, if desired, the second fixture can be used in place of the first fixture to directly mount the closure head to the second support ring. The second fixture can be used in the early stages of manufacturing, for example, as a supplemental welding or cladding fixture, or to accommodate changes in the IHP configuration, such as changes in the height of control latch assemblies, which may only be determined later during the design phase. Indeed, the second fixture can be configured to hold the first fixture off the ground before the closure head reaches the first fixture. Thus, the second fixture can be attached to and detached from the first fixture as needed. Even when detached, it can be transported through the factory on the same guided vehicle as the first fixture and closure head. The second fixture can be securely fastened to the first fixture via a bolting arrangement. This can be in the form of multiple nuts and bolts, each with a clevis pin-type safety, i.e., if the threads of a given bolt fail, each clevis pin is configured to retain the weight transmitted through the bolt.

[0018] In a third aspect, the present disclosure provides a fixture system including the fixture of the first aspect as a first fixture and a third fixture that fits over the closure head when the closure head is attached to the first fixture, the third fixture being located on a peripheral section of the closure head and protruding upward from the closure head. The third fixture can be clamped to the peripheral section, for example, via the screw holes of the fastening points formed by the upstanding protrusions, allowing the closure head and the third fixture to be inverted so that the closure head is attached to the third fixture, and the third fixture can then be placed on the ground. If the support ring has the first and second fastening points, the first fixture can be placed on top of the closure head by clamping the third fixture to the peripheral section via the first fastening point, allowing the first fixture to be inverted together with the closure head and the third fixture. Alternatively, by clamping the third fixture to the peripheral section via only the second fastening point, the first fixture can be removed from the closure head when it is inverted. For the former operation, the inversion can be performed using the rotational configuration of the first fixture described above. For the latter operation, the third fixture may have additional rotational formations, such as trunnions at its upper and lower ends, configured to facilitate rotation of the third fixture. Advantageously, the third fixture allows operations to be performed on the closure head, for example, with the dome-shaped central section of the closure head positioned with its convex side facing up. The third fixture can also help protect the peripheral section of the closure head from accidental damage in a factory environment.

[0019] In a fourth aspect, the present disclosure provides a method of manufacturing a closure head for a reactor pressure vessel having a central section through which control rod guide tubes pass and a peripheral section including a circumferential row of through holes corresponding to the locations of bolt holes through which the closure head will be bolted to the reactor pressure vessel in use, the method comprising the steps of: attaching the closure head to the support ring of the first aspect of the fitting by fitting the upstanding projections of the support ring through respective holes in the peripheral section of the closure head; Moving the closure head while attached to the support ring during a manufacturing operation.

[0020] Protective sleeves are placed around the upstanding projections of the support ring, which then fit through holes in the peripheral section of the closure head. These sleeves can be modified as needed to accommodate the diameter of the holes, which can be increased in size as the head is manufactured.

[0021] If the support ring has a first group of upstanding protrusions and a second group of upstanding protrusions, the method may further include the steps of: securing the lifting gear to the first fastening point, thereby lifting the closure head and the fixture; A step of moving the lifted closure head and the fixture to perform a manufacturing operation on the closure head while the closure head is supported by the support ring of the fixture.

[0022] Similarly, if the support ring has a first group of upstanding protrusions and a second group of upstanding protrusions, the method may further include the steps of: fastening the lifting gear only to the second fastening point, thereby lifting the closure head and the second portion of the support ring; Moving the elevated closure head and the second portion of the support ring to perform a manufacturing operation on the closure head.

[0023] If the fixture is a first fixture of the fixture system of the second aspect and the upstanding protrusion forms a fastening point, the method may further comprise the steps of: fastening the lifting gear to a fastening point (e.g., a first fastening point), thereby lifting the closure head and the support ring and the first fixture; Lowering the closure head, support ring, and first fixture onto the second fixture to hold the first fixture off the ground.

[0024] If the fixture is a first fixture of the fixture system of the third aspect and the upstanding protrusion forms a fastening point, the method may further comprise the steps of: the third fitting is located on the peripheral section of the closure head and projects upwardly from the closure head, and mounting the third fitting on the closure head; clamping a third fixture to a peripheral section of the closure head (e.g., via the first or second fastening points); Inverting the third fixture and closure head.

[0025] The present invention includes combinations of the described embodiments and preferred features except where such combinations are expressly not permitted or explicitly avoided.

[0026] BRIEF DESCRIPTION OF THE DRAWINGS Embodiments and examples illustrating the principles of the present invention will now be described with reference to the accompanying drawings. [Brief explanation of the drawings]

[0027] [Figure 1] 1 is a schematic diagram of a reactor pressure vessel. [Figure 2] FIG. 2 is a bottom perspective view of the head of the reactor pressure vessel lid. [Figure 3] FIG. 2 is a schematic perspective view of a first fitting for a closure head. [Figure 4] 1 is a schematic side view of the closure head attached to the support ring of the first fitting. FIG. [Figure 5] FIG. 10 is a schematic perspective view of the closure head being lifted from the first fixture. [Figure 6A] 10A and 10B show schematic views of the upstanding protrusions provided by the support ring with a protective sleeve sized to fit one of the holes. [Figure 6B]10A and 10B are schematic diagrams showing upstanding protrusions with protective sleeves for larger sized holes. [Figure 7A] 1 is a schematic side view of a first fitting and a closure head attached to a second fitting. FIG. [Figure 7B] 1 is a schematic side view of these components after installation is complete. [Figure 7C] FIG. 10 is a schematic side view of a second fixture that directly supports the closure head. [Figure 8A] FIG. 10 is a schematic side view of a first fitting and a closure head attached to a third fitting. [Figure 8B] 1 is a schematic side view of these components when the installation process is complete. [Figure 8C] 10 is a schematic side view of a third fixture supporting the closure head after the closure head has been inverted and removed from the first fixture. FIG. DETAILED DESCRIPTION OF THE INVENTION

[0028] Aspects and embodiments of the present invention will now be described with reference to the accompanying drawings. Further aspects and embodiments will be apparent to those skilled in the art. All documents mentioned herein are incorporated by reference.

[0029] 1 shows a reactor pressure vessel (RPV) 1 used in a PWR-type nuclear power generation system. The RPV 1 is housed within a containment structure 12 and has a removable closure head assembly 2. This closure head assembly 2 is an IHP having a closure head 3 for closing the top opening of an RPV body 4, thereby sealing the fuel assemblies / core (not shown) within a cavity 5 inside the RPV body. The IHP also has a control rod drive mechanism 10 within a shroud 11.

[0030] As shown in Figure 2, the closure head 3 has a sealing surface 6 at its lower axial end for sealing against the RPV body 4. The sealing surface 6 is annular and is surrounded by a circumferential annular flange 7 having a circumferential row of through holes 14 for receiving bolts which, in use, are tightened to seal the closure head onto the RPV body 4 via one or more seals located at the interface between the flange and the body. A central dome-shaped portion 16 of the closure head includes further holes 15 for the passage of control rod guide tubes and vent lines.

[0031] During manufacture of the closure head 3, the closure head 3 is moved from station to station within the factory to undergo successive manufacturing processes such as machining, welding, heat treating, fitting, testing, etc. To facilitate this movement, and to protect the closure head from accidental damage while it is within the factory, the closure head is mounted to a first fixture 20, shown schematically in Figure 3, which can be transported, moved by a guided vehicle, or moved via an overhead crane as needed.

[0032] The first fixture 20 includes a support ring 21 providing a circumferential row of upstanding projections 22 matingly engageable through respective holes 14 in the circumferential row of holes in the peripheral annular flange 7 of the closure head 3 for mounting the closure head thereon. Below the support ring is a lower frame 23 for holding the support ring and closure head when mounted off the ground. The lower frame includes a guard portion 24 extending from the base 25 of the lower frame to the support ring to help protect the closure head, and particularly the control rod guide tubes when mounted thereon, from accidental damage in a factory environment. The guard portion may be solid-walled or may have an opening formed therein (e.g., the guard portion may be formed from a circumferential array of posts or bars) so that the underside of the closure head is visible from outside the first fixture. Conveniently, the guard portion includes at least one opening 26 to allow the bottom-facing side of the closure head to be machined while it is positioned on the first fixture.

[0033] 4 shows diagrammatically a side view (albeit in cross section through the guard part 24) of the closure head 3 mounted on the support ring 21 with the convex side of the dome-shaped part 16 of the head facing downwards. A control rod guide tube 17, fixed in a further hole 15 in the closure head, extends downwards from the dome-shaped part in the space formed by the guard part 24 of the lower frame 23. The support ring 21 and base 25 of the lower frame 23 therefore support trunnions 34 at either end of the lower frame which facilitate rotation of the lower frame and support ring when a rotated position of the closure head is required for a particular manufacturing operation.

[0034] As best shown in FIG. 3, the upright protrusions 22 are divided into two groups. For example, a first group consisting of half of the protrusions is provided by a first portion 26 of the support ring 21, which is rigidly coupled to the lower frame 23. A second group consisting of the other half of the protrusions is provided by a plurality of second portions 27 of the support ring, which are liftably removable from corresponding recesses formed in the first portion. The first group is formed by an integer number N (3 in FIG. 3) of adjacent first subgroups of upright protrusions. Similarly, the second group is formed by N second subgroups of adjacent upright protrusions, each second subgroup being provided by a respective second portion 27 of the support ring. These first and second subgroups are arranged in an alternating pattern around the support ring.

[0035] The upright protrusions 22 each include a threaded hole for securing an eyebolt or other mounting device for a lifting gear. However, the threaded holes in the first group of protrusions have a different size and / or thread compared to the threaded holes in the second group of protrusions. Therefore, a lifting gear that can be engaged with the first group of protrusions cannot be engaged with the second group of protrusions, and vice versa. Thus, when the closure head 3 arrives to perform a manufacturing operation and it is desired that the head remain on the first fixture 20 (e.g., because the first fixture provides a convenient working platform for the head), the lifting gear is secured to the first group of protrusions, and the first fixture as well as the head are moved into position for the operation. In contrast, when the closure head 3 arrives for the performance of a manufacturing operation that does not require the first fixture, the lifting gear is fixed to the second group of protrusions (as shown diagrammatically in Figure 5) so that the head is moved into position together with the second part 27 of the support ring 21, but also together with the rest of the support ring and the lower frame 23. Once the manufacturing operation is completed, the lifting operation can then be reversed so that the second part 27 returns to the respective recesses in the first part 26 of the support ring and the first group of protrusions engage in the respective holes 14 in the annular flange 7.

[0036] The fastening points of the lifting gear provided by the first group of protrusions are configured to be different from the fastening points of the lifting gear provided by the second group of protrusions, so that depending on whether the first fixture 20 needs to be raised together with the closing head 3 or not, the lifting gear for each production operation can be arranged to a matching type, thereby ensuring that the correct lifting operation is always performed.

[0037] 3 and 5 show the first fixture 20 having an equal number of protrusions 22 in the first and second groups, but this is not necessary. For example, having more protrusions in the first group may be desirable because the lifting load when lifting the closure head 3 with the first fixture is significantly greater than when lifting without the closure head 3. Additionally or alternatively, to configure the fastening points of the lifting gear provided by the protrusions in the first group differently from those provided by the protrusions in the second group, the protrusions in the first group may have larger diameter threaded holes to accept larger eyebolts, which can carry a larger load. Another option is to have different integer values ​​for the first and second subgroups of protrusions (e.g., N=4).

[0038] The threaded holes in the protrusions 22 are not only used for lifting operations but can also be conveniently used to receive headed bolts for clamping the closure head 3 to the support ring 21 of the first fixture 20 when the closure head 3 is moved from place to place within the factory.

[0039] To protect the hole 14 in the peripheral annular flange 7 of the closure head 3, the upstanding protrusion 22 can carry a protective sleeve, for example made of nylon or a similar plastic material. However, during some manufacturing operations, the size of the hole 14 in the peripheral annular flange 7 of the closure head 3 may increase. To accommodate this, the outer diameter of the sleeve can be increased as needed. For example, FIG. 6A schematically shows a protrusion with a protective sleeve 31 sized to fit one hole, while FIG. 6B schematically shows the same upstanding protrusion with a replacement protective sleeve 31' sized to fit a larger hole.

[0040] The lower frame 23 can be configured to lift the support ring 21 high enough above the ground for all manufacturing operations. Another option, however, is to provide a removable second fixture 30 that can lift the first fixture 20 when needed, as shown diagrammatically in FIGS. 7A and 7B. Advantageously, this arrangement allows the center of gravity of the closure head 3 to be maintained relatively low for work and transport operations where extra height is not required, thereby reducing the risk of the head accidentally falling. The second fixture 30 can be particularly adapted for performing high-temperature operations such as welding, cladding, and heat treatment, and thus can be deployed as a work platform for such operations. In this case, the second fixture 30, together with the first fixture 20 and the closure head, can be easily moved around the factory as a separate item on a guided vehicle or via an overhead crane. The second fixture 30 can then be secured to a manipulator before the first fixture and the closure head are attached thereto. Like the first fixture, the second fixture 30 may have trunnions (not shown) at its upper and lower ends to facilitate its rotation.

[0041] As shown in FIGS. 7A and 7B, the second fixture 30 may have a second support ring 32 at its upper end, providing an additional circumferential row of upstanding protrusions 33. These protrusions 33 can be used to hold the two fixtures 20, 30 together if the base of the lower frame 23 has corresponding holes. For example, the ends of the protrusions 33 can be threaded to accept bolts that are tightened onto the base of the lower frame 23. These bolts can be supplemented with clevis pin-type safety devices inserted laterally through the protrusions 33 to maintain the two fixtures attached in the event that the bolt threads break. However, as shown in FIG. 7C, the protrusions 33 allow the second fixture 30 to directly support the closure head 3 by itself, i.e., without the intervention of the first fixture 20. This can be advantageous if the second fixture 30 is particularly suited as a mount for a closure head for a particular manufacturing operation.

[0042] Yet another option, as shown schematically in FIGS. 8A and 8B , provides a third fixture 40 that can be raised to fit over the lower frame 23 of the first fixture 20 and the closure head 3 attached to the support ring 21, so that the third fixture protrudes upward from the closure head. More specifically, the third fixture 40 may have an additional support ring 41 with a circumferential row of holes in which the upstanding protrusions 22 are disposed. Thus, the third fixture can protect the upper surface of the closure head from accidental damage. Furthermore, as shown in FIG. 8C , the third fixture 40, closure head 3, and second part 27 can be removed from the first fixture 20 and inverted by tightening bolts fitted into threaded holes in a second group of protrusions provided by the second part 27 of the support ring 21, so that the closure head rests on the third fixture 40 with the convex side of the dome-shaped portion 16 of the head facing upward. In this orientation, manufacturing operations can be performed on the closure head, but it would be difficult to perform manufacturing operations with the head upside down. To assist in inversion, the third fixture 40 may have trunnions (not shown) at its upper and lower ends. Instead of removing the third fixture 40 and closure head 3 from the first fixture 20 before inversion, the first fixture 20 together with the third fixture 40 and closure head 3 can be inverted by means of tightening bolts fitted into threaded holes in the first group of protrusions provided by the first portion 26 of the support ring 21, whereby the first fixture forms a protective cover over the top surface of the closure head.

[0043] The features disclosed in the foregoing description, or the following claims, or the accompanying drawings, are expressed in their specific form, or in terms of means for performing a disclosed function, or methods or processes for obtaining a disclosed result, and may, where appropriate, be utilized separately or in any combination of such features to realize the invention in its various forms.

[0044] While the present invention has been described in conjunction with the above exemplary embodiments, many equivalent modifications and variations will become apparent to those skilled in the art given this disclosure. Accordingly, the exemplary embodiments of the invention set forth above are considered to be illustrative and not limiting. Various changes can be made to the described embodiments without departing from the spirit and scope of the invention.

[0045] For the avoidance of doubt, the theoretical explanations provided herein are provided for the purpose of enhancing the reader's understanding, and the inventors do not wish to be bound by any of these theoretical explanations.

[0046] Any section headings used herein are for organizational purposes only and should not be construed as limiting the subject matter described.

[0047] Throughout this specification, including the claims that follow, and unless the context requires otherwise, the words "comprise" and "include," and variations such as "comprises," "comprising," and "including," will be understood to imply the inclusion of a stated integer or step or group of integers or steps, but not the exclusion of any other integer or step or group of integers or steps.

[0048] It should be noted that, as used in this specification and the appended claims, the singular forms "a," "an," and "the" include plural referents unless the context clearly dictates otherwise. Ranges may be expressed herein as from "about" one particular value and / or to "about" another particular value. When such a range is expressed, another embodiment includes from the one particular value and / or to the other particular value. Similarly, when values ​​are expressed as approximations, it is understood that the particular value forms another embodiment by use of the antecedent "about." The term "about" with respect to numerical values ​​is arbitrary and means, for example, + / - 10%.

Claims

1. 1. A fixture (20) for attaching a closure head (3) to a reactor pressure vessel during manufacture of the closure head, the closure head having a central section (16) for passage of control rod guide tubes (17) therethrough and a peripheral section (7) including a circumferential row of through holes (14) corresponding to the locations of bolt holes through which the closure head will be bolted to the reactor pressure vessel in use, the fixture comprising: a support ring (21) providing a circumferential row of upstanding projections (22) matingly engageable through respective holes in a circumferential row of holes in a peripheral section of said closure head for mounting said closure head to said support ring; a lower frame (23) for holding the support ring and the closure head mounted thereon off the ground; A fixture comprising:

2. 2. The fixture of claim 1, wherein the upstanding projections (22) form fastening points for fastening lifting gear, the lifting being performed by means of the lifting gear fastened to the fastening points, thereby lifting the closure head.

3. 3. The fixture of claim 2, wherein the fastening point is a threaded hole formed in the upstanding protrusion for fastening an eyebolt of a lifting gear to the upstanding protrusion.

4. 4. The fixture of claim 2 or 3, wherein a first group of the upright protrusions is provided by a first portion (26) rigidly connected to the lower frame, and a second group of the upright protrusions is provided by a plurality of second portions (27) of the support ring that are liftably removable from the first portion, the first and second groups of upright protrusions forming first and second fastening points for the lifting gear, respectively, so that when a closure head is attached to the support ring, lifting performed using the lifting gear fixed to the first fastening points lifts the closure head as well as the support ring and the lower frame, and lifting performed using the lifting gear fixed only to the second fastening points lifts only the closure head and the second portion of the support ring.

5. 5. The fixture of claim 4, wherein the first fastening point and the second fastening point are configured differently such that an elevator gear securable to the first fastening point is not securable to the second fastening point and an elevator gear securable to the second fastening point is not securable to the first fastening point.

6. 6. A fitting according to claim 5 when dependent on claim 3, wherein the threaded holes formed in the first group of upstanding protrusions have a different thread and / or a different diameter than the threaded holes formed in the second group of upstanding protrusions.

7. A fitting according to any one of claims 4 to 6, wherein the second portions of the support ring are received within respective recesses formed in the first portion of the support ring.

8. 8. A fixture as claimed in any one of claims 4 to 7, wherein the first group of upstanding protrusions is formed by first subgroups of an integer number N of adjacent upstanding protrusions and the second group of upstanding protrusions is formed by second subgroups of N adjacent upstanding protrusions, the first subgroups and second subgroups being arranged in an alternating pattern around the support ring.

9. A fitting according to any preceding claim, wherein the lower frame is configured to shield control rod guide tubes passing through the central section from accidental damage.

10. 10. A fixture system comprising a fixture according to any one of claims 1 to 9 as a first fixture, and further comprising a removable second fixture (30) that holds the first fixture off the ground.

11. 10. A fitting system comprising a fitting according to any one of claims 1 to 9 as a first fitting, and further comprising a third fitting (40) that fits over the closure head when the closure head is attached to the first fitting, the third fitting being located at the peripheral section of the closure head and protruding upwardly from the closure head.

12. 1. A method of manufacturing a closure head for a reactor pressure vessel, the closure head having a central section through which control rod guide tubes extend and a peripheral section including a circumferential array of through holes corresponding to the locations of bolt holes through which the closure head will be bolted to the reactor pressure vessel in use; - mounting the closure head on the support ring of a fitting according to any one of claims 1 to 9 by fitting the upstanding projections of the support ring through respective holes in the peripheral section of the closure head; moving the closure head while attached to the support ring during a manufacturing operation; A manufacturing method comprising:

13. 13. A method according to claim 12, wherein a protective sleeve (31, 31') is placed around the upstanding projection of the support ring before the upstanding projection fits through a hole in the peripheral section of the closure head.

14. securing a lifting gear to the first fastening point, thereby lifting the closure head and the fixture; moving the lifted closure head and the fixture to perform a manufacturing operation on the closure head while the closure head is supported by the support ring of the fixture; The method of claim 12 or 13, carried out using the fixture of claim 4, further comprising:

15. fastening a lifting gear only to the second fastening point, thereby lifting the closure head and the second portion of the support ring; moving the raised closure head and the second portion of the support ring to perform a manufacturing operation on the closure head; The method of claim 12 or 13, carried out using the fixture of claim 4, further comprising:

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